C++ Dialect Options (Using the GNU Compiler Collection (GCC))

### 3.5 Options Controlling C++ Dialect ¶

This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in. For example, you might compile a file `firstClass.C` like this:

    g++ -g -fstrict-enums -O -c firstClass.C

In this example, only `-fstrict-enums` is an option meant only for C++ programs; you can use the other options with any language supported by GCC.

Some options for compiling C programs, such as `-std`, are also relevant for C++ programs. See Options Controlling C Dialect.

Here is a list of options that are only for compiling C++ programs:

`--compile-std-module` ¶

  Build the compiled module interfaces (see C++ Modules) for the ‘`<bits/stdc++.h>`’ header unit and the ‘`std`’ and ‘`std.compat`’ modules before compiling any source files explicitly specified on the command line. This is intended to be useful for building simple programs that use ‘`import std;`’ with a single command.

`-fabi-version=n` ¶

  Use version n of the C++ ABI. The default is version 0.

  Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.

  Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

  Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.

  Version 3 corrects an error in mangling a constant address as a template argument.

  Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

  Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

  Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and –, and a class scope function used as a template argument.

  Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

  Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

  Version 9, which first appeared in G++ 5.2, corrects the alignment of `nullptr_t`.

  Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. ‘`stdcall`’).

  Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies `-fnew-inheriting-ctors`.

  Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor.

  Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.

  Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

  Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.

  Version 16, which first appeared in G++ 11, changes the mangling of `__alignof__` to be distinct from that of `alignof`, and dependent operator names.

  Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.

  Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.

  Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, handling of cv-qualified [[no_unique_address]] members, and adds mangling of C++20 constraints on function templates.

  Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.

  Version 21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231).

  See also `-Wabi`.

`-fabi-compat-version=n` ¶

  On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias.

  With `-fabi-version=0` (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use `-fabi-compat-version=2`.

  If this option is not provided but `-Wabi=n` is, that version is used for compatibility aliases. If this option is provided along with `-Wabi` (without the version), the version from this option is used for the warning.

`-fno-access-control` ¶

  Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.

`-faligned-new` ¶
`-faligned-new=alignment`

  Enable support for C++17 `new` of types that require more alignment than `void* ::operator new(std::size_t)` provides. A numeric argument such as `-faligned-new=32` can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default of `alignof(std::max_align_t)`.

  This flag is enabled by default for `-std=c++17`.

`-fno-assume-sane-operators-new` ¶

  The C++ standard allows replacing the global `new`, `new[]`, `delete` and `delete[]` operators, though a lot of C++ programs don’t replace them and just use the implementation provided version. Furthermore, the C++ standard allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed if `__builtin_operator_new` or `__builtin_operator_delete` functions are used). This option allows control over some optimizations around calls to those operators. With `-fassume-sane-operators-new-delete` option GCC may assume that calls to the replaceable global operators from new or delete expressions or from `__builtin_operator_new` or `__builtin_operator_delete` calls don’t read or modify any global variables or variables whose address could escape to the operators (global state; except for `errno` for the `new` and `new[]` operators). This allows most optimizations across those calls and is something that the implementation provided operators satisfy unless `malloc` implementation details are observable in the code or unless `malloc` hooks are used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default. With `-fno-assume-sane-operators-new-delete` option GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and code needs to expect such behavior.

`-fchar8_t` ¶
`-fno-char8_t`

  Enable support for `char8_t` as adopted for C++20. This includes the addition of a new `char8_t` fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, associated standard library updates, and new `__cpp_char8_t` and `__cpp_lib_char8_t` feature test macros.

  This option enables functions to be overloaded for ordinary and UTF-8 strings:

      int f(const char *);    // #1
      int f(const char8_t *); // #2
      int v1 = f("text");     // Calls #1
      int v2 = f(u8"text");   // Calls #2

  and introduces new signatures for user-defined literals:

      int operator""_udl1(char8_t);
      int v3 = u8'x'_udl1;
      int operator""_udl2(const char8_t*, std::size_t);
      int v4 = u8"text"_udl2;
      template<typename T, T...> int operator""_udl3();
      int v5 = u8"text"_udl3;

  The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when `-fchar8_t` is specified.

      const char *cp = u8"xx";// error: invalid conversion from
                              //        `const char8_t*' to `const char*'
      int f(const char*);
      auto v = f(u8"xx");     // error: invalid conversion from
                              //        `const char8_t*' to `const char*'
      std::string s{u8"xx"};  // error: no matching function for call to
                              //        `std::basic_string<char>::basic_string()'
      using namespace std::literals;
      s = u8"xx"s;            // error: conversion from
                              //        `basic_string<char8_t>' to non-scalar
                              //        type `basic_string<char>' requested

`-fcheck-new` ¶

  Check that the pointer returned by `operator new` is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that `operator new` only returns `0` if it is declared `throw()`, in which case the compiler always checks the return value even without this option. In all other cases, when `operator new` has a non-empty exception specification, memory exhaustion is signalled by throwing `std::bad_alloc`. See also ‘`new (nothrow)`’.

`-fconcepts` ¶

  Enable support for the C++ Concepts feature for constraining template arguments. With `-std=c++20` and above, Concepts are part of the language standard, so `-fconcepts` defaults to on.

  Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn’t make it into the standard, could additionally be enabled by `-fconcepts-ts`. The option `-fconcepts-ts` was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20 concepts.

`-fconcepts-diagnostics-depth=n` ¶

  Specify maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is 1.

`-fconstexpr-depth=n` ¶

  Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.

`-fconstexpr-cache-depth=n` ¶

  Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you.

`-fconstexpr-fp-except` ¶

  Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.

      constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except

`-fconstexpr-loop-limit=n` ¶

  Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).

`-fconstexpr-ops-limit=n` ¶

  Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25).

`-fcontracts` ¶

  Enable support for the C++ Contracts feature, as specified in the C++26 working draft (N5003).

  On violation of an enforced or observed contract (see `-fcontract-evaluation-semantic` below), a violation handler is called; the standard library provides a default handler that emits information about the contract that failed.

  Users can replace the default violation handler by defining

      void
      handle_contract_violation (const std::contracts::contract_violation&);

`-fcontract-evaluation-semantic=semantic` ¶

  Set the semantic with which contracts will be evaluated to semantic.

  Available values for the evaluation mode are:

  ’`ignored`’ The contract checks will be elided, with no checking added at either compile or runtime.

  ’`observed`’ The contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, the execution of the code will continue. At compile-time the checks are performed for `constexpr` evaluations, in this case a diagnostic is emitted if the check fails.

  ’`enforce`’ The contract checks are performed and the handler is called if one fails. When the handler returns the execution of the code is terminated preventing it from continuing past the failed case. At compile-time the checks are applied to `constexpr` and, if one fails, the program is ill-formed; an error will be emitted.

  ’`quick_enforce`’ The contract checks are performed (at runtime) and, if one fails, the execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the `enforce` case, since no handler is invoked at compile time.

`-fcontracts-conservative-ipa` ¶

  This prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs which means that such actions would be potentially incorrect.

`-fcontract-checks-outlined` ¶

  By default, contract checks for `pre` and `post` condition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body.

`-fcontract-disable-optimized-checks` ¶

  When `pre` and `post` condition checks are outlined (using `-fcontract-checks-outlined`), this option disables the optimisation steps for those functions which can avoid unwanted elision of checking steps.

`-fcontracts-client-check=mode` ¶

  This option enables insertion of checks at call sites (caller-side checking).

  The value of mode determines which contract checks are made: ’`none`’ No caller/client side checking (default)

  ’`pre`’ Preconditions are checked at the caller/client side.

  ’`all`’ Both pre and post-conditions are checked at the caller/client side.

`-fcontracts-definition-check=mode` ¶

  This option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active.

  The variable mode has the values ’`on`’ and ’`off`’.

`-fcoroutines` ¶

  Enable support for the C++ coroutines extension. With `-std=c++20` and above, coroutines are part of the language standard, so `-fcoroutines` defaults to on.

`-fdiagnostics-all-candidates` ¶

  Permit the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected.

`-fdump-lang-`
`-fdump-lang-switch`
`-fdump-lang-switch-options`
`-fdump-lang-switch-options=filename`

  Control the dumping of C++-specific information. The options and filename portions behave as described in the `-fdump-tree` option. The following switch values are accepted:

  ‘`all`’

    Enable all of the below.

  ‘`class`’ ¶

    Dump class hierarchy information. Virtual table information is emitted unless ’`slim`’ is specified.

  ‘`module`’ ¶

    Dump module information. Options `lineno` (locations), `graph` (reachability), `blocks` (clusters), `uid` (serialization), `alias` (mergeable), `asmname` (Elrond), `eh` (mapper) & `vops` (macros) may provide additional information.

  ‘`raw`’ ¶

    Dump the raw internal tree data.

  ‘`tinst`’ ¶

    Dump the sequence of template instantiations, indented to show the depth of recursion. The `lineno` option adds the source location where the instantiation was triggered, and the `details` option also dumps pre-instantiation substitutions such as those performed during template argument deduction.

    Lines in the .tinst dump start with ‘`I`’ for an instantiation, ‘`S`’ for another substitution, and ‘`R[IS]`’ for the reopened context of a deferred instantiation.

`-fno-elide-constructors` ¶

  The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

  In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.

`-fno-enforce-eh-specs` ¶

  Don’t generate code to check for violation of exception specifications at run time. This option violates the C++ standard, but may be useful for reducing code size in production builds, much like defining `NDEBUG`. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time.

`-fextern-tls-init` ¶
`-fno-extern-tls-init`

  The C++11 and OpenMP standards allow `thread_local` and `threadprivate` variables to have dynamic (runtime) initialization. To support this, any use of such a variable goes through a wrapper function that performs any necessary initialization. When the use and definition of the variable are in the same translation unit, this overhead can be optimized away, but when the use is in a different translation unit there is significant overhead even if the variable doesn’t actually need dynamic initialization. If the programmer can be sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is statically initialized, or a use of the variable in the defining TU will be executed before any uses in another TU), they can avoid this overhead with the `-fno-extern-tls-init` option.

  On targets that support symbol aliases, the default is `-fextern-tls-init`. On targets that do not support symbol aliases, the default is `-fno-extern-tls-init`.

`-ffold-simple-inlines` ¶
`-fno-fold-simple-inlines`

  Permit the C++ frontend to fold calls to `std::move`, `std::forward`, `std::addressof`, `std::to_underlying` and `std::as_const`. In contrast to inlining, this means no debug information will be generated for such calls. Since these functions are rarely interesting to debug, this flag is enabled by default unless `-fno-inline` is active.

`-fno-gnu-keywords` ¶

  Do not recognize `typeof` as a keyword, so that code can use this word as an identifier. You can use the keyword `__typeof__` instead. This option is implied by the strict ISO C++ dialects: `-ansi`, `-std=c++98`, `-std=c++11`, etc.

`-fno-immediate-escalation` ¶

  Do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified in P2564R3. For example:

      consteval int id(int i) { return i; }

      constexpr int f(auto t)
      {
        return t + id(t); // id causes f<int> to be promoted to consteval
      }

      void g(int i)
      {
        f (3);
      }

  compiles in C++20: `f` is an immediate-escalating function (due to the `auto` it is a function template and is declared `constexpr`) and `id(t)` is an immediate-escalating expression, so `f` is promoted to `consteval`. Consequently, the call to `id(t)` is in an immediate context, so doesn’t have to produce a constant (that is the mechanism allowing consteval function composition). However, with `-fno-immediate-escalation`, `f` is not promoted to `consteval`, and since the call to consteval function `id(t)` is not a constant expression, the compiler rejects the code.

  This option is turned on by default; it is only effective in C++20 mode or later.

`-fimplicit-constexpr` ¶

  Make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and other optimizations.

`-fno-implicit-templates` ¶

  Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. If you use this option, you must take care to structure your code to include all the necessary explicit instantiations to avoid getting undefined symbols at link time. See Where’s the Template?, for more information.

`-fno-implicit-inline-templates` ¶

  Don’t emit code for implicit instantiations of inline templates, either. The default is to handle inlines differently so that compiles with and without optimization need the same set of explicit instantiations.

`-fno-implement-inlines` ¶

  To save space, do not emit out-of-line copies of inline functions controlled by `#pragma implementation`. This causes linker errors if these functions are not inlined everywhere they are called.

`-fmodules` ¶
`-fno-modules`

  Enable support for C++20 modules (see C++ Modules). The `-fno-modules` is usually not needed, as that is the default. Even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version.

`-fmodule-header` ¶
`-fmodule-header=user`
`-fmodule-header=system`

  Compile a header file to create an importable header unit.

`-fmodule-implicit-inline` ¶

  Member functions defined in their class definitions are not implicitly inline for modular code. This is different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code porting. This option makes such function definitions implicitly inline. It does however generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.)

`-fno-module-lazy` ¶

  Disable lazy module importing and module mapper creation.

`-fmodule-mapper=[hostname]:port[?ident]` ¶
`-fmodule-mapper=|program[?ident] args...`
`-fmodule-mapper==socket[?ident]`
`-fmodule-mapper=<>[inout][?ident]`
`-fmodule-mapper=<in>out[?ident]`
`-fmodule-mapper=file[?ident]`

  An oracle to query for module name to filename mappings. If unspecified the `CXX_MODULE_MAPPER` environment variable is used, and if that is unset, an in-process default is provided.

`-fmodule-only` ¶

  Only emit the Compiled Module Interface, inhibiting any object file.

`-fms-extensions` ¶

  Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member function via non-standard syntax.

`-fnew-inheriting-ctors` ¶

  Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless `-fabi-version=10` or lower is specified.

`-fnew-ttp-matching` ¶

  Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for `-std=c++17`.

`-fno-nonansi-builtins` ¶

  Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include `ffs`, `alloca`, `_exit`, `index`, `bzero`, `conjf`, and other related functions.

`-fnothrow-opt` ¶

  Treat a `throw()` exception specification as if it were a `noexcept` specification to reduce or eliminate the text size overhead relative to a function with no exception specification. If the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller because the EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to `terminate` rather than `unexpected`.

`-fno-operator-names` ¶

  Do not treat the operator name keywords `and`, `bitand`, `bitor`, `compl`, `not`, `or` and `xor` as synonyms as keywords.

`-fno-optional-diags` ¶

  Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class.

`-fno-pretty-templates` ¶

  When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. `void f(T) [with T = int]` rather than `void f(int)`) so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use `-fno-pretty-templates` to disable them.

`-frange-for-ext-temps` ¶

  Enable lifetime extension of C++ range based for temporaries. With `-std=c++23` and above this is part of the language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard.

`-freflection` ¶

  Enable experimental C++26 Reflection.

`-fno-rtti` ¶

  Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (`dynamic_cast` and `typeid`). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. The `dynamic_cast` operator can still be used for casts that do not require run-time type information, i.e. casts to `void *` or to unambiguous base classes.

  Mixing code compiled with `-frtti` with that compiled with `-fno-rtti` may not work. For example, programs may fail to link if a class compiled with `-fno-rtti` is used as a base for a class compiled with `-frtti`.

`-fsized-deallocation` ¶

  Enable the built-in global declarations

      void operator delete (void *, std::size_t) noexcept;
      void operator delete[] (void *, std::size_t) noexcept;

  as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster. Enabled by default under `-std=c++14` and above. The flag `-Wsized-deallocation` warns about places that might want to add a definition.

`-fstrict-enums` ¶

  Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type.

`-fstrong-eval-order` ¶
`-fstrong-eval-order=kind`

  Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. `-fstrong-eval-order` is equivalent to `-fstrong-eval-order=all`, and is enabled by default with `-std=c++17` or later.

  `-fstrong-eval-order=some` enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.

  `-fstrong-eval-order=none` is equivalent to `-fno-strong-eval-order`.

`-ftemplate-backtrace-limit=n` ¶

  Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10.

`-ftemplate-depth=n` ¶

  Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations.

`-fno-threadsafe-statics` ¶

  Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this option to reduce code size slightly in code that doesn’t need to be thread-safe.

`-fuse-cxa-atexit` ¶

  Register destructors for objects with static storage duration with the `__cxa_atexit` function rather than the `atexit` function. This option is required for fully standards-compliant handling of static destructors, but only works if your C library supports `__cxa_atexit`.

`-fno-use-cxa-get-exception-ptr` ¶

  Don’t use the `__cxa_get_exception_ptr` runtime routine. This causes `std::uncaught_exception` to be incorrect, but is necessary if the runtime routine is not available.

`-fvisibility-inlines-hidden` ¶

  This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

  The effect of this is that GCC may, effectively, mark inline methods with `__attribute__ ((visibility ("hidden")))` so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.

  The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

  You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

  Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. See Where’s the Template?.

`-fvisibility-ms-compat` ¶

  This flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio.

  The flag makes these changes to GCC’s linkage model:

  1. It sets the default visibility to `hidden`, like `-fvisibility=hidden`.
  2. Types, but not their members, are not hidden by default.
  3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

  In new code it is better to use `-fvisibility=hidden` and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.

  Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

`-fno-weak` ¶

  Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++.

`-fext-numeric-literals (C++ and Objective-C++ only)` ¶

  Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: `-std=c++98`, `-std=gnu++98`, `-std=gnu++11`, `-std=gnu++14`. This option is off by default for ISO C++11 onwards (`-std=c++11`, ...).

`-nostdinc++` ¶

  Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.)

`-flang-info-include-translate` ¶
`-flang-info-include-translate-not`
`-flang-info-include-translate=header`

  Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form `"user"` or `<system>` it will be resolved to a specific user or system header using the include path.

`-flang-info-module-cmi` ¶
`-flang-info-module-cmi=module`

  Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will note reading a specific module’s CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in `<>` or `""`).

`-stdlib=libstdc++,libc++` ¶

  When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit `-lstdc++` or `-lc++` respectively, when a C++ runtime is required for linking.

In addition, these warning options have meanings only for C++ programs:

`-Wabi-tag (C++ and Objective-C++ only)` ¶

  Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++-Specific Variable, Function, and Type Attributes for more information about ABI tags.

`-Wno-abbreviated-auto-in-template-arg` ¶

  Disable the error for an `auto` placeholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.

      void f(S<auto>);

  This feature was proposed in the Concepts TS, but was not adopted into C++20; in the standard, a placeholder in a parameter declaration must appear as a decl-specifier. The error can also be reduced to a warning by `-fpermissive` or `-Wno-error=abbreviated-auto-in-template-arg`.

`-Wcomma-subscript (C++ and Objective-C++ only)` ¶

  Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in `( )` is not deprecated. Example:

      void f(int *a, int b, int c) {
          a[b,c];     // deprecated in C++20, invalid in C++23
          a[(b,c)];   // OK
      }

  In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with `-Wno-comma-subscript`.

  Enabled by default with `-std=c++20` unless `-Wno-deprecated`, and after `-std=c++23` regardless of `-Wno-deprecated`. Before `-std=c++20`, enabled with explicit `-Wdeprecated`.

  This warning is upgraded to an error by `-pedantic-errors` in C++23 mode or later.

`-Wctad-maybe-unsupported (C++ and Objective-C++ only)` ¶

  Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:

      struct allow_ctad_t; // any name works
      template <typename T> struct S {
        S(T) { }
      };
      // Guide with incomplete parameter type will never be considered.
      S(allow_ctad_t) -> S<void>;

`-Wctor-dtor-privacy (C++ and Objective-C++ only)` ¶

  Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor.

`-Wdangling-reference (C++ and Objective-C++ only)` ¶

  Warn when a reference is bound to a temporary whose lifetime has ended. For example:

      int n = 1;
      const int& r = std::max(n - 1, n + 1); // r is dangling

  In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference `r` is dangling. This warning also detects dangling references in member initializer lists:

      const int& f(const int& i) { return i; }
      struct S {
        const int &r; // r is dangling
        S() : r(f(10)) { }
      };

  Member functions are checked as well, but only their object argument:

      struct S {
         const S& self () { return *this; }
      };
      const S& s = S().self(); // s is dangling

  Certain functions are safe in this respect, for example `std::use_facet`: they take and return a reference, but they don’t return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a `#pragma`:

      #pragma GCC diagnostic push
      #pragma GCC diagnostic ignored "-Wdangling-reference"
      const T& foo (const T&) { ... }
      #pragma GCC diagnostic pop

  The `#pragma` can also surround the class; in that case, the warning will be disabled for all the member functions.

  `-Wdangling-reference` also warns about code like

      auto p = std::minmax(1, 2);

  where `std::minmax` returns `std::pair<const int&, const int&>`, and both references dangle after the end of the full expression that contains the call to `std::minmax`.

  The warning does not warn for `std::span`-like classes. We consider classes of the form:

      template<typename T>
      struct Span {
        T* data_;
        std::size len_;
      };

  as `std::span`-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor.

  The warning can be disabled by using the `gnu::no_dangling` attribute (see C++-Specific Variable, Function, and Type Attributes).

  This warning is enabled by `-Wextra`.

`-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)` ¶

  Warn when `delete` is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by `-Wall`.

`-Wdeprecated-copy (C++ and Objective-C++ only)` ¶

  Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by `-Wextra`.

`-Wdeprecated-copy-dtor (C++ and Objective-C++ only)` ¶

  Similar to `-Wdeprecated-copy`, but also deprecate if the class has a user-provided destructor.

`-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only)` ¶

  Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:

      enum E1 { e };
      enum E2 { f };
      int k = f - e;

  `-Wdeprecated-enum-enum-conversion` is enabled by default with `-std=c++20`. In pre-C++20 dialects, this warning can be enabled by `-Wenum-conversion` or `-Wdeprecated`.

`-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only)` ¶

  Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:

      enum E1 { e };
      enum E2 { f };
      bool b = e <= 3.7;

  `-Wdeprecated-enum-float-conversion` is enabled by default with `-std=c++20`. In pre-C++20 dialects, this warning can be enabled by `-Wenum-conversion` or `-Wdeprecated`.

`-Wdeprecated-literal-operator (C++ and Objective-C++ only)` ¶

  Warn that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit `-Wdeprecated`.

      string operator "" _i18n(const char*, std::size_t); // deprecated
      string operator ""_i18n(const char*, std::size_t); // preferred

`-Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only)` ¶

  Warn that omitting a comma before the varargs `...` at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit `-Wdeprecated`.

      void f1(int...); // deprecated
      void f1(int, ...); // preferred
      template <typename ...T>
      void f2(T...); // ok
      template <typename ...T>
      void f3(T......); // deprecated

`-Wno-elaborated-enum-base` ¶

  For C++11 and above, warn if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with -pedantic-errors.

`-Wno-init-list-lifetime (C++ and Objective-C++ only)` ¶

  Do not warn about uses of `std::initializer_list` that are likely to result in dangling pointers. Since the underlying array for an `initializer_list` is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array’s lifetime. For example:

  - If a function returns a temporary `initializer_list`, or a local `initializer_list` variable, the array’s lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
  - If a new-expression creates an `initializer_list`, the array only lives until the end of the enclosing full-expression, so the `initializer_list` in the heap has a dangling pointer.
  - When an `initializer_list` variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the `initializer_list` variable has a dangling pointer.

        // li's initial underlying array lives as long as li
        std::initializer_list<int> li = { 1,2,3 };
        // assignment changes li to point to a temporary array
        li = { 4, 5 };
        // now the temporary is gone and li has a dangling pointer
        int i = li.begin()[0] // undefined behavior

  - When a list constructor stores the `begin` pointer from the `initializer_list` argument, this doesn’t extend the lifetime of the array, so if a class variable is constructed from a temporary `initializer_list`, the pointer is left dangling by the end of the variable declaration statement.
`-Winvalid-constexpr` ¶

  Warn when a function never produces a constant expression. In C++20 and earlier, for every `constexpr` function and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it’s possible to have a function or a function template marked `constexpr` for which no invocation satisfies the requirements of a core constant expression.

  This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, `-Winvalid-constexpr` can be turned on, in which case it will be an ordinary warning. For example:

      void f (int& i);
      constexpr void
      g (int& i)
      {
        // Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
        f(i);
      }

`-Winvalid-imported-macros` ¶

  Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it requires additional processing to determine. It may be useful when preparing sets of header-units to ensure consistent macros.

`-Wno-literal-suffix (C++ and Objective-C++ only)` ¶

  Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from `<inttypes.h>`. For example:

      #define __STDC_FORMAT_MACROS
      #include <inttypes.h>
      #include <stdio.h>

      int main() {
        int64_t i64 = 123;
        printf("My int64: %" PRId64"\n", i64);
      }

  In this case, `PRId64` is treated as a separate preprocessing token.

  This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn’t begin with an underscore. Literal suffix identifiers that don’t begin with an underscore are reserved for future standardization.

  These warnings are enabled by default.

`-Wno-narrowing (C++ and Objective-C++ only)` ¶

  For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but `-Wno-narrowing` suppresses the diagnostic. Note that this does not affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts.

  With `-Wnarrowing` in C++98, warn when a narrowing conversion prohibited by C++11 occurs within ‘`{ }`’, e.g.

      int i = { 2.2 }; // error: narrowing from double to int

  This flag is included in `-Wall` and `-Wc++11-compat`.

`-Wnoexcept (C++ and Objective-C++ only)` ¶

  Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-throwing exception specification (i.e. `throw()` or `noexcept`) but is known by the compiler to never throw an exception.

`-Wnoexcept-type (C++ and Objective-C++ only)` ¶

  Warn if the C++17 feature making `noexcept` part of a function type changes the mangled name of a symbol relative to C++14. Enabled by `-Wabi` and `-Wc++17-compat`.

  As an example:

      template <class T> void f(T t) { t(); };
      void g() noexcept;
      void h() { f(g); }

  In C++14, `f` calls `f<void(*)()>`, but in C++17 it calls `f<void(*)()noexcept>`.

`-Wclass-memaccess (C++ and Objective-C++ only)` ¶

  Warn when the destination of a call to a raw memory function such as `memset` or `memcpy` is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to `memset` below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.

      std::string str = "abc";
      memset (&str, 0, sizeof str);

  The `-Wclass-memaccess` option is enabled by `-Wall`. Explicitly casting the pointer to the class object to `void *` or to a type that can be safely accessed by the raw memory function suppresses the warning.

`-Wnon-virtual-dtor (C++ and Objective-C++ only)` ¶

  Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if `-Weffc++` is specified. The `-Wdelete-non-virtual-dtor` option (enabled by `-Wall`) should be preferred because it warns about the unsafe cases without false positives.

`-Wregister (C++ and Objective-C++ only)` ¶

  Warn on uses of the `register` storage class specifier, except when it is part of the GNU Variables in Specified Registers extension. The use of the `register` keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with `-std=c++17`.

`-Wreorder (C++ and Objective-C++ only)` ¶

  Warn when the order of member initializers given in the code does not match the order in which they must be executed. For instance:

      struct A {
        int i;
        int j;
        A(): j (0), i (1) { }
      };

  The compiler rearranges the member initializers for `i` and `j` to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by `-Wall`.

`-Wno-pessimizing-move (C++ and Objective-C++ only)` ¶

  This warning warns when a call to `std::move` prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.

      struct T {
      ...
      };
      T fn()
      {
        T t;
        ...
        return std::move (t);
      }

  But in this example, the `std::move` call prevents copy elision.

  This warning is enabled by `-Wall`.

`-Wno-redundant-move (C++ and Objective-C++ only)` ¶

  This warning warns about redundant calls to `std::move`; that is, when a move operation would have been performed even without the `std::move` call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn’t applicable. Consider:

      struct T {
      ...
      };
      T fn(T t)
      {
        ...
        return std::move (t);
      }

  Here, the `std::move` call is redundant. Because G++ implements Core Issue 1579, another example is:

      struct T { // convertible to U
      ...
      };
      struct U {
      ...
      };
      U fn()
      {
        T t;
        ...
        return std::move (t);
      }

  In this example, copy elision isn’t applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue.

  This warning is enabled by `-Wextra`.

`-Wrange-loop-construct (C++ and Objective-C++ only)` ¶

  This warning warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the range declaration is not a reference, but probably should be. For example:

      struct S { char arr[128]; };
      void fn () {
        S arr[5];
        for (const auto x : arr) { ... }
      }

  It does not warn when the type being copied is a trivially copy constructible type whose size is less than 64 bytes.

  This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type resulting in a copy. For example:

      void fn() {
        int arr[10];
        for (const double &x : arr) { ... }
      }

  In the example above, in every iteration of the loop a temporary value of type `double` is created and destroyed, to which the reference `const double &` is bound.

  This warning is enabled by `-Wall`.

`-Wredundant-tags (C++ and Objective-C++ only)` ¶

  Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:

      struct foo;
      struct foo *p;   // warn that keyword struct can be eliminated

  On the other hand, in this example there is no warning:

      struct foo;
      void foo ();   // "hides" struct foo
      void bar (struct foo&);  // no warning, keyword struct is necessary

`-Wno-subobject-linkage (C++ and Objective-C++ only)` ¶

  Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn’t give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions. `-Wsubobject-linkage` is enabled by default.

`-Weffc++ (C++ and Objective-C++ only)` ¶

  Warn about violations of the following style guidelines from Scott Meyers’ Effective C++ series of books:

  - Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  - Prefer initialization to assignment in constructors.
  - Have `operator=` return a reference to `*this`.
  - Don’t try to return a reference when you must return an object.
  - Distinguish between prefix and postfix forms of increment and decrement operators.
  - Never overload `&&`, `||`, or `,`.

  This option also enables `-Wnon-virtual-dtor`, which is also one of the effective C++ recommendations. However, the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too.

  When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use ‘`grep -v`’ to filter out those warnings.

`-Wno-exceptions (C++ and Objective-C++ only)` ¶

  Disable the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers.

`-Wsfinae-incomplete (C++ and Objective-C++ only)` ¶

  Warn about a class that is found to be incomplete, or a function with auto return type that has not yet been deduced, in a context where that causes substitution failure rather than an error, and then the class or function is defined later in the translation unit. This is problematic because template instantiations or concept checks could have different results if they first occur either before or after the definition.

  This warning is enabled by default. `-Wsfinae-incomplete=2` adds a warning at the point of substitution failure, to make it easier to track down problems flagged by the default mode.

`-Wstrict-null-sentinel (C++ and Objective-C++ only)` ¶

  Warn about the use of an uncasted `NULL` as sentinel. When compiling only with GCC this is a valid sentinel, as `NULL` is defined to `__null`. Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.

`-Wno-non-c-typedef-for-linkage (C++ and Objective-C++ only)` ¶

  Disable pedwarn for unnamed classes with a typedef name for linkage purposes containing C++ specific members, base classes, default member initializers or lambda expressions, including those on nested member classes.

      typedef struct {
        int a; // non-static data members are ok
        struct T { int b; }; // member classes too
        enum E { E1, E2, E3 }; // member enumerations as well
        int c = 42; // default member initializers are not ok
        struct U : A { int c; }; // classes with base classes are not ok
        typedef int V; // typedef is not ok
        using W = int; // using declaration is not ok
        decltype([](){}) x; // lambda expressions not ok
      } S;

  In all these cases, the tag name S should be added after the struct keyword.

`-Wno-non-template-friend (C++ and Objective-C++ only)` ¶

  Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such as ‘`friend int foo(int)`’, where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default.

`-Wold-style-cast (C++ and Objective-C++ only)` ¶

  Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts (`dynamic_cast`, `static_cast`, `reinterpret_cast`, and `const_cast`) are less vulnerable to unintended effects and much easier to search for.

`-Woverloaded-virtual (C++ and Objective-C++ only)` ¶
`-Woverloaded-virtual=n`

  Warn when a function declaration hides virtual functions from a base class. For example, in:

      struct A {
        virtual void f();
      };

      struct B: public A {
        void f(int); // does not override
      };

  the `A` class version of `f` is hidden in `B`, and code like:

      B* b;
      b->f();

  fails to compile.

  In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add `using A::f;` to `B`.

  The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:

      struct C {
        virtual void f();
        virtual void f(int);
      };

      struct D: public C {
        void f(int); // does override
      }

  This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.

  At level 1, this case does not warn; at level 2, it does. `-Woverloaded-virtual` by itself selects level 2. Level 1 is included in `-Wall`.

`-Wno-pmf-conversions (C++ and Objective-C++ only)` ¶

  Disable the diagnostic for converting a bound pointer to member function to a plain pointer.

`-Wsign-promo (C++ and Objective-C++ only)` ¶

  Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior.

`-Wtemplates (C++ and Objective-C++ only)` ¶

  Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates.

`-Wmismatched-new-delete (C++ and Objective-C++ only)` ¶

  Warn for mismatches between calls to `operator new` or `operator delete` and the corresponding call to the allocation or deallocation function. This includes invocations of C++ `operator delete` with pointers returned from either mismatched forms of `operator new`, or from other functions that allocate objects for which the `operator delete` isn’t a suitable deallocator, as well as calls to other deallocation functions with pointers returned from `operator new` for which the deallocation function isn’t suitable.

  For example, the `delete` expression in the function below is diagnosed because it doesn’t match the array form of the `new` expression the pointer argument was returned from. Similarly, the call to `free` is also diagnosed.

      void f ()
      {
        int *a = new int[n];
        delete a;   // warning: mismatch in array forms of expressions

        char *p = new char[n];
        free (p);   // warning: mismatch between new and free
      }

  The related option `-Wmismatched-dealloc` diagnoses mismatches involving allocation and deallocation functions other than `operator new` and `operator delete`.

  `-Wmismatched-new-delete` is included in `-Wall`.

`-Wmismatched-tags (C++ and Objective-C++ only)` ¶

  Warn for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.

  For example, the declaration of `struct Object` in the argument list of `draw` triggers the warning. To avoid it, either remove the redundant class-key `struct` or replace it with `class` to match its definition.

      class Object {
      public:
        virtual ~Object () = 0;
      };
      void draw (struct Object*);

  It is not wrong to declare a class with the class-key `struct` as the example above shows. The `-Wmismatched-tags` option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with `-Wredundant-tags`.

`-Wmultiple-inheritance (C++ and Objective-C++ only)` ¶

  Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use multiple inheritance.

`-Wvirtual-inheritance` ¶

  Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also define classes that indirectly use virtual inheritance.

`-Wno-virtual-move-assign` ¶

  Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled.

`-Wnamespaces` ¶

  Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names.

`-Wno-template-body (C++ and Objective-C++ only)` ¶

  Disable diagnosing errors when parsing a template, and instead issue an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings with `Wno-error=` or `-fpermissive`.

`-Wno-template-id-cdtor (C++ and Objective-C++ only)` ¶

  Disable the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example:

      template<typename T> struct S {
        S<T>(); // should be S();
        ~S<T>();  // should be ~S();
      };

  `-Wtemplate-id-cdtor` is enabled by default with `-std=c++20`; it is also enabled by `-Wc++20-compat`.

`-Wtemplate-names-tu-local` ¶

  Warn when a template body hides an exposure of a translation-unit-local entity. In most cases, referring to a translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a module’s CMI is an error. However, within the initializer of a variable, or in the body of a non-inline function, this is not an exposure and no error is emitted.

  This can cause variable or function templates to accidentally become unusable if they reference such an entity, because other translation units that import the template will never be able to instantiate it. This warning attempts to detect cases where this might occur. The presence of an explicit instantiation silences the warning.

  This flag is enabled by `-Wextra`.

`-Wno-expose-global-module-tu-local` ¶

  An exposure of a translation-unit-local entity from a module interface is invalid, as this may cause ODR violations and manifest in link errors or other unexpected behaviour. However, many existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities may be difficult in some cases.

  As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning indicates when such an invalid exposure has occurred, and can be silenced using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to `export using` declarations naming such entities.

  When combined with `-Wtemplate-names-tu-local`, GCC will also warn about non-exposure references to TU-local entities in template bodies. Such templates can still be instantiated in other TUs but the above risks regarding exposures of translation-unit-local entities apply.

  This warning is enabled by default, and is upgraded to an error by `-pedantic-errors`.

`-Wno-external-tu-local` ¶

  Warn when naming a TU-local entity outside of the translation unit it was declared in. Such declarations will be ignored during name lookup. This can occur when performing ADL from a template declared in the same TU as the internal function:

      export module M;
      template <typename T> void foo(T t) {
        bar(t);
      }
      struct S {} s;
      static void bar(S) {}  // internal linkage

      // instantiating foo(s) from outside this TU can see ::bar,
      // but naming it there is ill-formed.

  This can be worked around by making `bar` attached to the global module, using `extern "C++"`.

  This warning is enabled by default, and is upgraded to an error by `-pedantic-errors`.

`-Wno-terminate (C++ and Objective-C++ only)` ¶

  Disable the warning about a throw-expression that will immediately result in a call to `terminate`.

`-Wno-vexing-parse (C++ and Objective-C++ only)` ¶

  Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:

      void f(double a) {
        int i();        // extern int i (void);
        int n(int(a));  // extern int n (int);
      }

  Another example:

      struct S { S(int); };
      void f(double a) {
        S x(int(a));   // extern struct S x (int);
        S y(int());    // extern struct S y (int (*) (void));
        S z();         // extern struct S z (void);
      }

  The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead.

  This warning is enabled by default.

`-Wno-class-conversion (C++ and Objective-C++ only)` ¶

  Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called.

`-Wvolatile (C++ and Objective-C++ only)` ¶

  Warn about deprecated uses of the `volatile` qualifier. This includes postfix and prefix `++` and `--` expressions of `volatile`-qualified types, using simple assignments where the left operand is a `volatile`-qualified non-class type for their value, compound assignments where the left operand is a `volatile`-qualified non-class type, `volatile`-qualified function return type, `volatile`-qualified parameter type, and structured bindings of a `volatile`-qualified type. This usage was deprecated in C++20.

  Enabled by default with `-std=c++20`. Before `-std=c++20`, enabled with explicit `-Wdeprecated`.

`-Waligned-new` ¶
`-Waligned-new=[none|global|all]`

  Warn about a new-expression of a type that requires greater alignment than the `alignof(std::max_align_t)` but uses an allocation function without an explicit alignment parameter. This option is enabled by `-Wall`.

  Normally this only warns about global allocation functions, but `-Waligned-new=all` also warns about class member allocation functions.

`-Wno-placement-new` ¶
`-Wplacement-new=n`

  Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large.

      char buf [64];
      new (buf) int[64];

  This warning is enabled by default.

  `-Wplacement-new=1`

    This is the default warning level of `-Wplacement-new`. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following `new` expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array.

        struct S { int n, a[1]; };
        S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
        new (s->a)int [32]();

  `-Wplacement-new=2`

    At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.

        struct S { int n, a[]; };
        S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
        new (s->a)int [32]();

`-Wcatch-value` ¶
`-Wcatch-value=n (C++ and Objective-C++ only)`

  Warn about catch handlers that do not catch via reference. With `-Wcatch-value=1` (or `-Wcatch-value` for short) warn about polymorphic class types that are caught by value. With `-Wcatch-value=2` warn about all class types that are caught by value. With `-Wcatch-value=3` warn about all types that are not caught by reference. `-Wcatch-value` is enabled by `-Wall`.

`-Wconditionally-supported (C++ and Objective-C++ only)` ¶

  Warn for conditionally-supported (C++11 [intro.defs]) constructs.

`-Wno-defaulted-function-deleted (C++ and Objective-C++ only)` ¶

  Warn when an explicitly defaulted function is deleted by the compiler. That can occur when the function’s declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.

`-Wno-delete-incomplete (C++ and Objective-C++ only)` ¶

  Do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default.

`-Wextra-semi (C++, Objective-C++ only)` ¶

  Warn about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):

      struct S {
        void foo () {};
      };

  Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98):

      struct S {
      };
      ;

  And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98):

      struct S {
        int a;
        ;
      };

`-Wno-global-module (C++ and Objective-C++ only)` ¶

  Disable the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives.

`-Wno-inaccessible-base (C++, Objective-C++ only)` ¶

  This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the `-Wextra` option.

      struct A { int a; };

      struct B : A { };

      struct C : B, A { };

`-Wno-inherited-variadic-ctor` ¶

  Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited.

`-Wno-invalid-offsetof (C++ and Objective-C++ only)` ¶

  Suppress warnings from applying the `offsetof` macro to a non-POD type. According to the 2014 ISO C++ standard, applying `offsetof` to a non-standard-layout type is undefined. In existing C++ implementations, however, `offsetof` typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it.

  The restrictions on `offsetof` may be relaxed in a future version of the C++ standard.

`-Wsized-deallocation (C++ and Objective-C++ only)` ¶

  Warn about a definition of an unsized deallocation function

      void operator delete (void *) noexcept;
      void operator delete[] (void *) noexcept;

  without a definition of the corresponding sized deallocation function

      void operator delete (void *, std::size_t) noexcept;
      void operator delete[] (void *, std::size_t) noexcept;

  or vice versa. Enabled by `-Wextra` along with `-fsized-deallocation`.

`-Wsuggest-final-types` ¶

  Warn about types with virtual methods where code quality would be improved if the type were declared with the C++11 `final` specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete.

`-Wsuggest-final-methods` ¶

  Warn about virtual methods where code quality would be improved if the method were declared with the C++11 `final` specifier, or, if possible, its type were declared in an anonymous namespace or with the `final` specifier. This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete. It is recommended to first consider suggestions of `-Wsuggest-final-types` and then rebuild with new annotations.

`-Wsuggest-override` ¶

  Warn about overriding virtual functions that are not marked with the `override` keyword.

`-Wno-conversion-null (C++ and Objective-C++ only)` ¶

  Do not warn for conversions between `NULL` and non-pointer types. `-Wconversion-null` is enabled by default.