Statement Exprs (Using the GNU Compiler Collection (GCC))

#### 6.12.1 Statements and Declarations in Expressions ¶

A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.

Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:

    ({ int y = foo (); int z;
       if (y > 0) z = y;
       else z = - y;
       z; })

is a valid (though slightly more complex than necessary) expression for the absolute value of `foo ()`.

The last thing in the compound statement should be an expression followed by a semicolon; the value of this subexpression serves as the value of the entire construct. (If you use some other kind of statement last within the braces, the construct has type `void`, and thus effectively no value.)

This feature is especially useful in making macro definitions “safe” (so that they evaluate each operand exactly once). For example, the “maximum” function is commonly defined as a macro in standard C as follows:

    #define max(a,b) ((a) > (b) ? (a) : (b))

But this definition computes either a or b twice, with bad results if the operand has side effects. In GNU C, if you know the type of the operands (here taken as `int`), you can avoid this problem by defining the macro as follows:

    #define maxint(a,b) \
      ({int _a = (a), _b = (b); _a > _b ? _a : _b; })

Note that introducing variable declarations (as we do in `maxint`) can cause variable shadowing, so while this example using the `max` macro produces correct results:

    int _a = 1, _b = 2, c;
    c = max (_a, _b);

this example using maxint will not:

    int _a = 1, _b = 2, c;
    c = maxint (_a, _b);

This problem may for instance occur when we use this pattern recursively, like so:

    #define maxint3(a, b, c) \
      ({int _a = (a), _b = (b), _c = (c); maxint (maxint (_a, _b), _c); })

Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.

If you don’t know the type of the operand, you can still do this, but you must use `typeof` or `__auto_type` (see Referring to a Type with `typeof`).

In G++, the result value of a statement expression undergoes array and function pointer decay, and is returned by value to the enclosing expression. For instance, if `A` is a class, then

            A a;

            ({a;}).Foo ()

constructs a temporary `A` object to hold the result of the statement expression, and that is used to invoke `Foo`. Therefore the `this` pointer observed by `Foo` is not the address of `a`.

In a statement expression, any temporaries created within a statement are destroyed at that statement’s end. This makes statement expressions inside macros slightly different from function calls. In the latter case temporaries introduced during argument evaluation are destroyed at the end of the statement that includes the function call. In the statement expression case they are destroyed during the statement expression. For instance,

    #define macro(a)  ({__typeof__(a) b = (a); b + 3; })
    template<typename T> T function(T a) { T b = a; return b + 3; }

    void foo ()
    {
      macro (X ());
      function (X ());
    }

has different places where temporaries are destroyed. For the `macro` case, the temporary `X` is destroyed just after the initialization of `b`. In the `function` case that temporary is destroyed when the function returns.

These considerations mean that it is probably a bad idea to use statement expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement expressions that lead to precisely this bug.)

Jumping into a statement expression with `goto` or using a `switch` statement outside the statement expression with a `case` or `default` label inside the statement expression is not permitted. Jumping into a statement expression with a computed `goto` (see Labels as Values) has undefined behavior. Jumping out of a statement expression is permitted, but if the statement expression is part of a larger expression then it is unspecified which other subexpressions of that expression have been evaluated except where the language definition requires certain subexpressions to be evaluated before or after the statement expression. A `break` or `continue` statement inside of a statement expression used in `while`, `do` or `for` loop or `switch` statement condition or `for` statement init or increment expressions jumps to an outer loop or `switch` statement if any (otherwise it is an error), rather than to the loop or `switch` statement in whose condition or init or increment expression it appears. In any case, as with a function call, the evaluation of a statement expression is not interleaved with the evaluation of other parts of the containing expression. For example,

      foo (), (({ bar1 (); goto a; 0; }) + bar2 ()), baz();

calls `foo` and `bar1` and does not call `baz` but may or may not call `bar2`. If `bar2` is called, it is called after `foo` and before `bar1`.