Keywords: ISO C++, library

Associative
Prev | Chapter 9. Containers | Next

## Associative

### Insertion Hints

Section [23.1.2], Table 69, of the C++ standard lists this function for all of the associative containers (map, set, etc):

          a.insert(p,t);

where 'p' is an iterator into the container 'a', and 't' is the item to insert. The standard says that “`t` is inserted as close as possible to the position just prior to `p`.” (Library DR #233 addresses this topic, referring to N1780. Since version 4.2 GCC implements the resolution to DR 233, so that insertions happen as close as possible to the hint. For earlier releases the hint was only used as described below.

Here we'll describe how the hinting works in the libstdc++ implementation, and what you need to do in order to take advantage of it. (Insertions can change from logarithmic complexity to amortized constant time, if the hint is properly used.) Also, since the current implementation is based on the SGI STL one, these points may hold true for other library implementations also, since the HP/SGI code is used in a lot of places.

In the following text, the phrases greater than and less than refer to the results of the strict weak ordering imposed on the container by its comparison object, which defaults to (basically) “<”. Using those phrases is semantically sloppy, but I didn't want to get bogged down in syntax. I assume that if you are intelligent enough to use your own comparison objects, you are also intelligent enough to assign “greater” and “lesser” their new meanings in the next paragraph. *grin*

If the `hint` parameter ('p' above) is equivalent to:

- `begin()`, then the item being inserted should have a key less than all the other keys in the container. The item will be inserted at the beginning of the container, becoming the new entry at `begin()`.

- `end()`, then the item being inserted should have a key greater than all the other keys in the container. The item will be inserted at the end of the container, becoming the new entry before `end()`.

- neither `begin()` nor `end()`, then: Let `h` be the entry in the container pointed to by `hint`, that is, `h = *hint`. Then the item being inserted should have a key less than that of `h`, and greater than that of the item preceding `h`. The new item will be inserted between `h` and `h`'s predecessor.

For `multimap` and `multiset`, the restrictions are slightly looser: “greater than” should be replaced by “not less than”and “less than” should be replaced by “not greater than.” (Why not replace greater with greater-than-or-equal-to? You probably could in your head, but the mathematicians will tell you that it isn't the same thing.)

If the conditions are not met, then the hint is not used, and the insertion proceeds as if you had called `a.insert(t)` instead. (Note that GCC releases prior to 3.0.2 had a bug in the case with `hint == begin()` for the `map` and `set` classes. You should not use a hint argument in those releases.)

This behavior goes well with other containers' `insert()` functions which take an iterator: if used, the new item will be inserted before the iterator passed as an argument, same as the other containers.

Note also that the hint in this implementation is a one-shot. The older insertion-with-hint routines check the immediately surrounding entries to ensure that the new item would in fact belong there. If the hint does not point to the correct place, then no further local searching is done; the search begins from scratch in logarithmic time.

### bitset

#### Size Variable

No, you cannot write code of the form

          #include <bitset>

          void foo (size_t n)
          {
    	  std::bitset<n>   bits;
    	  ....
          }

because `n` must be known at compile time. Your compiler is correct; it is not a bug. That's the way templates work. (Yes, it is a feature.)

There are a couple of ways to handle this kind of thing. Please consider all of them before passing judgement. They include, in no particular order:

- A very large N in `bitset<N>`.

- A container<bool>.

- Extremely weird solutions.

A very large N in `bitset<N>`. It has been pointed out a few times in newsgroups that N bits only takes up (N/8) bytes on most systems, and division by a factor of eight is pretty impressive when speaking of memory. Half a megabyte given over to a bitset (recall that there is zero space overhead for housekeeping info; it is known at compile time exactly how large the set is) will hold over four million bits. If you're using those bits as status flags (e.g., “changed”/“unchanged” flags), that's a lot of state.

You can then keep track of the “maximum bit used” during some testing runs on representative data, make note of how many of those bits really need to be there, and then reduce N to a smaller number. Leave some extra space, of course. (If you plan to write code like the incorrect example above, where the bitset is a local variable, then you may have to talk your compiler into allowing that much stack space; there may be zero space overhead, but it's all allocated inside the object.)

A container<bool>. The Committee made provision for the space savings possible with that (N/8) usage previously mentioned, so that you don't have to do wasteful things like `Container<char>` or `Container<short int>`. Specifically, `vector<bool>` is required to be specialized for that space savings.

The problem is that `vector<bool>` doesn't behave like a normal vector anymore. There have been journal articles which discuss the problems (the ones by Herb Sutter in the May and July/August 1999 issues of C++ Report cover it well). Future revisions of the ISO C++ Standard will change the requirement for `vector<bool>` specialization. In the meantime, `deque<bool>` is recommended (although its behavior is sane, you probably will not get the space savings, but the allocation scheme is different than that of vector).

Extremely weird solutions. If you have access to the compiler and linker at runtime, you can do something insane, like figuring out just how many bits you need, then writing a temporary source code file. That file contains an instantiation of `bitset` for the required number of bits, inside some wrapper functions with unchanging signatures. Have your program then call the compiler on that file using Position Independent Code, then open the newly-created object file and load those wrapper functions. You'll have an instantiation of `bitset<N>` for the exact `N` that you need at the time. Don't forget to delete the temporary files. (Yes, this can be, and has been, done.)

This would be the approach of either a visionary genius or a raving lunatic, depending on your programming and management style. Probably the latter.

Which of the above techniques you use, if any, are up to you and your intended application. Some time/space profiling is indicated if it really matters (don't just guess). And, if you manage to do anything along the lines of the third category, the author would love to hear from you...

Also note that the implementation of bitset used in libstdc++ has some extensions.

#### Type String

Bitmasks do not take char* nor const char* arguments in their constructors. This is something of an accident, but you can read about the problem: follow the library's “Links” from the homepage, and from the C++ information “defect reflector” link, select the library issues list. Issue number 116 describes the problem.

For now you can simply make a temporary string object using the constructor expression:

          std::bitset<5> b ( std::string("10110") );

instead of

          std::bitset<5> b ( "10110" );    // invalid

Prev | Up | Next
Chapter 9. Containers | Home | Unordered Associative