Keywords: ISO C++ , library

Chapter 17. Associative
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## Chapter 17. 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.

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