Why does multimap allow duplicate key-value pairs?

EDIT: Please note, I'm NOT asking why multimap can't contain duplicate keys.

What's the rationale behind multimap allowing duplicate key-value pairs? (not keys)

#include <map>
#include <string>
#include <iostream>

int
main(int argc, char** argv)
{
    std::multimap<std::string, std::string> m;
    m.insert(std::make_pair("A", "B"));
    m.insert(std::make_pair("A", "B"));
    m.insert(std::make_pair("A", "C"));
    std::cout << m.size() << std::endl;
    return 0;
}

This printed 3, which somewhat surprised me, I expected multimap to behave like a set of开发者_StackOverflow pairs, so I was expecting 2.

Intuitively, it's not consistent with C++ std::map behaviour, where insert does not always change the map (as opposed to operator[]).

Is there a rationale behind it, or it's just arbitrary?

Multimap only has a predicate ordering the keys. It has no method to determine whether the values are equal. Is value "A" a duplicate of value "a"? Without a second predicate for the values, there's no telling. Therefore, it doesn't even make sense to talk about duplicate values in a multimap.

If you would like a container that stores pairs, and enforces the unique-ness of both parts of the pair, look at boost::multi_index_container. It's very flexible, but takes a load of arguments as a result.

EDIT: This answer does not answer the current question anymore. I'll keep it as it is because it got upvoted a lot so it must be useful for some.

The multi in multimap stands for the fact that the same key can occur multiple times.

The standard puts no limit on the type used as value, so one cannot assume that operator==() is defined. Because we don't want the result of your code depend on whether the operator==() is defined or not, it is never used.

std::multimap is not a replacement for std::map. As you noticed, it behaves differently when the same key is inserted multiple times. If you want std::map's behaviour, use std::map.

There is also a std::multiset.

The rational: sometimes one would like to keep all old entries for the same key around as well. [TBD: Insert some example here]

Personally, I barely ever use std::multimap. If I want multiple entries for the same key, I usually rely on std::map<std::vector<T> >.

The values are allowed to be duplicates because they are not required to be comparable to each other. The container cannot do anything with the values besides copy them in. This enables types like multimap< int, my_class >.

If duplicate key-value pairs are undesirable, then use set< pair< T, U > > and use lower_bound to find the first match to a given key.

As you know, multimap allows to have multiple keys. Since it does not place any constraints on values comparability, it is unable to check, if values haven't been doubled.

If you want to have some dictionary data structure which allows for duplicate keys, but not key-value pairs, you would have to ensure that values are comparable.

Let's say we have a game of some sort, where there is 2D world of sqaure fields, and you can put items on fields. You can have multimap<Field, Item>, which will allow you to keep two identical items on the field. Items don't have to be comparable here.

My reasoning is multimap is based on the Key lookup/insertion and not on the value. So whether the value on duplicate keys is the same or not does not play a part when elements are being inserted.

23.3.2 Class template multimap

1 A multimap is a kind of associative container that supports equivalent keys (possibly containing multiple copies of the same key value) and provides for fast retrieval of values of another type T based on the keys.

"multimap" is meant to support 'multiple' keys unlike simple "map". Since it allows multiple keys, it won't bother for their values, so it shows 3 elements in your example. The other difference is, one can not have operator [] for multimap.

A use of duplicate [map,value] pairs is to count the number of occurrences of say a word on a page of a book, be it no times, thus no entry in the multimap for that word, be it once with one entry, or more than once with the number of occurrences in multimap for make_pair(word, page_number). It was more by accident that design that I found this usage.