可以std :: map键地址的值是多少？

Question

It may be a little bit stupid question, but assuming the std::map defined as follows:

 std::map<int, int> m;

Is there any way to have more than one value stored and be accessible for a single key ?

Motivation of asking:

std::map has methods like count() and equal_range() that get a key as parameters, that kind of give a sense that there is more than one value may be specified by a single key.

Answer 1

Those methods exist so as to provide a common interface with other associative containers that do allow multiple values per key (such as std::multimap which is exactly what you're looking for).

This makes implementing algorithms generically (ie with templates) much easier than it would otherwise be, and nothing of value is lost by designing it this way.

It's true that, in the case of std::map , count() can only give you zero or one (unless you're using transparent keys, which are a whole other kettle of fish).

C++20 will introduce std::map::contains() , which is more or less a check that count() == 1 — this seems to have been intended to address concerns that the function count() is kind of a weird thing to have for a std::map specifically.

Answer 2

Is there any way to have more than one value stored and be accessible for a single key ?

Not with std::map , these objects store only one value per key, but std::multimap can store a variable number of values per key.

Similarities between both types (eg std::map::count , std::multimap::count ) are due to establishing similar interfaces between STL containers.

Answer 3

No. std::map is designed to have one value per key. If you want multiple values for one key, you should use std::multimap .