Does std::map::iterator return a copy of value or a value itself?

Question

I'm trying to create a map inside a map:

typedef map<float,mytype> inner_map; typedef map<float,inner_map> outer_map; 

Will I be able to put something inside inner map, or does iterator::second returns a copy?

stl_pair.h suggests the latter:

74: _T2 second;          ///< @c second is a copy of the second object 

but my test program run fine with the code like this:

it = my_map.lower_bound(3.1415); (*it).second.insert(inner_map::value_type(2.71828,"Hello world!"); 

So where is the truth? Is this a copy or not?

Answer 1

I want to add a follow up answer to this question for people using C++11 iterators...

The following code:

std::map<std::string, std::string> m({{"a","b"},{"c","d"}}); for (auto i : m) {     std::cout << i.first << ": " << i.second << std::endl; } 

does copy the key and value since "auto" is a value by default, not a const reference (at least that's how it behaves clang 3.1).

Additionally, the code:

std::map<std::string, std::string> m({{"a","b"},{"c","d"}}); for (const std::pair<std::string,std:string>& i : m) {     std::cout << i.first << ": " << i.second << std::endl; } 

also copies the key and value since the correct code should be:

std::map<std::string, std::string> m({{"a","b"},{"c","d"}}); for (const auto& i : m) {     std::cout << i.first << ": " << i.second << std::endl; } 

or

std::map<std::string, std::string> m({{"a","b"},{"c","d"}}); for (const std::pair<const std::string,std:string>& i : m) {     std::cout << i.first << ": " << i.second << std::endl; } 

Answer 2

The comment in stl_pair.h is misleading in this specific case.

There will be no copy, since the map::iterator actually refers to the original data inside the map (the value_type, which itself is a pair), it’s not a copy. Thus iterator::second also refers to the original data.