Why is a re-constructed map type different from the original one?

Question

Consider the following code:

#include <iostream>
#include <typeinfo>
#include <map>


int main(int argc, const char* argv[]) {
    typedef std::map<int,float> testmap;
    typedef std::map<int,float> testmap2;
    typedef std::map<typename testmap::value_type::first_type, typename testmap::value_type::second_type> rebuiltMap;
    std::cout << "map samenes: " << std::is_same<testmap, rebuiltMap>::value << "\n";
    std::cout << "map samenes: " << std::is_same<testmap, testmap2>::value << "\n";
    std::cout << "original map type name " << typeid(testmap).name() << "\n";
    std::cout << "same     map type name " << typeid(testmap2).name() << "\n";
    std::cout << "rebuilt  map type name " << typeid(rebuiltMap).name() << "\n";
    std::cout << "original map valuetype " << typeid(testmap::value_type).name() << "\n";
    std::cout << "rebuilt  map valuetype " << typeid(rebuiltMap::value_type).name() << "\n";
}

This produces the following output:

map samenes: 0
map samenes: 1
original map type name St3mapIifSt4lessIiESaISt4pairIKifEEE
same     map type name St3mapIifSt4lessIiESaISt4pairIKifEEE
rebuilt  map type name St3mapIKifSt4lessIS0_ESaISt4pairIS0_fEEE
original map valuetype St4pairIKifE
rebuilt  map valuetype St4pairIKifE

Why is the "rebuilt" map type different from the simple map type, although both have the same value_type? Background: i want to test if a container containing pairs is a map or not with a construct like

std::is_same<std::map<typename Container::value_type::first_type,
                      typename Container::value_type::second_type>,
             Container>::value

Answer 1

This is because std::map::value_type is std::pair<const Key, Value>, not std::pair<Key, Value>. The reason for that is you are never allowed to modify the key of a pair that is in the map.

To get the rebuilt map to be the same using value_type::first_type you need to remove that const using std::remove_const_t like

typedef std::map<std::remove_const_t<typename testmap::value_type::first_type>, typename testmap::value_type::second_type> rebuiltMap;

Answer 2

testmap::value_type::first_type is const int rather than int.

You could use testmap::key_type instead to get the correct type. Based on the pair type, it is not possible to distinguish const and non-const key type. That said, a const key would not satisfy the requirements of std::map, so you could simply assume that it is non-const and remove const from testmap::value_type::first_type.