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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
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those which describe ⁄ comment on specific features using the landscape as a background or for context (all other maps – usually called thematic maps). For ease of describing functionally quite different maps; and to explain what can be confusing differences; we have opted to use a greater number of map types.
A thematic map is not the same as a general reference map since these maps don’t just show natural features like rivers, cities, political subdivisions, elevation, and highways.
These maps show minor and major highways and roads (based on detail) along with things like airports, city locations, and interesting attractions, such as, parks, campgrounds, and monuments. Major highways with a road map are usually red and bigger than other roads, while minor roads can be a lighter color and a smaller line.
One common point of view is that there are two main types of maps: those which summarise the actual landscape (topographic and general reference maps); and; those which describe ⁄ comment on specific features using the landscape as a background or for context (all other maps – usually called thematic maps).
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;
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.
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