declare a vector through decltype

Question

#include "stdafx.h"
#include <iostream>

#include <vector>
#include <map>

template <typename T>
auto Copy(T c)
{
    std::vector<decltype(c.begin()->first)> lc;

   //Copying

    return lc;

}

int main()
{
    std::map<int, int> map;

    Copy(map);


    return 0;
}

In the above code I am trying declare a vector from the datatype of keys of map but I am getting following error -

"The C++ Standard forbids containers of const elements allocator<const T> is ill-formed." 

Answer 1

The problem is that decltype(c.begin()->first) returns a const int (when using libstdc++ - your example compiles with libc++).

As your error tells you...

The C++ Standard forbids containers of const elements because allocator< const T > is ill-formed.

A possible solution is using std::decay_t:

std::vector<std::decay_t<decltype(c.begin()->first)>> lc;

This will guarantee that your example works both with libstdc++ and libc++. Alternatively, std::remove_const_t also works in this particular situation.

Here's a working example on wandbox.