C++ template static member instantiation

Question

#include <map>
#include <iostream>
template <typename T>
class A 
{
 static std::map<int, int> data;
public:
 A()
 {
  std::cout << data.size() << std::endl;
  data[3] = 4;
 }
};

template <typename T>
std::map<int, int> A<T>::data;

//std::map<int, int> A<char>::data;

A<char> a;

int main()
{
 return 0;
}

What is wrong with this? Without explicit instantiation it breaks at

 data[3] = 4; 

Explicit instantiation solves the problem but the program breaks after

std::cout << data.size() << std::endl;

what means that the static class template memeber data was instantiated.

Answer 1

I don't have Visual C++ handy, but I can see the same problem with your code compiling with GCC. You need the initialize the data member:

template<> std::map<int, int> A<char>::data = std::map<int, int>();

With this change, it compiles and runs correctly (for me on GCC on Linux).

Answer 2

There is no explicit instantiation in your code.

There is no order of initialization of instantiated static data members among other static data members. So your code has effectively undefined behavior: Depending on whether the compiler first initializes the map or a, the reference to the map is valid or not.

See C++ Static member initialization.