C++静态成员初始化(模板乐趣在里面)

For static member initialization I use a nested helper struct, which works fine for non templated classes. However, if the enclosing class is parameterized by a template, the nested initialization class is not instantiated, if the helper object is not accessed in the main code. For illustration, a simplified example (In my case, I need to initialize a vector).

#include <string>
#include <iostream>

struct A
{
    struct InitHelper
    {
        InitHelper()
        {
            A::mA = "Hello, I'm A.";
        }
    };
    static std::string mA;
    static InitHelper mInit;

    static const std::string& getA(){ return mA; }
};
std::string A::mA;
A::InitHelper A::mInit;


template<class T>
struct B
{
    struct InitHelper
    {
        InitHelper()
        {
            B<T>::mB = "Hello, I'm B."; // [3]
        }
    };
    static std::string mB;
    static InitHelper mInit;

    static const std::string& getB() { return mB; }
    static InitHelper& getHelper(){ return mInit; }
};
template<class T>
std::string B<T>::mB; //[4]
template<class T>
typename B<T>::InitHelper B<T>::mInit;


int main(int argc, char* argv[])
{
    std::cout << "A = " << A::getA() << std::endl;

//    std::cout << "B = " << B<int>::getB() << std::endl; // [1]
//    B<int>::getHelper();    // [2]
}

使用 g++ 4.4.1:

A = Hello, I'm A.

A = Hello, I'm A.
B = 

A = Hello, I'm A.
B = Hello, I'm B.

struct C { C(int n) { printf("%d
", n); } };

template<int N>
struct A {
  static C c;
}; 

template<int N>
C A<N>::c(N); 

A<1> a; // implicit instantiation of A<1> and 2
A<2> b;

int main() { 
  A<1>::c; // reference them
  A<2>::c; 
}