对于静态成员初始化，我使用嵌套的帮助器结构，它适用于非模板化类。
但是，如果封闭类由模板参数化，则如果在主代码中未访问辅助对象，则不会实例化嵌套的初始化类。
为了说明，一个简化的例子(在我的例子中，我需要初始化一个 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]
}

Unless a member of a class template or a member template has been explicitly instantiated or explicitly specialized, the specialization of the member is implicitly instantiated when the specialization is referenced in a context that requires the member definition to exist; in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist.

最佳答案

前段时间在 usenet 上讨论过这个问题，当时我正在尝试回答有关 stackoverflow 的另一个问题:Point of Instantiation of Static Data Members .我认为减少测试用例并单独考虑每个场景是值得的，所以让我们先更一般地看一下它:

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

"... in particular, the initialization (and any associated side-effects) of a static data member does not occur unless the static data member is itself used in a way that requires the definition of the static data member to exist."

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

"Objects with static storage duration defined in namespace scope in the same translation unit and dynamically initialized shall be initialized in the order in which their definition appears in the translation unit."

Each translated translation unit is examined to produce a list of required instantiations. [Note: this may include instantiations which have been explicitly requested (14.7.2). ] The definitions of the required templates are located. It is implementation-defined whether the source of the translation units containing these definitions is required to be available. [Note: an implementation could encode sufficient information into the translated translation unit so as to ensure the source is not required here. ] All the required instantiations are performed to produce instantiation units. [Note: these are similar to translated translation units, but contain no references to uninstantiated templates and no template definitions. ] The program is ill-formed if any instantiation fails.

Dynamic initialization of a non-local object with static storage duration is either ordered or unordered. Definitions of explicitly specialized class template static data members have ordered initialization. Other class template static data members (i.e., implicitly or explicitly instantiated specializations) have unordered initialization.

关于C++静态成员初始化(模板好玩里面)，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/1819131/