c++ - 模板类的静态变量初始化，c++

Question

考虑以下代码:

//header.h
template<class T>
class A
{
    static int x;
};

template<class T>
int A<T>::x = 0;

//source1.cpp
#include "header.h"
void f(){} // dummy function


//main.cpp
#include "header.h"
int main(){}

在这种情况下，代码可以完美无误地编译，但是如果我从类中删除模板限定符

class A
{
    static int x;
};

int A::x = 0;

1. 在这种情况下，编译器因 x 的多重定义而出错。谁能解释这种行为？
2. 以及何时初始化/实例化模板类的静态变量？

Answer 1

编译器将自行删除重复的模板实例化。如果您将模板类变成常规类，那么您有责任确保只存在一个静态变量定义(否则将出现链接器错误)。还要记住，静态数据成员不会在不同类型的模板实例化之间共享。使用 c++11，您可以使用外部模板自行控制实例化:using extern template (C++11) .

关于静态成员的实例化点:

14.6.4.1 Point of instantiation [temp.point] 1 For a function template specialization, a member function template specialization, or a specialization for a member function or static data member of a class template, if the specialization is implicitly instantiated because it is referenced from within another template specialization and the context from which it is referenced depends on a template parameter, the point of instantiation of the specialization is the point of instantiation of the enclosing specialization. Otherwise, the point of instantiation for such a specialization immediately follows the namespace scope declaration or definition that refers to the specialization.

所以实例化点应该是 ie。如果您是第一次在 main() 中使用您的类型，则紧接在 main() 之后。