类模板实例化 [英] class template instantiation

问题描述

我刚读了关于 CRTP 的wiki文章，我对模板实例化有点困惑。

I just read the wiki article about CRTP, and I'm a little confused about template instantiation.

根据wiki，

成员函数体（定义）
。

member function bodies (definitions) are not instantiated until long after their declarations.

我不太明白这是什么意思。

I don't quite understand what it means.

假设我有一个类模板：

template <typename T>
class A
{
    public:
        void foo(T t)
        {
            //...
        };
};

当我实例化类模板A时，是否实例化成员函数foo（）？

When I instantiate the class template A, does it instantiate the member function foo()?

例如：

//in .cpp file
int main()
{
    A<int> a; //question 1
              //class template is instantiated here, isn't it? 
              //What about foo(), is it instantiated too?

    a.foo(10); //question 2
               //according to the quotation, foo() will not be instantiated until it is used.
               //if so, foo() is instantiated right here, not in question 1, right?
}

推荐答案

事情：

实例化在编译期间发生，而不是在运行时。因此，你不能说在哪一行上类模板或函数模板被实例化。

Instantiation happens during compilation, not during runtime. Hence you can't say "on which line" a class template or a function template was instantiated.

也就是说，对于成员函数模板不能与类模板一起实例化，这是正确的。

That said, you're right about the fact that member function templates aren't instantiated together with class templates.

您可以在这种情况下观察它：

You could observe it in such a case: You have the following files

• template.h（定义类A和函数A： ：foo）


• a.cpp（使用A）


• b.cpp（使用A和A :: foo）

然后在编译a.cpp的过程中，只有A被实例化。但是，在b.cpp的编译过程中，两者都将被实例化。

Then during compilation of a.cpp, only A would be instantiated. However, during compilation of b.cpp, both would be instantiated.

因为这一点，如果A :: foo包含给定模板集合的一些语义无效的代码参数，你会得到b.cpp中的编译错误，但不会a.cpp。

Because of this, in case A::foo contained some semantically invalid code for a given set of template parameters, you would get compile errors in b.cpp, but not a.cpp.

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