为什么 C++ 模板定义需要在头文件中?

问题描述

可能的重复:
为什么应该模板类的实现和声明在同一个头文件中?

例如在定义模板类时，为什么类方法的实现需要在标题中?为什么它们不能在实现文件 (cpp/cxx) 中?

e.g when defining a template class why do the implementations of the class methods need to be in the header? Why can't they be in a implementation file (cpp/cxx)?

推荐答案

模板类不是类，而是可用于创建类的模板.当你实例化这样一个类时，例如MyTemplate，编译器当场创建类.为了创建它，它必须查看所有模板化的成员函数(以便它可以使用模板创建实际的成员函数，例如 MyTemplate::foo() )，因此这些模板化的成员函数必须在头文件中.

A template class is not a class, it's a template that can be used to create a class. When you instantiate such a class, e.g. MyTemplate<int>, the compiler creates the class on the spot. In order to create it, it has to see all the templated member functions (so that it can use the templates to create actual member functions such as MyTemplate<int>::foo() ), and therefore these templated member functions must be in the header.

如果成员不在头文件中，编译器将简单地假设它们存在于其他地方，并从模板化的函数声明中创建实际的函数声明，这会导致链接器错误.

If the members are not in the header, the compiler will simply assume that they exist somewhere else and just create actual function declarations from the templated function declarations, and this gives you linker errors.

"export" 关键字应该可以解决这个问题，但很少有编译器支持它(我只知道 Comeau).

The "export" keyword is supposed to fix this, but few compilers support it (I only know of Comeau).

您也可以显式实例化 MyTemplate - 然后编译器会在编译包含 MyTemplate 成员函数定义模板.

You can also explicitly instantiate MyTemplate<int> - then the compiler will create actual member functions for MyTemplate<int> when it compiles the cpp files containing the MyTemplate member function definition templates.

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