模板类中的C ++非模板方法 [英] C++ Non Template Method in Template Class

问题描述

是否可以在.cpp文件的模板类（struct）中编写非模板方法的实现？我已经阅读，模板方法应该写在.h，但我的方法不是模板方法，虽然它属于模板类。这是我的.h中的代码：

Is it posible to write implementation of non template method in template class(struct) at .cpp file? I have read that template method should be written on .h, but my method is not template method although it belongs to template class. Here is the code in my .h:

#include <iostream>

#ifndef KEY_VALUE_H
#define KEY_VALUE_H

using namespace std;

namespace types
{
    template <class T, class U>
    struct key_value
    {
        T key;
        U value;

        static key_value<T, U> make(T key, U value)
        {
            key_value<T, U> kv;

            kv.key = key;
            kv.value = value;

            return kv;
        };

        string serialize()
        {
            // Code to serialize here I want to write in .cpp but fails.
        }
    };
}


#endif /* KEY_VALUE_H */

我尝试在.cpp文件中写入方法 serialize（）的实现，如下所示：

I tried to write the implementation of method serialize() in .cpp file like this:

#include "key_value.h"

using namespace types;

template <class T, class U>
string key_value<T, U>::serialize()
{
    // Code here returning string
}

结束时出现错误：重新定义'serialize'

如何正确地执行此操作？

How is the proper way to doing this?

推荐答案

这不可能 ^* 。想想为什么模板首先需要在头文件中：所以每个使用从模板实例化的代码的.cpp文件可以访问它（模板仅根据需求实例化）。

It's not possible^*. Think about why templates need to be in header files in the first place: so that every .cpp file which uses code instantiated from a template can access it (templates are instantiated on demand only).

因此，在某种程度上，你可以将类模板视为数据布局（数据成员）的模板加上一组模板，每个成员函数一个模板。因此，模板类的所有成员都被视为模板。

So in a way, you can think of a class template as a template for data layout (data members) plus a set of templates, one for each member function. Therefore, all members of a template class are treated as templates. You can even explicitly specialise a member function of a class template.

^* 一如既往，如果显式实例化是一个选项，你可以定义一个类模板的成员函数。成员函数，并在.cpp文件中提供所有必需的实例化实例。

^* As always, if explicit instantiation is an option, you can define the member function in a .cpp file and provide all required expicit instantiations in that .cpp file.

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