与参数匹配的朋友模板函数实例化 [英] Friend template function instantiations that match parameter

问题描述

我有一个模板类，应该有一个朋友:一个 make_object 函数，该函数允许推导某些类型和值.我希望只与模板类类型匹配的那些实例化成为朋友.我的代码的简化版本如下:

I have a template class that should have a friend: a make_object function that allows to deduct certain types and values. I wish to make friends only those instantiations that match the types of the template class. The simplified version of my code is below:

template<size_t S, typename T>
class Object {
private:
    Object(T t);

    template<typename U, typename... Args>
    friend auto make_object(U, Args... args);
};


template<typename U, typename... Args>
inline auto make_object(U u, Args... args)
{
    constexpr size_t S = sizeof...(Args);
    return std::unique_ptr<Object<S, U>>(new Object<S, U>(u));
}

以该代码为例，我希望只将 typename U 与对象的 typename T 匹配的 make_object 的那些实例化作为朋友.那有可能吗?

Taking this code as an example I wish to make friends only those instantiations of make_object whose typename U matches the typename T of the object. Is that even possible?

推荐答案

如果您想拥有一个朋友模板，则可以像您的示例一样将模板的所有实例设为一个朋友，或者可以将其完全专门化.一个朋友.

If you want to have a friend template, you can make all the instatiations of the template a friend, like in your example, or you can make a full specialization a friend.

不幸的是，两者之间没有任何东西.

There is nothing in between unfortunately.

要解决此问题，您可以将 make_object 函数包装在模板类中，并将该模板类作为朋友.

To work around that you could wrap your make_object function in a template class and make that template class a friend.

#include <type_traits>
#include <iostream>
#include <memory>

template<typename U>
struct object_maker;

template<std::size_t S, typename T>
class Object {
private:
    Object(T) {}

    friend struct object_maker<T>;
};


template<typename U>
struct object_maker
{
    template<typename... Args>
    static auto make_object(U u, Args... args)
    {
        constexpr std::size_t S = sizeof...(Args);
        return std::unique_ptr<Object<S, U>>(new Object<S, U>(u));
    }
};

int main()
{
    auto obj = object_maker<int>::make_object(7);
    static_assert(std::is_same_v<decltype(obj), std::unique_ptr<Object<0,int>>>);
}

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