如何在命名空间std中转发声明模板类? [英] How to forward declare a template class in namespace std?

问题描述

#ifndef __TEST__
#define __TEST__

namespace std
{
    template<typename T>
    class list;
}

template<typename T>
void Pop(std::list<T> * l)
{
    while(!l->empty())
        l->pop();
}

#endif

并在我的主要功能中使用了该功能.我得到错误.当然，我知道 std::list 有更多的模板参数(我认为是分配器).但是，这不是重点.我是否必须知道模板类的完整模板声明才能转发声明它?

and used that function in my main. I get errors. Of course, I know that there are more template params for std::list (allocator I think). But, that is beside the point. Do I have to know the full template declaration of a template class to be able to forward declare it?

我之前没有使用指针 - 它是一个参考.我用指针试试看.

I wasn't using a pointer before - it was a reference. I'll try it out with the pointer.

推荐答案

问题不在于不能前向声明模板类.是的，您确实需要知道所有模板参数及其默认值才能正确前向声明它:

The problem is not that you can't forward-declare a template class. Yes, you do need to know all of the template parameters and their defaults to be able to forward-declare it correctly:

namespace std {
  template<class T, class Allocator = std::allocator<T>>
  class list;
}

但是，即使在 namespace std 中进行这样的前向声明，标准也明确禁止:only 允许您放入 std 是一个模板specialisation，通常是用户定义类型的std::less.如有必要，其他人可以引用相关文字.

But to make even such a forward declaration in namespace std is explicitly prohibited by the standard: the only thing you're allowed to put in std is a template specialisation, commonly std::less on a user-defined type. Someone else can cite the relevant text if necessary.

只需#include <list>，不用担心.

哦，顺便说一句，任何包含双下划线的名称都保留供实现使用，因此您应该使用 TEST_H 之类的名称，而不是 __TEST__.它不会产生警告或错误，但如果您的程序与实现定义的标识符发生冲突，则不能保证正确编译或运行:它是格式错误的.除其他外，还禁止以下划线后跟大写字母开头的名称.一般来说，除非你知道你在处理什么魔法，否则不要用下划线开头.

Oh, incidentally, any name containing double-underscores is reserved for use by the implementation, so you should use something like TEST_H instead of __TEST__. It's not going to generate a warning or an error, but if your program has a clash with an implementation-defined identifier, then it's not guaranteed to compile or run correctly: it's ill-formed. Also prohibited are names beginning with an underscore followed by a capital letter, among others. In general, don't start things with underscores unless you know what magic you're dealing with.

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