如何在名称空间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__ 
 
命名空间std 
 {
模板< typename T> 
班级名单; 
} 
 
模板< typename T> 
 void Pop（std :: list< T> * l）
 {
 while（！l-> empty（））
 l-> pop（）; 
} 
 
 #endif 
  

主要。我收到错误。当然，我知道 std :: list （我认为是分配器）有更多的模板参数。但是，这是重要的。我必须知道模板类的完整模板声明才能够转发声明吗？

编辑：我之前没有使用指针 - 它是一个参考。我会用指针来试试它。

解决方案

问题不在于您无法转发声明模板类。是的，您需要知道所有模板参数及其默认值才能正确地转发声明：

  namespace std {
 template< class T，class Allocator = std :: allocator< T>> 
班级名单; 
} 
  

但是为了在命名空间中创建这样的前向声明std 被标准明确禁止：允许放入 的东西 std 是一个模板 specialization ，通常 std :: less 在用户定义的类型上。

只要 #include< list> 并且不用担心，如果有必要的话，其他人可以引用相关文本。

呵呵，顺便说一句，任何包含双下划线的名字都会被保留供实现使用，所以你应该使用类似于 TEST_H / code>而不是 __ TEST __ 。它不会生成警告或错误，但是如果程序与实现定义的标识符发生冲突，则不能保证能够正确编译或运行：它是不正确的 。还禁止使用以下划线开头的名称，后面跟着大写字母等等。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始。

#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

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?

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

解决方案

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;
}

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.

Just #include <list> and don't worry about it.

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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