C ++类名和内部类 [英] C++ typename and inner classes

问题描述

我尝试搜寻这个，但我无法提出合适的答案。任何C ++专家能告诉我为什么C ++要求你用typename关键字声明OuterClass < T> :: Innerclass？

I tried googling this, but I was unable to come up with a suitable answer. Could any C++ gurus tell me why C++ requires you to declare OuterClass<T>::Innerclass with the typename keyword?

我是数据结构课程的TA，我一直看到这个错误。我知道告诉我的学生，他们需要把typename放在返回类型的前面，但我无法解释为什么这是必需的。

I am a TA for a data structures course and I see this error all of the time. I know to tell my students that they need to put typename in front of the return type, but I am unable to explain why this is required.

谢谢。 >

Thanks.

推荐答案

这是因为模板中的两阶段名称查找。当编译器看到Innerclass时，它必须知道该名称是否是一个类型（例如，可以是OuterClass的一些特殊化的int类型的静态成员）。所以它假设它不是一个类型名称，除非你这样说。 typename必须在模板中使用，并且仅在依赖于模板参数的名称上使用。 HTH

That's because of the two-phase name lookup in templates. When the compiler sees Innerclass it must know whether that name is a type or not (is could, for example, be a static member of type int for some specialization of OuterClass). So it supposes it is NOT a type name unless you say so. typename must be used in templates and only on names dependent on the template parameter. HTH

示例：

template <class T>
class X
{ 
   typedef T XXX;
};
template<>
class X<char>
{
   static int XXX;
};

template<class T>
class Y
{        
   // X<T>::XXX member; invalid XXX is not assumed to be a type! 
   typename X<T>::XXX member; 
   //we explicitly specify that XXX is a type; Later, upon instantiation, we will verify that
};

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