正式地说，typename 是做什么用的?

问题描述

有时我会看到在使用模板时 gcc 吐出一些非常难以理解的错误消息......具体来说，我遇到过看似正确的声明导致非常奇怪的编译错误的问题，这些错误很神奇通过在声明的开头添加 typename 关键字前缀......(例如，就在上周，我将两个迭代器声明为另一个模板化类的成员，我不得不这样做)...

On occasion I've seen some really indecipherable error messages spit out by gcc when using templates... Specifically, I've had problems where seemingly correct declarations were causing very strange compile errors that magically went away by prefixing the typename keyword to the beginning of the declaration... (For example, just last week, I was declaring two iterators as members of another templated class and I had to do this)...

typename 有什么故事?

推荐答案

以下是 Josuttis 书中的引述:

Following is the quote from Josuttis book:

关键字typename被引入指定标识符下面是一个类型.考虑下面的例子:

The keyword typename was introduced to specify that the identifier that follows is a type. Consider the following example:

template <class T>
Class MyClass
{
  typename T::SubType * ptr;
  ...
};

这里，typename 用于说明SubType 是class T 的一种类型.因此，ptr 是指向类型的指针T::SubType.没有typename，SubType将被视为静态成员.于是

Here, typename is used to clarify that SubType is a type of class T. Thus, ptr is a pointer to the type T::SubType. Without typename, SubType would be considered a static member. Thus

T::SubType * ptr

将是价值的倍增SubType 类型为 T 和 ptr.

would be a multiplication of value SubType of type T with ptr.

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