为什么对于依赖类型，单词"typedef"后面需要"typename"? [英] Why does the word 'typedef' need 'typename' after it for dependent types?

问题描述

从属类型通常需要typename来告诉编译器成员是类型，而不是函数或变量.

Dependent types generally need typename to tell the compiler the member is a type, not a function or variable.

但是，并非总是如此.
例如，基类不需要此，因为它只能是类型:

However, this is not always the case.
For example, a base class doesn't require this, because it can only ever be a type:

template<class T> struct identity { typedef T type; }
template<class T> class Vector : identity<vector<T> >::type { };  // no typename

现在我的问题是，为什么typedef在之后需要typename ?

Now my question is, why does typedef ever require typename after it?

template<class T> class Vector
{
    typedef typename /* <-- why do we need this? */ vector<T>::iterator iterator;
};

推荐答案

typedef不需要出现在类型之前.

typedef does not need to appear before the type.

template <typename T>
struct S { typedef T type; };
template <typename T>
void f() { typename S<T>::type typedef t; }

这是完全正确的，在这种情况下，我希望您能理解，如果typename是可选的，则解析会很复杂.

This is perfectly valid, and in this case, I hope you can understand that parsing would be complicated if typename were optional.

我能理解

template <typename T>
void f() { typedef S<T>::type t; }

的解释可能有所不同，但这会引入意想不到的情况，其中typedef关键字的位置突然变得很重要.

could be interpreted differently, but that would introduce unexpected cases where the position of the typedef keyword suddenly becomes significant.

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