Dependent types generally need typename to tell the compiler the member is a type, not a function or variable.
依赖类型通常需要typename来告诉编译器成员是类型，而不是函数或变量。
However, this is not always the case.
For example, a base class doesn't require this, because it can only ever be a type:
然而，情况并非总是如此。例如，基类不需要这个，因为它只能是一个类型:
template<class T> struct identity { typedef T type; }
template<class T> class Vector : identity<vector<T> >::type { }; // no typename
Now my question is, why does typedef ever require typename after it?
现在我的问题是，为什么typedef会在它之后需要typename ?
template<class T> class Vector
{
typedef typename /* <-- why do we need this? */ vector<T>::iterator iterator;
};
1 个解决方案
#1
7
typedef does not need to appear before the type.
类型定义不需要出现在类型之前。
template <typename T>
struct S { typedef T type; };
template <typename T>
void f() { typename S<T>::type typedef t; }
This is perfectly valid, and in this case, I hope you can understand that parsing would be complicated if typename were optional.
这是完全有效的，在这种情况下，我希望您能够理解，如果typename是可选的，那么解析将会很复杂。
I can understand that
我能理解,
template <typename T>
void f() { typedef S<T>::type t; }
could be interpreted differently, but that would introduce unexpected cases where the position of the typedef keyword suddenly becomes significant.
可能会有不同的解释，但这将引入一些意想不到的情况，即typedef关键字的位置突然变得非常重要。
#1
7
typedef does not need to appear before the type.
类型定义不需要出现在类型之前。
template <typename T>
struct S { typedef T type; };
template <typename T>
void f() { typename S<T>::type typedef t; }
This is perfectly valid, and in this case, I hope you can understand that parsing would be complicated if typename were optional.
这是完全有效的，在这种情况下，我希望您能够理解，如果typename是可选的，那么解析将会很复杂。
I can understand that
我能理解,
template <typename T>
void f() { typedef S<T>::type t; }
could be interpreted differently, but that would introduce unexpected cases where the position of the typedef keyword suddenly becomes significant.
可能会有不同的解释，但这将引入一些意想不到的情况，即typedef关键字的位置突然变得非常重要。