使用成员类型的模板类作为类成员变量的类型 [英] Using a member type of templated class as the type of a class member variable

问题描述

为什么下面没有编译（MSVC10 - 但我怀疑它不是有效的C ++），并有一个解决方法？

Why does the following not compile (MSVC10 - but I suspect it's not valid C++), and is there a workaround?

template <typename M>
struct MyClass
{
    typedef std::vector<M>::iterator iteratorT;

    iteratorT myIterator;

};

错误是错误C2146：语法错误： iteratorT'。我已经尝试了一组具有相同结果的变体：可以使用 std :: vector< M> :: iterator 作为成员函数中的类型，

Error is error C2146: syntax error : missing ';' before identifier 'iteratorT'. I've tried a bunch of variations with the same result: you can use std::vector<M>::iterator as a type in a member function, but not as a type of a member variable.

推荐答案

这是一个 typename 。简单的答案，你需要这样做：

It's a case of the typename. Short answer, you need to do this instead:

 typedef typename std::vector< M >::iterator iteratorT;

答案很长，编译器不知道 std :: vector< M> :: iterator 解析为 M 可以是任何东西，可以有 std :: vector 。具体来说，它不能判断 std :: vector< M> :: iterator 是一个类型或值，它相信它的值。你必须通过插入 typename 来显式地告诉编译器它的类型。

Long answer, the compiler doesn't know what std::vector< M >::iterator resolves to as M can be anything and there could be a specialization of std::vector for it. Specifically, it cannot tell if std::vector< M >::iterator is a type or a value, and it believes its a value. You have to explicitly tell the compiler its a type by inserting typename.

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