使用std :: iterator_traits <>时出现不清楚的编译时错误

Question

There's a code that gives an error, which is completely unclear to me. What I want is to pass a pointer to struct/class member through an iterator_traits .

template<typename Iter, typename S>
S mean(Iter begin, const Iter& end, S std::iterator_traits<Iter>::value_type::* v)
{
    //...
}

But even at the declaration of this templated function, MSVS2015 gives following error on the code above:

Error C2653 'value_type': is not a class or namespace name

However, without iterator_trais<...> ie

template<typename Iter, typename S>
S mean(Iter begin, const Iter& end, S Iter::value_type::* v)
{
    //...
}

everything compiles properly.

Why it cannot recognize value_type as a class name ?

Compiler bug ?

Answer 1

It's a simple syntax error. You mean:

template<typename Iter, typename S>
S mean(
    Iter begin,
    const Iter& end,
    typename std::iterator_traits<Iter>::value_type S::* v)
{
    // ...
}