c++ - operator<< 不能使用其 friend 数组的 IT 成员

Question

我想，如果我让 operator<< 成为他的 friend 数据结构(按名称排列)；

//Forward Declarations
template<typename S, typename T> 
struct array;

template<typename U, typename V>
ostream& operator<< (ostream& ous, const array<U, V>& t);

然后，我就可以做这样的事情了；在 operator <<

的实现中

//operator<< is a friend of struct array{} already
template<typename T, typename U>
ostream& operator<< (ostream& os, const array<T, U>& var){

    if(var){
        /*Error: 'IT' was not declared in this scope*/

        for(IT it = var.data.begin(); it != var.data.end(); it++){
            /*and i thought i need not redeclare IT before using it
             since operator<< is a friend of array already*/
        }
    }
    else{cout << "empty";}

    return os;
}

现在，这是数组的实现:

/*explicit (full) specialization of array for <type, char>*/
template<>
template<typename Key>
struct array<Key, char>{ 

     //data members
     map<const Key, string> data; 
     typedef map<const Key, string>::iterator IT;

     //member function
     friend ostream& operator<< <>(ostream& ous, const array& t);

     //other stuff/functions
};

最后，当我这样试驾时，编译器很生气；

void Test(){
     array<int, char> out;
     out[1] = "one";            //Never mind. this has been taken care of
     out[2] = "two";            
     cout << out;               //Error: 'IT' was not declared in this scope
}

问题:我究竟做错了什么，或者，为什么我不能直接访问和使用IT(数组中的 typedef)，即使在我声明了运算符 <<(请求 IT)之后作为数组结构的 friend ？

Answer 1

写

for( typename array<T, U>::IT it = var.data.begin(); it != var.data.end(); it++){

并改变

typedef map<const Key, string>::iterator IT;

到

typedef typename std::map<const Key, string>::const_iterator IT;

这是一个演示程序，为了简单起见，我使用了 std::array 而不是 std::map。我认为它可以帮助你。

#include <iostream>
#include <array>

template <typename T, size_t N>
struct A
{
    std::array<T, N> a;

    typedef typename std::array<T, N>::const_iterator IT;
};

template <typename T, size_t N>
std::ostream & operator <<( std::ostream &os, const A<T, N> &a )
{
    for ( typename A<T, N>::IT it = a.a.begin(); it != a.a.end(); ++it ) os << *it << ' ';

    return os;
}

int main() 
{
    A<int, 10> a  = { { { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 } } };

    std::cout << a << std::endl;

    return 0;
}

程序输出为

0 1 2 3 4 5 6 7 8 9