重载矢量模板的ostream与迭代器

Question

Why I can't use iterator in ostream overloading?

If I use the same declaration using iterative approach it works.

Consider the following code:

template <class T>
class List {
    template <class U>
    friend ostream &operator<<(ostream &os, const List<U> &rhs);
private:
    vector<T> v;
};

template<class U>
ostream & operator<<(ostream & os, const List<U>& rhs)
{
    vector<U>::iterator it = rhs.v.begin();
    return os;
}

int main()
{
    List<int> list;
    cout << list << endl;
    return 0;
}

Answer 1

1. Note that rhs is declared as reference to const , then rhs.v will be const too, then rhs.v.begin() will return a std::vector<U>::const_iterator , which can't be converted to std::vector<U>::iterator directly.

2. You should use typename for dependent type names .

So change it to

typename vector<U>::const_iterator it = rhs.v.begin();

BTW: void main() should be int main() .

Answer 2

Try with

typename vector<U>::const_iterator it = rhs.v.begin();

If your rsh is const , you should use a const_iterator