Question

I’m having a problem when trying to output STL iterator values in C++. The following code generates an error – no matter what template target I’m going to use:

template <typename T>
void outputVector(vector<T> &v)
{
    typename vector<T>::const_iterator iter;
    for(iter = v.begin(); iter != v.end(); iter++)
    {
        cout << *iter << endl;
    }

The message is

no match for ‘operator<<‘ in ‘std::cout << iter.__gnu_cxx::__normal_iterator<_Iterator, _Container>::operator* with _Iterator = const Node*, _Container = std::vector >’

When replacing the for loop with

for(iter = v.begin(); iter != v.end(); iter++)
{
    T t = *iter;
    cout << t << endl;
}

everything works just fine.

Do you have any problem what the problem might be? The used class “Node” overloads the “<<” operator by

ostream & operator<<(ostream &o, Node &n)
{
o << "Hello World" << endl;
return o;
}

Answer 1

Your operator<< fails to take a reference to const for the second argument:

std::ostream& operator<<(std::ostream& os, MyType& obj);

This means the temporary *iter cannot bind to that argument.

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Get into the habit of using const any place you don’t need to modify an object:

std::ostream& operator<<(std::ostream& os, MyType const& obj);

Now, the reference argument can bind to the temporary. Huzzah!

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