std library map and template functions

Question

I really don't understand why this code

#include <map>

template<typename T, typename U> std::ostream& operator<<(std::ostream& o,
        const std::map<T,U>& input)
{
    for (std::map<typename T,typename U>::iterator it=input.begin(); it!=input.end(); ++it)
    {
        o << it->first << " => " << it->second << '\n';
    }
    return o;
}

returns this compilation error:

error: wrong number of template arguments (1, should be 4)
error: provided for ‘template<class _Key, class _Tp, class _Compare, class _Alloc> class std::map’

can anyone help me please??

score 4 · Answer 1 · edited May 23 '17 at 12:14

4

You should write typename before the iterator declaration and use const_iterator:

for (typename std::map<T,U>::const_iterator it=input.begin(); it!=input.end(); ++it

The argument of the operator << requires const objects. So elements of the map must be const. To achieve this one uses a const_iterator. The typename in the declaration of the iterator is required to indicate that the following expression is a nested template class depending on the types T and U.

See also this question: Making sense of arguments to a function template

edited May 23 '17 at 12:14

Community

1
1

answered Apr 14 '15 at 16:48

StefanW

197
1
4

Or even better, use `auto`: `for (auto it = input.begin(); ...` or even the range-based for loop: `for (auto& pair : input)` – Angew is no longer proud of SO Apr 14 '15 at 16:49

std library map and template functions

1 Answers1