No matching function call when passing iterators as arguments

Question

I want to create a Sum() function that calculates the sum of a STL container. The function uses the template argument for the iterator type and accepts two iterators as arguments as below:

template <typename T>
double Sum(typename T::const_iterator start, typename T::const_iterator end)
{
    typename T::const_iterator i3;
    double sum2=0.0;
    for (i3=start; i3 != end; ++i3)
    {
        sum2 += (i3);
    }
    return sum2;
}

and in the main() I call the function like:

vector<double>::const_iterator vec_start=myvector.begin();
vector<double>::const_iterator vec_end=myvector.end();
cout<<"The sum of the vector is "<<Sum(vec_start, vec_end)<<endl;

But it says that no matching function call for Sum(). I read some discussions that it's because the function signature is T but I pass iterators and that I need to specify data type before passing iterator parameters.

Answer 1

Just as the discussions you've found said, the template parameter T can't be deduced automatically because of non-deduced contexts , so you need to specify it explicitly. eg

cout << "The sum of the vector is " << Sum<vector<double>>(vec_start, vec_end) << endl;
//                                        ~~~~~~~~~~~~~~~~

But you don't need to do that in fact, you could change the declaration of Sum to:

template <typename I>
double Sum(I start, I end)
{
    I i3;
    double sum2=0.0;
    for (i3=start; i3 != end; ++i3)
    {
        sum2 += *i3;
    }
    return sum2;
}

I could be deduced then it's fine to use it originally as

cout << "The sum of the vector is " << Sum(vec_start, vec_end) << endl;