在模板函数中使用const迭代器

Question

I'm using template functions in order that it may work with list or vector of different numerical data types. But one of them has an issue:

// This function should take 2 iterators of a vector/list and sum the elements between them
template <typename T >
double Sum1(typename T::const_iterator start, typename T::const_iterator end){
    double sum = 0;
    // For g++, "typename is needed before T::const_iterator"
    for(typename T::const_iterator I = start; I <= end;I++){
        sum += *I;
    }
    return sum;
}

in the main(), it is called like this:

vector<double> b;// and it is pushed by numbers 1-9 later

cout<<Sum1(b.begin(), b.end())<<endl;

but the compiler gives me errors

no matching function for call to 'Sum1(std::vector::iterator, >std::vector::iterator)'

couldn't deduce template parameter 'T'

However, this function works fine:

// T can be vector<int>, list<double>, etc. 
template <typename T >
double Sum(T& values){
    double sum = 0;
    // For g++, "typename is needed before T::const_iterator"
    for(typename T::const_iterator I = values.begin(); I != values.end();I++){
        sum += *I;
    }
    return sum;
}

Any idea why this happens? How should I revise the first function so it can take 2 iterators of a vector/list and calculate the sum in between?

Answer 1

Instead of templatizing on the container type, templateize on the iterator. And don't restrict your users to const_iterator -- any iterator should do is you don't actually modify the contents through the iterator:

template <typename Iter> double Sum1(Iter start, Iter end)

By doing this you can also use any type that is dereferenceable and incrementable -- such as a simple pointer.