Question

What I mean is the following. I want a template function that takes two vector iterators (or two pointers to array of double) and returns a double that is somehow related to the vector iterators or array pointers that I pass. However, I want this to work for double or int, or any arithmetic type.

I think I’m not allowed to say:

template <class T> 
T* func(T Begin, T End)

 T new_variable = Begin + 5;

 return (*new_variable);
}

because the compiler won’t understand what T* means. A solution I thought of is to take what I’m trying to return and make it a third argument:

template <class T> 
void func(T Begin, T End, T* new_variable)

 new_variable = Begin + 5;

 return (*new_variable);
}

Will this work? Even if so, is there another way of doing what I’m trying to do? (Sorry if I haven’t been clear enough.)

Answer 1

If you want to return a double (i.e the type that you would get when dereferencing), you can use the iterator traits:

template<typename RandomAccessIterator>
typename std::iterator_traits<RandomAccessIterator>::value_type 
func(RandomAccessIterator a, RandomAccessIterator b) {
    typedef typename std::iterator_traits<RandomAccessIterator>::value_type 
      value_type;

    // use value_type now, when you want to save some temporary
    // value into a local variable, for instance
    value_type t = value_type();
    for(; a != b; ++a) t += *a;
    return t;
}

These work for all iterators, including pointers:

int main() {
  int d[3] = { 1, 2, 3 };
  assert(func(d, d + 3) == 6);
}