Represent sum of random access iterators as a random access iterator

Question

Random access iterators are constrained to the difference between two iterators, or to adding or subtracting an integer from an iterator. Rules do not allow addition of two iterators.

I stumbled on a situation where adding two iterators could actually be useful:

#include <iostream>
#include <vector>

using std::vector;
using std::cout;            using std::endl;

template<typename RndIterator> 
RndIterator rndIteratorsSum(RndIterator left, RndIterator right)
{
    //return (left + right) / 2;        // forbidden
    return left + (right - left) / 2;   // workaround
}

A sum of pointers has been reduced to a sum of a pointer + (std::ptrdiff_t / int = int) so it is legal now. The test function could be:

int main()
{
// outputs median of input
    vector<int> test = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    vector<int>::iterator iter = rndIteratorsSum(test.begin(), test.end());
    
    cout << "median of input vector: " << *iter << endl;

return 0;
}

Is using the above trick allowed or discouraged? If discouraged, why?

Answer 1

You are asking to allow (left + right) / 2 as a shorter notation for left + (right - left) / 2 because these expressions are equivalent for numbers in mathematics. However, to do that, you would need to define the addition of 2 iterators and division of iterator by a number. Neither of these operations seems to make sense on their own.