Question

I’ve managed to wrap my head around some of C++’s functional capacities (for_each, mapping functions, using iterators…) but the construction of the templates and function argument lists for taking in generic containers and iterators still eludes me. I have a practical example I’m hoping someone can illustrate for me:

Take the following function that processes an incoming std::vector and builds a running total of many data-points/iterations of a process:

/* the for-loop method - not very savvy */
void UpdateRunningTotal (int_vec& total, int_vec& data_point) {
  for (int i = 0; i < V_SIZE; i++) {
    total[i] += data_point[i];
  }
}

typedef int_vec std::vector<int>;
int_vec running_total (V_SIZE, 0);  // create a container to hold all the "data points" over many iterations
/* further initialization, and some elaborate loop to create data points */

UpdateRunningTotal (running_total, iteration_data);
/* further processing */

The above works, but I’d much rather have a function that takes iterators and performs this summation. Even better, have a generic parameter list with the type deduced instead of specifying the container type, i.e.:

UpdateRunningTotal (iteration_data.begin(), iteration_data.end(), running_total.begin());

I’m really lost at this point and need a little guidance to find how to define the template and argument lists to make the function generic. What would the template and function definition look like? I’m already familiar with a way to perform this specific task using STL functionality – I’m looking for illustration of the generic function/template definition.

Answer 1

You could use std::transform and std::plus:

std::transform(iteration_data.begin(), iteration_data.end(),
                running_total.begin(), iteration_data.begin(), std::plus<int>());

And in your function, that would be:

template <typename Iter1, typename Iter2>
void UpdateRunningTotal(Iter1 pBegin, Iter1 pEnd, Iter2 pBegin2)
{
    typedef typename std::iterator_traits<Iter1>::value_type value_type;

    std::transform(pBegin, pEnd, pBegin2, pBegin, std::plus<value_type>());
}