In C++, how do I create a function that accepts an InputIterator?

Question

C++ transform , accumulate , and similar functions accept an InputIterator as an argument, so they can be used with many types of containers. I want to write a function that accepts InputIterator so I can use the function with any type of container. How do I do that? What headers do I need, etc.

Answer 1

You don't need any headers. You just make a function template, and document your template to require an argument that is an input iterator. You may like to use the iterator_traits template from the <iterator> header to extract adherent data, though. For example:

#include <iterator>

//  Requires: InputIterator is an input iterator
//  Returns: sum of the range, starting at acc

template <typename InputIterator>
typename std::iterator_traits<InputIterator>::value_type
sum(InputIterator it,
    InputIterator last,
    typename std::iterator_traits<InputIterator>::value_type acc)
{
    while (it != last) { acc += *it; ++it; }
    return acc;
}

For some algorithms you don't need traits at all, and therefore no headers. For example:

// Requires: Iter is an input iterator, F is a callable and copyable
// Returns: a copy of f
// Effects: calls f with every value in the range
template <typename Iter, typename F>
F for_each(Iter it, Iter last, F f)
{
    while (it != last) { f(*it); ++it; }
    return f;
}

Answer 2

InputIterator is a concept more than a type.

Just write

template<typename InputIterator>
void myFunc(InputIterator begin, InputIterator end) {
    /* … */
}

and you are good to go, if you apply on the begin and end variables only actions that correspond to the InputIterator concept (incrementation, dereference…).

Answer 3

I'd suspect you already did have a look at eg std::transform() documentation yet.

You should have noticed that they're taking these iterator types as template parameters:

template< class InputIt, class OutputIt, class UnaryOperation >
OutputIt transform( InputIt first1, InputIt last1, OutputIt d_first,
                    UnaryOperation unary_op );

The particular requirements are checked at compile time, using the various concepts defined in the standard language, and being defined as eg std::iterator_traits<It> .