The output of an iterator that takes into account only the number of increments?

Question

Is there a ready-made output iterator in STL / Boost that counts the number of times it increased? When the algorithm executes *iter = value, the value will simply be discarded.

If not, rolling my own seems easy enough. Although it may be useful for others, if someone posted an example of such an iterator.

Answer 1

There is no such iterator in standard C ++, and as far as I know, Boost iterator has this exact functionality. There are many ways you could do this using these libraries, but not folding your own. For example, using Boost function_output_iterator, you can build a counter as follows:

struct Counter {
    size_t* out;

    explicit Counter(size_t* where) : out(where) {
        // Handled in initializer list
    }

    /* Either make this a template, or make the class itself a template. */
    template <typename T> void operator()(T& value) {
        ++ *out;
    }
};

This type of functor takes a pointer to a counter variable, and then whenever it is called operator(), the counter is incremented. If you then wrap this in function_output_iterator, as shown here:

size_t count;
your_algorithm(begin, end,
              boost::make_function_output_iterator(Counter(&count)));

Then, whenever the created iterator is written, your call operator()will be called and the counter will increment.

Hope this helps!

Answer 2

Yeah. In boost:

Iterator Count