In our previous post, we created an inserter iterator for unhinted insertion, and now we’re taking it a step further by generalizing it into a boilerplate-only version. This generic output iterator allows for custom insertion logic using a lambda, but as we’ll see, it doesn’t fully satisfy iterator requirements—something we’ll attempt to fix next time.

Creating a Generic Insertion Iterator, Part 1

by Raymond Chen

From the article:

Last time, we created an inserter iterator that does unhinted insertion. We noticed that most of the iterator is just boilerplate, so let’s generalize it into a version that is all-boilerplate.

// Do not use: See discussion
template<typename Lambda>
struct generic_output_iterator
{
    using iterator_category = std::output_iterator_tag;
    using value_type = void;
    using pointer = void;
    using reference = void;
    using difference_type = void;

    generic_output_iterator(Lambda&& lambda) :
        insert(std::forward<Lambda>(lambda)) {}

    generic_output_iterator& operator*() noexcept
        { return *this; }
    generic_output_iterator& operator++() noexcept
        { return *this; }
    generic_output_iterator& operator++(int) noexcept
        { return *this; }

    template<typename Value>
    generic_output_iterator& operator=(
        Value&& value)
    {
        insert(std::forward<Value>(value));
        return *this;
    }

protected:
    std::decay_t<Lambda> insert;

};

template<typename Lambda>
generic_output_iterator<Lambda>
generic_output_inserter(Lambda&& lambda) {
    return generic_output_iterator<Lambda>(
        std::forward<Lambda>(lambda));
}

template<typename Lambda>
generic_output_iterator(Lambda&&) ->
    generic_output_iterator<Lambda>;

For convenience, I provided both a deduction guide and a maker function, so you can use whichever version appeals to you. (The C++ standard library has a lot of maker functions because they predate class template argument deduction (CTAD) and deduction guides.)