The std::transform-like function that returns transformed container

Question

I\'m trying to implement a function similar to std::transform algorithm but instead of taking the output iterator by an argument I want to create and return a c

Answer 1

I wrote a blog post to solve a similar problem recently. Using templates and the iterator interface was the route I chose to follow.

for_each:

To cut down on the amount of boilerplate, we're going to create a using clause that allows us to grab the type contained within an iterator:

template 
using ItemType = typename std::iterator_traits::value_type;

With that in place, we can implement a helper function for_each like so:

template 
void for_each(IteratorType &items, std::function const &item)> forEachCb)
{
    for (typename IteratorType::iterator ptr = items.begin(); ptr != items.end(); ++ptr)
        forEachCb(*ptr);
}

transform_container:

Finally transform_container, could be implemented like so:

template 
ReturnType transform_container(IteratorType &items, std::function(ItemType const &item)> mapCb)
{
    ReturnType mappedIterator;
    for_each(items, [&mappedIterator, &mapCb](auto &item) { mappedIterator.insert(mappedIterator.end(), mapCb(item)); });
    return mappedIterator;
}

Which will allow us to call your two examples in the following way:

std::vector vi{ 1, 2, 3, 4, 5 };
auto vs = transform_container, std::vector>(vi, [](int i){return std::to_string(i);});
assert(vs == std::vector({"1", "2", "3", "4", "5"}));

std::set si{ 5, 10, 15 };
auto sd = transform_container, std::set>(si, [] (int i) { return i / 2.; }); 
assert(sd == std::set({5/2., 10/2., 15/2.}));

My blog post also goes into a little more detail if that's helpful.