Question

Good day!

In his “Effective STL” Scott Meyers wrote

A third is to use the information in an ordered container of iterators to iteratively splice the list’s elements into the positions you’d like them to be in. As you can see, there are lots of options. (Item 31, second part)

Can someone explain me this way?

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More text (to understand the context):

The algorithms sort, stable_sort, partial_sort, and nth_element require random access iterators, so they may be applied only to vectors, strings, deques, and arrays. It makes no sense to sort elements in standard associative containers, because such containers use their comparison functions to remain sorted at all times. The only container where we might like to use sort, stable_sort, partial_sort, or nth_element, but can’t, is list, and list compensates somewhat by offering its sort member function. (Interestingly, list::sort performs a stable sort.) If you want to sort a list, then, you can, but if you want to use partial_sort, or nth_element on the objects in a list, you have to do it indirectly. One indirect approach is to copy the elements into a container with random access iterators, then apply the desired algorithm to that. Another is to create a container of list::iterators, use the algorithm on that container, then access the list elements via the iterators. A third is to use the information in an ordered container of iterators to iteratively splice the list’s elements into the positions you’d like them to be in. As you can see, there are lots of options.

Answer 1

I’m not sure what the confusion is but I suspect that it is what “splicing” refers to: the std::list<T> has an splice() member function (well, actually several overloads) which transfer nodes between lists. That is, you create a std::vector<std::list<T>::const_iterator> and apply the sorting algorithm (e.g. std::partial_sort()) to this. Then you create a new std::list<T> and use the splice() member with the iterators from the sorted vector to put the nodes into their correct order without moving the objects themselves.

This would look something like this:

std::vector<std::list<T>::const_iterator> tmp;
for (auto it(list.begin()), end(list.end()); it != end; ++it) {
    tmp.push_back(it);
}
some_sort_of(tmp);
std::list<T> result;
for (auto it(tmp.begin()), end(tmp.end()); it != end; ++it) {
    result.splice(result.end(), list, it);
}