Why didn't the C++17 standard bring partial class template argument deductions? [duplicate]

Question

One of the places I hoped I would be able to use the new template argument deduction, was in construction of std::set's / std::maps / any other containers with custom comparators - my goal is to create a one-line statement, that will create an efficient set with a lambda comparator. What I can do since C++11 is:

std::set<int, std::function<bool(int, int)>> s([](int a, int b) {return a > b;});

But since it uses std::function, it is noticeably slower.

Another option is:

auto mycomp = [](int a, int b) {return a > b; };
std::set<int, decltype(mycomp)> s(mycomp);

It gets the job done, but 1) It requires 2 lines, and creation of the mycomp variable 2) I need to pass mycomp's type explicitly.

As I read on reference pages, none of the standard containers has a deduction guide for this kind of situation. Unfortunately, I'm afraid it cannot be even done with the current language standard (C++17), as one can find:

Class template argument deduction is only performed if no template argument list is present. If a template argument list is specified, deduction does not take place.

What is the reasoning behind this? Why didn't they allow partial argument deduction? I guess there are some problems with it that I overlook, but in my opinion it would be more than helpful.

Answer 1

In one line (C++17)

std::set s(std::initializer_list<int>{}, [](int a, int b) {return a > b; });

Answer 2

As alternative, you can still make the old make_xxx

template <typename T, typename COMP>
std::set<T, COMP> make_set(COMP comp)
{
    return std::set<T, COMP>{comp};
}


auto s = make_set<int>([](int a, int b) {return a > b; });

Answer 3

What you should be doing is writing a comparator functor type, so you don't need to proxy to it when using your set. It will be a lot more lines of code (the atrocity!), but is better in virtually every way:

struct MyComparator
{
    bool operator()(int a, int b) const { ...; }
};
using MySet = std::set<int, MyComparator>;

And from then on, you'd just create your customly sorted set where you need it.