How to emplace object with no-argument constructor into std::map?

Question

I want to emplace an object into a std::map whose constructor does not take any arguments. However, std::map::emplace seems to require at least one

Answer 1

You could explicitly create a pair and pass that to map::emplace, or use the piecewise construction constructor of std::pair.

struct foo {};

std::map m;

m.emplace(std::pair(1, {}));
m.emplace(std::piecewise_construct,
          std::forward_as_tuple(2),
          std::forward_as_tuple());

Live demo