en.cppreference.com example for emplace construction in unordered maps

emplace-tuple-piecewise.cpp

#include <iostream>
#include <utility>

#include <unordered_map>

int main(){
	std::unordered_map<std::string, std::string> m;

	// uses pair's copy-constructor
	m.emplace(std::make_pair(std::string("a"), std::string("a")));

	// uses pair's converting copy constructor
	m.emplace(std::make_pair("b", "abcd"));

	// uses pair's template constructor
	m.emplace("d", "ddd");

	// uses pair's piecewise constructor
	m.emplace(std::piecewise_construct,
		std::forward_as_tuple("c"),
		std::forward_as_tuple(10, 'c'));

	for (const auto &p : m) {
		std::cout << p.first << " => " << p.second << '\n';
	}
}