This is a C++20 feature that allows aggregate initialization through the standard constructor syntax, rather than the typical braced-list initialization syntax. (Note that this only works if the parameters cannot be used in a valid call to a default or copy/move constructor. If they could, that would be called instead of performing aggregate initialization.) According … Read more
In the particular case of a map the old options were only two: operator[] and insert (different flavors of insert). So I will start explaining those. The operator[] is a find-or-add operator. It will try to find an element with the given key inside the map, and if it exists it will return a reference … Read more
The arguments you pass to map::emplace get forwarded to the constructor of map::value_type, which is pair<const Key, Value>. So you can use the piecewise construction constructor of std::pair to avoid intermediate copies and moves. std::map<int, Foo> m; m.emplace(std::piecewise_construct, std::forward_as_tuple(1), std::forward_as_tuple(2.3, “hello”)); Live demo