Suppose you have a C++ class that is non-copyable and non-movable. This can happen if it has a member which is non-copyable and non-movable, like a std::mutex. But how do you put an object of this class into a map?

struct weird
{
    int value;
    std::mutex mtx;
};

std::map<int, weird> table;

table.insert({ 1, {} }); // nope
table.insert_or_assign(1, weird{}); // nope
table.emplace({ 1, {} }); // nope
table.emplace(1, weird{}); // nope
table.try_emplace(1, weird{}); // nope

The problem with the insert method is that it takes a std::pair<int, weird> by value, which means that it cannot move the weird to its final destination.

The insert_or_assign, and emplace methods use the parameters to construct a std::pair<int, weird> at the final location, but since we passed a weird{} object as the second parameter, that would require moving the parameter into its final location.

The try_emplace method uses its first parameter as a key and the rest of the parameters to construct the weird, so if you pass a brand new weird, it’s going to use the rvalue or lvalue constructor (as appropriate) to create the weird at its final location.

Similar problems exist with unordered_map.

One solution is to use std::piecewise_construct, (which we learned about some time ago), so that you can specify constructor parameters for each of the halves of the std::pair:

table.emplace(std::piecewise_construct,
    std::forward_as_tuple(1),
    std::forward_as_tuple());

Passing an empty tuple for the second parameter uses the default constructor. If you want to pass constructor parameters for the weird object, you can put them inside the second tuple:

table.emplace(std::piecewise_construct,
    std::forward_as_tuple(1),
    std::forward_as_tuple("used to construct weird"));

The try_emplace method uses its first parameter to construct the key and the remaining parameters to construct the mapped type. In our case, we want a default constructor, so we can just say nothing:

table.try_emplace(1);

If we want to pass constructor parameters for weird, we can pass them after the key:

table.try_emplace(1, "parameters for constructing weird");

Bonus chatter: There are versions of these insertion and emplacement methods which take hints as the first parameter, ahead of the value or key. We learned about hints some time ago.