Last time, we investigated the puzzle of why the compiler wouldn’t let us put an object into a std::optional. It came down to the fact that the object is not copy-constructible, move-constructible, copy-assignable, or move-assignable, so there’s no way to put the temporary object into the std::optional.

Solving the Puzzle of Trying to Put an Object into a std::optional

by Raymond Chen

From the article:

What we have to do is construct the object in place inside the std::optional. And the C++ standard library term for “construct an object inside a container” is “emplace”.

struct Doodad
{
    Doodad();
    ~Doodad();
    std::unique_ptr<DoodadStuff> m_stuff;
};

struct Widget
{
    std::optional<Doodad> m_doodad;

    Widget()
    {
        if (doodads_enabled()) {
            m_doodad.emplace();
        }
    }
};

The parameters to emplace are whatever parameters you would have passed to the Doodad constructor. In our case, we wanted the default constructor, so that means that we pass nothing to emplace().