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

Last time, we investigated the puzzle of why the compiler wouldnΓÇÖt 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 thereΓÇÖs 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 ΓÇ£construct an object inside a containerΓÇ¥ is ΓÇ£emplaceΓÇ¥.

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().

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