Is it useful to construct an std::optional using std::in_place when returning from a function?

Question

I was just wondering, is it useful to return an std::optional from a function constructed using std::in_place? Should I be doing it blindly or does copy elision/changing C++ versions/the data structure being trivially constructible/destructible change anything?

Answer 1

The in-place constructor is useful when a class is not copyable or movable. For example:

#include <mutex>
#include <optional>

struct T { T(int, int); std::mutex m; };

std::optional<T> f() {
    // return T(1, 2);                            // Error: T not movable
    // return std::make_optional(T(1, 2));        // Error: T not movable
    return std::optional<T>(std::in_place, 1, 2); // works
}

Otherwise, if you already have the desired return value and the type is movable, you can just use the "normal" constructor-from-type:

T x = ...;
// ...
return x;

It'd perhaps be a bit unusual to spell this as return std::optional<T>(std::in_place, x);, even if just because it repeats the type name unnecessarily.