Return type std::optional<std::variant<...>>
I would consider this to be a useful use of std::monostate
. Specifically, variant<std::monostate, int, double, std::string, std::chrono::time_point>
. monostate
is useful for cases where a variant
may not contain a value.
The nice thing about using an actual type rather than optional<variant>
is that visitation works normally on it. You can write a functor that can take a monostate
parameter, thus allowing you to use visit
for even "empty" variants.
Just want to add that before C++17 and the standardization of variant
and monostate
, there is already boost::blank
to solve the exact same issue for boost::variant
.
By convention, if boost::blank
is used, it should always be the first template argument, so that a default-constructed variant is empty and checking for emptyness is done with .which() == 0
.