In one of the most respected stackoverflow answer I found an example of std::expected
template class usages:
What are coroutines in C++20?
At the same time I cannot find any mentioning of this class on cppreference.com. Could you please explain what it is?
Actually, the best way to learn about
std::expected
is a funny talk by the (in)famous Andrei Alexandrescu: "Expect the Expected!"What
std::expected
is, and when it's usedHere are three complementing explanations of what an
std::expected<T, E>
is:It is the return type of a function which is supposed to return a
T
value - but which may encounter some error, in which case it will return a descriptor of that error, of typeE
.An example:
It's an error-handling mechanism, being an alternative to throwing exceptions (in which case you always return the value you were supposed to), and to returning status/error codes (in which case you never return the value you want to, and have to use an out-parameter).
Here are the two alternative error-handling mechanisms applied to the function from the previous example:
It's a discriminated union of types
T
andE
with some convenience methods."How is it better than just an
std::variant<T,E>
?"It behaves somewhat like
std::optional<T>
, giving focus to the expected, rather than the unexpected, case:result.has_value()
- true if we got a value rather than an error.if (result)
- checks for the same thing*result
- gives us theT
value if it exists, undefined behavior otherwise (same asstd::optional
, although many don't like this).result.value()
, gives us theT
value if it exists, or throws otherwise.Actually, that last mode of access behaves differently than
std::optional
if we got an error: What it throws is abad_expected_access<E>
, with the returned error. This behavior can be thought of as a way to switch from expected-based to exception-based error-handling."Hey, I looked for it in the standard and it isn't there!"
std::expected
will be part of the upcoming C++23 standard. The proposal (P0323) has recently been accepted.This being said - it is quite usable already, since it requires no new language facilities: