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I was reviewing the interface for the C++-17 std::optional class template and noticed that the reset and assignment from nullopt are not marked as constexpr.
Was this an oversight or is there a reason that this operation cannot be marked constexpr?
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optional is required to not use dynamic allocation. If an optional contains a value, the value is guaranteed to be allocated as part of the optional object footprint, i.e. no dynamic memory allocation ever takes place.
(since C++17) The class template std::optional manages an optional contained value, i.e. a value that may or may not be present. A common use case for optional is the return value of a function that may fail.
Raymond Chen. October 4th, 20216 0. C++17 introduced std::optional<T> which lets you augment the values of a type T with a bonus value known as std::nullopt which semantically represents the absence of a value. A std::optional which holds the value std::nullopt is known as empty. The basic operations on std::optional ...
std::exchange was introduced in the C++ standard library in C++14 under the header <utility> . Its name suggests that it's a general-purpose and useful function, and its template prototype working with any type confirms this impression.
There was a reason, which was that [expr.const] had previously forbid:
an assignment expression or invocation of an assignment operator ([class.copy]) that would change the active member of a union;
That restriction no longer exists as a result of P1330: Changing the active member of a union inside constexpr, which makes all of these things much easier to implement (the paper literally just removes the bullet point I quoted above).
The reason that optional's copy and move assignment (but none of the other assignments) were constexpr was because they can just be defaulted for trivial types.
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