Apparently, consteval is going to be a keyword in C++20.
What is it going to be and how does it relate to constexpr?
1 Answer
It declares immediate functions, that is, functions that must be evaluated at compile time to produce a constant. (It used to be spelled constexpr! in a previous revision of the paper.) In contrast, constexpr functions may be evaluated at compile time or run time, and need not produce a constant in all cases.
The adopted paper is P1073R3, which is not yet publicly available, but a previous revision is available and the introductory (motivation and high-level description) portion is about the same (except that the "Source Locations" section is deleted in R3).
13 Comments
T.C.
No, you can't, just like
constexpr.
Davis Herring
It might be worth pointing out that the motivation is to have functions that rely on compiler data structures that need not be preserved in the binary.
geza
@Andry: it's not just about compile-timedness. It's about that the result is wrong. Check out: godbolt.org/z/12aZCH. The result depends on optimization level.
MarcusJ
Does this mean consteval functions aren't available at runtime?
jorgbrown
That's correct, @MarcusJ, consteval functions won't be available at runtime because there's no way to call them unless all their parameters are known at compile-time. And of course, if that's true, the compiler can figure out (at compile-time) what those routines return, with no need to actually generate code for them.
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decltype(std::declval<f()>)'ish but .. fun stuff.