unpopular opinion, I think constexpr and similar are anti patterns. if you want to memoize values, run a separate script to do that. a computation is not and cannot be constant, this is just some sugar so that people can pretend it is.
Also "a computation is not and cannot be constant" depends on what is meant precisely by "computation" and "constant." Is "23 * 48" a constant, or is it a computation? What about "2208/2"? What about "1.104e3"?
You can define "constant" as "something that is not computed but that exists in the program as an immediate value." In that case you're right but only by definition. And the compiler must still "interpret" the immediate value. It's a slightly fuzzy distinction in my eyes.
Sure it can - but you know that - a compile time computation can be a run time constant. There’s loads of reasons to want constant values built out of compile-time computation. You haven’t had to write any high performance code?
Start with something like
constexpr float angle_threshold = 45.0f;
constexpr float degToRad = M_PI / 180.0f;
constexpr float x_threshold = cosf( angle \* degToRad );
Maybe I need a small table of these, suddenly running a little code at compile time looks great. Doing it during compilation is a lot nicer than having a script dependency and having to write a build step for it.
The bigger issue with constexpr specifically is that it doesn’t actually guarantee compile time evaluation…
But I don't believe one can justify the existence of constexpr simply by pointing to code that absolutely any compiler would make into constants absent it.
Perhaps in your example one could say that the value is that you really want the compilation to abort should changes happen to the code that make it impossible for the compiler to turn it into constants, since doing so is performance critical-- I think that would be a fair point.
I wasn’t trying to justify the existence of constexpr, I was justifying the existence of compile time constant computation in response to what @38_14 said about there being no such thing as a computation that’s constant, which is, you probably agree, a bit silly. The top comment didn’t seem to be talking about constexpr specifically since the point was about computation and they said “constexpr and similar” which I assume includes const, consteval, as well as the general idea of baking the results of compute at compile time.
HN isn’t really the right place to demand justifying the existence of constexpr, but its existence has been debated at length by people who know a lot more about C++ than me. If you want to read about why it exists, there are lots of good places like cppreference.com and isocpp.org and even stack overflow.
constexpr auto size = 4;
constexpr std::array<int, size> arr = {1, 2, 3, 4};
constexpr auto i = 2;
constexpr auto x = arr[i]; constexpr auto x = 1+1;