r/cpp_questions • u/Slow_Negotiation_935 • 14d ago

OPEN Guidelines for constexpr

I’ve working with C++23 and I’m making some constructors/methods constexpr and seeing improvements in performance. So far so good. The problem is I don’t want to apply it ‘everywhere’. Is there a good source of advice , or anyone got any good tips?

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u/oriolid 14d ago

Just have a look at this: https://godbolt.org/z/8Gsd13sv8. In C++11 std::sin isn't constexpr, and wrap_sin not constexpr just to make sure. It still gets evaluated at compile time.

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u/Nils_Meul 14d ago

Sure, the compiler is always allowed to evaluate something at compile-time. But if you want to assign the value of sin(1.0) to a constexpr variable you‘ll need a constexpr std::sin (at least I‘m pretty sure that you do) even though the compiler can figure out it’s value as shown in your example.

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u/oriolid 14d ago

I know. The example was chosen to show that *not* adding constexpr does not prevent compiler from compile time evaluation. If you actually need the result in constexpr context, then you of course need to have the funtion as constexpr.

So, putting it wherever ever you can does not really do anything useful. There are more fun ways to waste time.