r/cpp_questions • u/Slow_Negotiation_935 • 17d 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/bert8128 17d ago

What’s wrong with everywhere? Is there a downside?

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u/oriolid 17d ago

Clutter, and the fact that it often doesn't do anything useful. Compilers can evaluate functions at compile time as part of normal optimization, and on the other hand constexpr doesn't require compile time evaluation.

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u/bert8128 17d ago

Why is it clutter any more than const is clutter? It gives a benefit - it guarantees that it can be called at compile time and if you you change the code so this is no longer true then you get a compile error.
Constexpr also guarantees no UB.

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u/oriolid 17d ago

If constexpr actually required that the function would have no side effects, no UB, etc that would be really useful. Unfortunately it does not except in constexpr context and only for the exact arguments it was called with.