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Reading through the constexpr specifier documentation on cppreference, I've noticed that the standard says the following:

[...] the function body [of constexpr function] must be either deleted or defaulted or contain only the following: [...] if the function is not a constructor, exactly one return statement.

What is the motivation behind imposing such a requirement? Although I can understand that this could potentially lead to a simpler implementation of the constexpr interpreter, I do not see the reason why this limitation had to be imposed.

Both clang 15.0.0 and gcc 12.2 compile constexpr functions with multiple return statements with no problem. Am I reading the standard wrong or was this just some "obsolete" decision that is no longer being followed?

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  • cppreference isn't the standard? Commented Feb 27, 2023 at 16:08
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    That part has a note saying (until C++14), so in newer standards this limitation doesn't exist. Commented Feb 27, 2023 at 16:09
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    @IlCapitano is correct, the note "until c++14" is bound to the whole list of bullet points, not only to the one bullet point you mentioned. Commented Feb 27, 2023 at 17:03

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It used to be a restriction when constexpr was introduced to the language in C++11. Basically, back then, constexpr functions could only consist of a single return statement (like a simple getter function). The scope of constexpr has been extended since then and it's now much more capable than before. I guess the restrictions were due to

  1. it was a new feature back then, so the committee was conservative in its scope, and
  2. in a constexpr context, UB is not allowed, which means the compiler has to prove that you're not invoking UB. I assume this was easier to implement from a compiler vendor perspective.
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I do not think that the second point matters. Even with just one return statement C++11 constexpr functions are already Turing-complete and can evaluate basically arbitrary complex expressions. The really tricky UB cases (e.g. the ones related to indeterminate/unspecified order of evaluation) are still open issues and would have been there with a single return statement as well.

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