CODEWITHSUNDEEP
c++language-lawyer
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Reputation: 21160

Are function declarations with abstract class types ill-formed?

Inspired by this question, I had a look around the standard. There is a note in [class.abstract]

[Note: An abstract class type cannot be used as a parameter or return type of a function being defined ([dcl.fct]) or called ([expr.call]), except as specified in [dcl.type.simple]. [...]

For definitions [dcl.fct.def.general]

The type of a parameter or the return type for a function definition shall not be a (possibly cv-qualified) class type that is incomplete or abstract within the function body unless the function is deleted ([dcl.fct.def.delete]).

But I couldn't find anything about declarations, by which I can only conclude there is nothing wrong with it.

Upvotes: 1

Views: 116

Answers (1)

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Reputation: 21160

This is P0929. Prior to C++20, function declarations are also ill-formed, but it lead to surprising semantics. Suppose

struct S;

S foo();  // 1

struct S { virtual void bar() = 0; };  // 2

At 1, the function is well-formed, yet 2 retroactively made it ill-formed. This is highly unintuitive, and so was changed.

Upvotes: 2

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