Why does a defaulted default constructor depend on whether it is declared inside or outside of class?

Question

In the following example, struct A does not have default constructor. So both struct B and struct C inherited from it cannot get compiler-generated default constructor:

struct A {
    A(int) {}
};

struct B : A {
    B() = default; //#1
};

struct C : A {
    C();
};

C::C() = default; //#2

#1. In struct B, defaulted default constructor is declared inside the class, and all compilers accept it, with only Clang showing a warning saying that explicitly defaulted default constructor is implicitly deleted [-Wdefaulted-function-deleted]

#2. But defaulted default constructor declared outside of struct C generates a compiler error. Demo: https://gcc.godbolt.org/z/3EGc4rTqE

Why does it matter where the constructor is defaulted: inside or outside the class?

*Why does it matter where the constructor is defaulted: inside or outside the class?* -- Maybe the simple answer is "those are the current rules of C++"? If you have a line or paragraph in the standard that allows your code to compile, but it doesn't compile for those compilers you tested for, you could state it. — PaulMcKenzie, Apr 22 '22 at 13:42
@PaulMcKenzie cop-out answer. *Why are those the current rules of C++?* — user253751, Apr 22 '22 at 14:10
@user253751 Follow-up. Go ask the committee. Or suggest it to them and see what happens. No different than asking "why static initializers are not allowed in classes" back in the C++98 days. — PaulMcKenzie, Apr 22 '22 at 14:11
@PaulMcKenzie Many questions about the C++ language design have reasonable answers. Like Alan's answer to this question. — user253751, Apr 22 '22 at 15:35
@PaulMcKenzie only in your mind and nobody else's. "Why can't I dereference an uninitialized pointer?" "Because the standard says so." — user253751, Apr 22 '22 at 16:34
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/244111/discussion-between-paulmckenzie-and-user253751). — PaulMcKenzie, Apr 22 '22 at 16:40

Alan Birtles · Accepted Answer · 2022-04-22T14:29:24.870

Note that if you actually try and instantiate B then you'll also get the error that B::B() is deleted: https://gcc.godbolt.org/z/jdKzv7zvd

The reason for the difference is probably that when you declare the C constructor, the users of C (assuming the definition is in another translation unit) have no way of knowing that the constructor is in fact deleted. At best this would lead to some confusing linker error, at worst it'd just crash at runtime. In the case of B all users of B will immediately be able to tell that the default constructor is deleted so it does no harm (even if it makes no sense as warned by clang) to declare it as defaulted.

Why does a defaulted default constructor depend on whether it is declared inside or outside of class?

1 Answers1