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I was wondering under which circumstances a class may have no default constructor but will still be value-initialized. The typical case for "no default ctor" is that a parametrized one is present and the default ctor is not defaulted (= default).

Quoting from the 2020 standard, chapter 9.4.1, Initializers/General (the latest draft contains equivalent wording):

To value-initialize an object of type T means:
(8.1) — if T is a (possibly cv-qualified) class type (Clause 11), then
(8.1.1) — if T has either no default constructor (11.4.5.2) or a default constructor that is user-provided or deleted, then the object is default-initialized;

And

11.4.5.2 Default constructors [class.default.ctor]: 1 A default constructor for a class X is a constructor of class X for which each parameter that is not a function parameter pack has a default argument (including the case of a constructor with no parameters). If there is no user-declared constructor for class X, a non-explicit constructor having no parameters is implicitly declared as defaulted (9.5)

I read this such that 8.1.1 does not refer to cases with no constructor at all (because, as 11.4.5.2 explains, a default constructor is then implicitly declared). Instead I understand it as "No default constructor at all" (including implicit ones).

Value-initialization happens with new()or brace-initialization, and for excess elements in arrays that are brace-initialized (as in struct T{}; T arr[1]{};).

Neither construct compiles when there is "no default constructor". Are there situations where objects of types without default constructor are value-initialized at all? Am I misreading 8.1.1?

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It’s just saying that value-initializing an object of a class type that lacks a default constructor tries and fails to default-initialize it. Note that the alternative in the next bullet still default-initializes it after zero-initializing it, so it’s not default-initialized any more for not having a default constructor.

Depending on one’s interpretation of [class.default.ctor], it might also be considered to cover the case where a class can be default-initialized via a variadic constructor template:

struct A {
  template<class ...TT> A(TT...);
};
auto a=A();  // value-initialization -> default-initialization
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5 Comments

The next bullet point starts with "otherwise", so it cannot refer to the same case. Regarding the "tries and fails": I'm missing the "fails", that's why I'm asking. But some obscure template issue may be a reason for the wording. (Although -- with the caveat of being ignorant about variadic templates -- I'd assume that the "no arguments" case would qualify as "default constructor".)
I think this means that the rules for "value initialization" doesn't forbid its use with non-default-constructible types, rather it delegates to the rules for "default initialization", which would in turn forbid it. I'm not sure why they would write it out like this, and I couldn't find what "default initialization" does with a type that doesn't have a default constructor (those rules seem to already assume there is a default constructor).
@Peter-ReinstateMonica: Of course the next bullet cannot apply; my point is that making it apply (by, say, removing "either no default constructor (11.4.5.2) or") wouldn't change whether such value initialization performs (or tries to perform) default initialization.
@FrançoisAndrieux: Right—and default initialization in turn fails in overload resolution. Lots of the standard is written in terms of “subroutine calls” like this, since it provides consistency; this one prevents performing zero-initialization and “then” being ill-formed, which might be even more confusing. (Really, it’s the “semantic constraints for default-initialization are checked” that’s weird here; there has been talk of simplifying that part.)
@DavisHerring I suppose the "let the default initialization clause spell out that it will fail" interpretation is right; but I'd had much preferred if they had said in the value-initialization definition "If T has no default constructor or a default constructor that is deleted, then the program is ill-formed", and in an additional paragraph covered the user-provided default ctor, referring initialization to default-initialization.

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