The standard says [dcl.init]:

"To value-initialize an object of type T means:

(9.1) if T is a (possibly cv-qualified) class type ([class]), then

(9.1.1) if T has either no default constructor ([class.default.ctor]) or a default constructor that is *user-provided* or deleted, then the object is default-initialized;

...

(9.3)

otherwise, the object is zero-initialized."

(note the emph on *user-provided*, as opposed to user-declared).

This causes very surprising behavior like the following (https://godbolt.org/z/rxYcsYdTd):

#include <iostream>

using namespace std;

int main() {

    struct Thingy1 {        // No user provided default ctor

        int i;

    };

    cout << Thingy1().i << endl;   // Zero

    Thingy1 t1;

    cout << t1.i << endl;          // Junk

    

    struct Thingy2 {

        Thingy2() {};       // User provided default ctor

        int i;

    };

    cout << Thingy2().i << endl;   // Junk

    

    struct Thingy3 {

        Thingy3() = default; // user DECLARED, not provided

        int i;

    };

    cout << Thingy3().i << endl;   // Zero again!

}

----------------------------

I've chased down the origin of this to defect report 1368 from 2011 (https://cplusplus.github.io/CWG/issues/1368.html). Quote:

"  struct A {

    int i;

    A() = default;

    A(int i): i(i) { }

  };

value-initialization leaves A::i uninitialized. This seems like an oversight."

I don't understand the "This seems like an oversight" bit. I spoke with quite a few people who were equally surprised and annoyed by this behavior - it seems the common expectation is that `Thingy3() = default` would express an intention identical to `Thingy3() {}`.

Now before I try to write a paper to promote (what I perceive to be) a fix - am I missing something? Is there reason to make A::i zero-initialized despite the intention expressed in `A() = default` ?

Thank you,

-Ofek Shilon