Can a trivial type class be copied when not all it

问题:

(I just realized I first need to solve a much more basic issue with copying unions: When a union object is copied, is a member subobject created?. Please see that other question first.)

The implicitly generated copy operations (constructor and assignment) of a class perform member by member copy (initialization or assignment). (For a trivial type these are the same.)

So a class with some members not initialized cannot be copied, as accessing uninitialized objects is illegal.

struct C {
  int m1, m2;
};

void f() {
  C c1, c2;

  c1.m1 = 1;
  c2 = c1; // not initialized
}

But a union can always be copied, even if it contains class members, some of which aren't initialized (because... by definition not two members of a unions are initialized).

Does that mean that copying a union of a class with uninitialized members is legal:

union U {
  C m;
};

void g() {
  U u1, u2;
  u1.m.m1 = 1;
  u2 = u1;
}

and if so, can classes be copied by casting to such union?

void f2() {
  C c1, c2;

  c1.m1 = 1;
  (U&)c2 = (U&)c1; // not initialized?
}

回答1:

Yes, you can copy the union with the uninitialized (indirect) member—via the default copy/move assignment operator that is defined to copy the object representation. (You could also write your own operator that used std::memcpy. It is obviously a wording defect if the defaulted operator doesn’t establish the correct active member, and is std::memcpy should do so as well.)

However, you cannot use cast-to-union as a safer copy for normal class objects; the call to the assignment operator has undefined behavior per a special rule needed because it doesn’t involve an access to any (scalar) object. Your own std::memcpy-based implementation would be fine, even via reinterpret_cast(!), but that’s not newsworthy—reading/copying indeterminate values via unsigned char (and perhaps char) and std::byte is always allowed).