Copying structs with uninitialized members

Tomek Czajka :

Is it valid to copy a struct some of whose members are not initialized?

I suspect it is undefined behavior, but if so, it makes leaving any uninitialized members in a struct (even if those members are never used directly) quite dangerous. So I wonder if there is something in the standard that allows it.

For instance, is this valid?

struct Data {
  int a, b;
};

int main() {
  Data data;
  data.a = 5;
  Data data2 = data;
}

walnut :

Yes, if the uninitialized member is not an unsigned narrow character type or std::byte, then copying a struct containing this indeterminate value with the implicitly defined copy constructor is technically undefined behavior, as it is for copying a variable with indeterminate value of the same type, because of [dcl.init]/12.

This applies here, because the implicitly generated copy constructor is, except for unions, defined to copy each member individually as if by direct-initialization, see [class.copy.ctor]/4.

This is also subject of the active CWG issue 2264.

I suppose in practice you will not have any problem with that, though.

If you want to be 100% sure, using std::memcpy always has well-defined behavior if the type is trivially copyable, even if members have indeterminate value.

---

These issues aside, you should always initialize your class members properly with a specified value at construction anyway, assuming you don't require the class to have a trivial default constructor. You can do so easily using the default member initializer syntax to e.g. value-initialize the members:

struct Data {
  int a{}, b{};
};

int main() {
  Data data;
  data.a = 5;
  Data data2 = data;
}