confusion about when embedded object's copy constructor gets called

Question

class WithCC { 
public:
  WithCC() {}
  WithCC(const WithCC&) {
    cout << "in WithCC's copy constructor" << endl;
  }
};


class Composite {
  WithCC withcc; // Embedded objects
public:
  Composite() {}
};

int main() {
  Composite c;
  Composite c2 = c;
} 

With the code above, withcc's copy constructor gets called and I get the output: in WithCC's copy constructor

But if I add a copy constructor to Composite like this...

class Composite {
  WithCC withcc; // Embedded objects
public:
  Composite() {}
  Composite(const Composite&) {
      cout << "in composite's copy constructor" << endl;
  }
};

withcc's copy constructor doesn't seem to get called because the output is: in composite's copy constructor

Why isn't withcc's copy constructor called here?

Answer 1

The copy constructor implicitly defined by the compiler performs a memberwise copy/move of its bases and members.

A user-defined copy-constructor defaault initializes subobjects of the derived class if their constructors were not called explicitly in the ctor-list.

So in your last code snippet you defined explicitly the copy constructor

  Composite(const Composite&) {
      cout << "in composite's copy constructor" << endl;
  }

but its ctor list is empty. So data member withcc will be default initialized.

I described this more detailed in my article

Implicitly defined copy constructor and explicitly defined copy constructor: what is the difference?

Though it is written in Russian but you will be able to read it using for example google service translare.

Answer 2

In the first sample you omitted a copy constructor for Composite and hence C++ generated a default one for you. This default one essentially runs a field by field copy and hence was running the copy constructor of WithCC.

When you explicitly defined a copy constructor C++ does no magic. It is up to you to copy the fields as necessary. For example

Composite(const Composite& other) : withcc(other.withcc) {
      cout << "in composite's copy constructor" << endl;
}