Copy constructor initialization lists

Question

I know that if you leave a member out of an initialization list in a no-arg constructor, the default constructor of that member will be called.

Do copy constructors likewise call the copy constructor of the members, or do they also call the default constructor?

class myClass {
  private:
    someClass a;
    someOtherClass b;
  public:
    myClass() : a(DEFAULT_A) {} //implied is b()
    myClass(const myClass& mc) : a(mc.a) {} //implied is b(mc.b)??? or is it b()?
}

Answer 1

Explicitly-defined copy constructors do not call copy constructors for the members.

When you enter the body of a constructor, every member of that class will be initialized. That is, once you get to { you are guaranteed that all your members have been initialized.

Unless specified, members are default-initialized in the order they appear in the class. (And if they can't be, the program is ill-formed.) So if you define your own copy constructor, it's now up to you to call any member copy constructors as desired.

Here is a small program you can copy-paste somewhere and mess around with:

#include <iostream>

class Foo {
public:
    Foo() {
        std::cout << "In Foo::Foo()" << std::endl;
    }

    Foo(const Foo& rhs) {
        std::cout << "In Foo::Foo(const Foo&)" << std::endl;
    }
};

class Bar {
public:
    Bar() {
        std::cout << "In Bar::Bar()" << std::endl;
    }

    Bar(const Bar& rhs) {
        std::cout << "In Bar::Bar(const Bar&)" << std::endl;
    }
};

class Baz {
public:
    Foo foo;
    Bar bar;

    Baz() {
        std::cout << "In Baz::Baz()" << std::endl;
    }

    Baz(const Baz& rhs) {
        std::cout << "In Baz::Baz(const Baz&)" << std::endl;
    }
};

int main() {
    Baz baz1;
    std::cout << "Copying..." << std::endl;
    Baz baz2(baz1);
}

As-is, this prints:

In Foo::Foo()
In Bar::Bar()
In Baz::Baz()
Copying...
In Foo::Foo()
In Bar::Bar()
In Baz::Baz(const Baz&)

Note that it's default-initializing the members of Baz.

By commenting out the explicit copy constructor, like:

/*
Baz(const Baz& rhs) {
    std::cout << "In Baz::Baz(const Baz&)" << std::endl;
}
*/

The output will become this:

In Foo::Foo()
In Bar::Bar()
In Baz::Baz()
Copying...
In Foo::Foo(const Foo&)
In Bar::Bar(const Bar&)

It calls the copy-constructor on both.

And if we reintroduce Baz's copy constructor and explicitly copy a single member:

Baz(const Baz& rhs) :
    foo(rhs.foo)
{
    std::cout << "In Baz::Baz(const Baz&)" << std::endl;
}

We get:

In Foo::Foo()
In Bar::Bar()
In Baz::Baz()
Copying...
In Foo::Foo(const Foo&)
In Bar::Bar()
In Baz::Baz(const Baz&)

As you can see, once you explicitly declare a copy-constructor you are responsible for the copying of all class members; it's your constructor now.

This applies for all constructors, including move constructors.

Answer 2

Yes. Ctors are ctors.

Answer 3

For any member variable having a default constructor that default constructor is invoked if you have not explicitly added any other constructor call for that member variable into the initialization list.

Answer 4

For details see: Is there an implicit default constructor in C++?

Short:

• Compiler Generated "Default Constructor": uses the default constructor of each member.
• Compiler Generated "Copy Constructor": uses the copy constructor of each member.
• Compiler Generated "Assignment Operator": uses the assignment operator of each member.

Answer 5

There is nothing magical about a copy constructor, other than that the compiler will add it in if needed. But in how it actually runs, there is nothing special - if you don't explicitly say "use such and such a constructor", it'll use the default.

Answer 6

Not in VC9. Not sure about the others.

// compiled as: cl /EHsc contest.cpp
//
//    Output was:
//    Child1()
//    -----
//    Child1()
//    Child2()
//    Parent()
//    -----
//    Child1(Child1&)
//    Child2()
//    Parent(Parent&)

#include <cstdio>

class Child1 {
    int x;
public:
    static Child1 DEFAULT;

    Child1(){
        x = 0;
        printf("Child1()\n");
    }

    Child1(Child1 &other){
        x = other.x;
        printf("Child1(Child1&)\n");
    }
};

Child1 Child1::DEFAULT;

class Child2 {
    int x;
public:
    Child2(){
        x = 0;
        printf("Child2()\n");
    }

    Child2(Child2 &other){
        x = other.x;
        printf("Child2(Child2&)\n");
    }
};

class Parent {
    int x;
    Child1 c1;
    Child2 c2;

public:
    Parent(){
        printf("Parent()\n");
    }

    Parent(Parent &other) : c1(Child1::DEFAULT) {
        printf("Parent(Parent&)\n");
    }
};

int main(){
    printf("-----\n");
    Parent p1;
    printf("-----\n");
    Parent p2(p1);

    return 0;
}

Answer 7

Depending on how the base call constructor is initiated, the member's constructors will be called the same way. For example, let's start with:

struct ABC{
    int a;
    ABC() : a(0)    {   printf("Default Constructor Called %d\n", a);   };

    ABC(ABC  & other )  
    {
        a=other.a;
        printf("Copy constructor Called %d \n" , a ) ;
    };
};

struct ABCDaddy{
    ABC abcchild;
};

You can do these tests:

printf("\n\nTest two, where ABC is a member of another structure\n" );
ABCDaddy aD;
aD.abcchild.a=2;

printf( "\n Test: ABCDaddy bD=aD;  \n" );
ABCDaddy bD=aD; // Does call the copy constructor of the members of the structure ABCDaddy ( ie. the copy constructor of ABC is  called)

printf( "\n Test: ABCDaddy cD(aD); \n" );
ABCDaddy cD(aD);    // Does call the copy constructor of the members of the structure ABCDaddy ( ie. the copy constructor of ABC is  called)

printf( "\n Test: ABCDaddy eD; eD=aD;  \n" );
ABCDaddy eD;
eD=aD;          // Does NOT call the copy constructor of the members of the structure ABCDaddy ( ie. the copy constructor of ABC is not called)

Output:

Default Constructor Called 0

Test: ABCDaddy bD=aD;
Copy constructor Called 2

Test: ABCDaddy cD(aD);
Copy constructor Called 2

Test: ABCDaddy eD; eD=aD;
Default Constructor Called 0

Enjoy.

Answer 8

When the compiler provides the default cctor, what do you think the compiler does for the member variables? It copy constructs it.

In the same vein, if the cctor is user-defined, and if one leaves out some members, those members cannot be left uninitialized. Class invariants are established during construction and have to be constantly maintained. So, the compiler does that for you.