Copy Constructor and default constructor

Question

Do we have to explicitly define a default constructor when we define a copy constructor for a class?? Please give reasons.

eg:

class A 
{
    int i;

    public:
           A(A& a)
           {
               i = a.i//Ok this is corrected....
           }

           A() { }   /Is this required if we write the above copy constructor??
};

Also, if we define any other parameterized constructor for a class other than the copy constructor, do we also have to define the default constructor?? Consider the above code without the copy constructor and replace it with

A(int z)
{
    z.i = 10;
}

Alrite....After seeing the answers I wrote the following program.

#include <iostream>

using namespace std;

class X
{
    int i;

    public:
            //X();
            X(int ii);
            void print();
};

//X::X() { }

X::X(int ii)
{
    i = ii;
}


void X::print()
{
    cout<<"i = "<<i<<endl;
}

int main(void)
{
    X x(10);
  //X x1;
    x.print();
  //x1.print();
}

ANd this program seems to be working fine without the default constructor. Please explain why is this the case?? I am really confused with the concept.....

Answer 1

Yes. Once you explicitly declare absolutely any constructor for a class, the compiler stops providing the implicit default constructor. If you still need the default constructor, you have to explicitly declare and define it yourself.

P.S. It is possible to write a copy constructor (or conversion constructor, or any other constructor) that is also default constructor. If your new constructor falls into that category, there's no need to provide an additional default constructor anymore :)

For example:

// Just a sketch of one possible technique    
struct S {
  S(const S&);
  S(int) {}
};

S dummy(0);

S::S(const S& = dummy) {
}

In the above example the copy constructor is at the same time the default constructor.

Answer 2

You don't have to define both. However, once you define any constructors for a class, all the default ones become unavailable. So - if you want to both copy-construct and to construct without copying, you need to define an non-default (ie explicit) default (ie no parameters) constructor too.

If you define a copy constructor, you should normally override the assignment operator too.

Answer 3

As AndreyT said, if you explicitly declare any constructor, including a copy constructor, the compiler will not implicitly declare or define a default constructor.

This is not always a problem.

If you do not want your class to be default-constructible, then it's perfectly fine not to declare a default constructor. But if you want it to be default-constructible (for instance, if you want to uncomment the X x1; line in your example), then you must declare and define a default constructor.

Also note that a default constructor is any constructor that can be called with no arguments, not just one with no arguments. X::X(int = 5) is a perfectly fine default constructor.