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: 
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.
1) Constructor with no argument is called 'default Constructor'.
2) If user have not provided any of the following constructor, then the compiler declares
 the default constructor for you. 

```
  2a) Copy Constructor
  2b) Non-default constructor
  2C) default constructor. 

```

3) If compilers declares the 'default constructor', then it is said to be 'implicitly declared default constructor', 

4) The 'implicitly declared default constructor' are of two types 

```
   4a) **trivial** Constructor
   4b) **non-trivial** Constructor. 

```

5) Implictly declared default constructor is said to be 'trivial', when there is 
 5a) No reference variable 
 5b) No Virtual Function
 5c) No Virtual Base Class.
 5d) Parent class has 'trivial' constructor, 

```
Else, it is said to be 'non-trivial'.

```

Hope this helps.