when to use copy constructors

[tropicaljava]

Do we need to write a copy constructor if my class has just one char pointer variable?

class Test
{
public:
	char *a;
	Test(void){ }
};

[vlad from moscow]

It is determined by how you want that the member of the class were copied.

[tropicaljava]

Ok., So if I don't write a copy constructor for above class then what will happen? Will it do just shallow copy?

Please see below code

	Test testObj;
	testObj.a ="no copycon";
	Test b = testObj; //will call default copycon
	cout << testObj.a << endl; //will print no copycon
	cout << b.a<< endl;; //will print no copycon

	testObj.a = "changed text";

	cout << testObj.a << endl;// will print changed text
	cout << b.a; // still prints no copycon 

Why the last line still prints "copycon"? As per my understanding it should changed and should display the changed string i.e. "changed text".
Isn't it?

[vlad from moscow]

1) "changed text" has its own address.
2) each object of the class Test his its own member 'a';
3) You have changed only the member of object testObj.

[tropicaljava]

ok thanks. But how to change above code so that I get an effect like if i change just the member of object testObj, and same change gets automatically reflected to the member of another object "b".

[vlad from moscow]

Define this mamber as static.

 class Test
{
public:
	static cont char *a;
	Test(void){ }
};

const char * Test::a;

Test testObj;
testObj.a ="no copycon";
Test b = testObj; //will call default copycon
cout << testObj.a << endl; //will print no copycon
cout << b.a<< endl;; //will print no copycon

testObj.a = "changed text";

cout << testObj.a << endl;// will print changed text
cout << b.a; // still prints no copycon  

[gtkano]

U need a copy destructor implemented, that will dealocate the pointers and free the memory

class Test
{
public:
	char *a;
	Test(void){ }
        ~Test() 
        {
           delete(a);// memory is freed 
           a = NULL; // pointer a points to nothing
        }
};

constructors are used when you do not want to have a separate function that will initialize the object.
example:

#include <iostream>
using namespace std;

class Test
{
public:
	char *a;
	Test(void)
        {
           a = new char; //pointer a is initialized
        }

        ~Test() 
        {
           delete(a);// memory is freed 
           a = NULL; // pointer a points to nothing
        }
};

void main ()
{
    char b='s';
    Test testObj;
    
    *(testObj.a) = b; // operator * is used to acces the data pointed by the pointer a
    cout<<*(testObj.a)<<endl;
	system("pause");
}

or u can create a separate function

#include <iostream>
using namespace std;

class Test
{
public:
	char *a;
	void Initialize(void) //insted of contructor we use a normal function
        {
           a = new char; //pointer a is initialized
        }

        ~Test() 
        {
           delete(a);// memory is freed 
           a = NULL; // pointer a points to nothing
        }
};

void main ()
{
    char b='s';
    Test testObj;
	testObj.Initialize(); //initialize the class
    
    *(testObj.a) = b; // operator * is used to acces the data pointed by the pointer a
    cout<<*(testObj.a)<<endl;
	system("pause");
}

[Disch]

Err....

           a = NULL; // if you set a to NULL
           delete(a);// then you are deleting NULL
           //  delete NULL does absolutely nothing. 

[vlad from moscow]

Moreover in the original code the pointer is assigned a string literal. So it can not be deleted.

[gtkano]

YES you are right

I will edit my previous post .