C++ Copy constructors


Copy constructor is

  • a constructor function with the same name as the class
  • used to make deep copy of objects.

There are 3 important places where a copy constructor is called.

  1. When an object is created from another object of the same type
  2. When an object is passed by value as a parameter to a function
  3. When an object is returned from a function

If a copy constructor is not defined in a class, the compiler itself defines one. This will ensure a shallow copy. If the class does not have pointer variables with dynamically allocated memory, then one need not worry about defining a copy constructor. It can be left to the compiler’s discretion.

But if the class has pointer variables and has some dynamic memory allocations, then it is a must to have a copy constructor.


For ex:

class A   //Without copy constructor
{
private:
int x;
public:
A() {A = 10;}
~A() {}
}

      
class B    //With copy constructor
{
private:
char *name;
public:
B()
{
name = new char[20];
}
~B()
{
delete name[];
}
//Copy constructor
B(const B &b)
{
name = new char[20];
strcpy(name, b.name);
}
};

Let us Imagine if you don’t have a copy constructor for the class B. At the first place, if an object is created from some existing object, we cannot be sure that the memory is allocated. Also, if the memory is deleted in destructor, the delete operator might be called twice for the same memory location.

This is a major risk. One happy thing is, if the class is not so complex this will come to the fore during development itself. But if the class is very complicated, then these kind of errors will be difficult to track.

In Windows this will lead to an application popup and unix will issue a core dump. A careful handling of this will avoid a lot of nuisance.