Copy Constructor
A copy constructor is a constructor that executes when you initialize a new object of the class with an existing object of the same class. You don't need to create a constructor for this; one is already built into all classes. It's called default copy constructor. It's a one argument constructor whose argument is an object of the same class.
For example,
String s1 ( " hi " ) ;
String s2 ( s1 ) ;
String s3 = s1 ;
The object s2 is initialized in the statement String s2 ( s1 ) ;
This causes the default copy constructor for the String class to perform a member-by-member copy of s1 into s2. Surprisingly, a different format has exactly the same effect, causing s1 to be copied member-by-member into s3:
String s3 = s1 ;
Although this looks like an assignment statement, it is not. Both formats invoke the default copy constructor, and can be used interchangeably
If we are not satisfied with the default copy constructor we can define a copy constructor on our own. It takes the form of
class_name ( const class_name & object_name ) ;
For example,
String ( const String & tmp ) ;
Difference between initialization & assignment
The difference between initialization of an object with another object, and assignment of one object to another is this: Assignment assigns the value of an existing object to another existing object; initialization creates a new object and initializes it with the contents of the existing object. The compiler can distinguish between the two by using your overloaded assignment operator for assignments and your copy constructor for initializers.
For example, the statement
String s2 = s1 ;
would define the object s2 and at the same time initialize it to the values of s1.
Remember the statement
s2 = s1 ;
will not invoke the copy constructor. However, if s1 and s2 are objects, this statement is legal and simply assigns the values of s1 to s2, member-by-member. This is the task of the overloaded assignment operator ( = ).