6.1. What is a copy constructor?

A copy constructor is a special constructor in C++ that initializes a new object as a copy of an existing object. Given a is an object of Student class. It is called when:

• Student b(a);

  Here, a is an existing object, and b is being created as a copy of a. We are passing a to the constructor.

• Student b = a;

  Though = syntax is used, but as b is being created in this same line, this calls the copy constructor. = operator is not called here.

• An object is passed by value to a function.

  For example, when the function prototype is void foo(Student s); and the function call is foo(a);. Passing a to the function creates a new object s in the function’s scope, and the copy constructor is called to initialize s with the value of a.

• An object is returned by value from a function.

  For example, when the function prototype is Student foo(); and the function return statement is return a;. Since the return type is Student, a new object is created to hold the returned object, and the copy constructor is called to initialize this new object with the value of a.

What does the copy constructor do? The copy constructor initializes the data of the new object being created with the values of the existing object’s data member variables. For example, consider the following class definition:

class Student {
  private:
    int id;
    string name;
  public:
    // Copy constructor
    Student(const Student &s) {
        id = s.id;
        name = s.name;
    }
};

(work in progress)