In C++, the copy constructor is used to create a new object as a copy of an existing object. The copy constructor is a special member function that takes a reference to an object of the same class as an argument, and initializes a new object with the same values as the original object.

Here's an example of a class with a copy constructor:

class MyClass {
  public:
    int myInt;
    double myDouble;
    MyClass(int value = 0, double dvalue = 0.0) {
      myInt = value;
      myDouble = dvalue;
    }
    MyClass(const MyClass& other) {
      myInt = other.myInt;
      myDouble = other.myDouble;
    }
};

In this example, the class MyClass has a copy constructor that takes a const reference to another object of the same class as an argument. The copy constructor initializes the new object's member variables with the values of the member variables in the original object.

You can create a new object of the MyClass class using the copy constructor like this:

MyClass obj1(42, 3.14);
MyClass obj2 = obj1; // Uses the copy constructor

In this example, we've created two objects of the MyClass class: obj1 and obj2. obj1 is initialized with the values 42 and 3.14, and obj2 is initialized by using the copy constructor to create a new object with the same values as obj1.

When you create a new object using the copy constructor, a new object is created with its own memory, and the values of the member variables are copied from the original object. If the original object has dynamically allocated memory, the copy constructor should allocate its own memory and copy the values from the original object.

The copy constructor is used in many situations, such as when you pass an object to a function by value, when you return an object from a function, or when you assign an object to another object using the assignment operator. By default, C++ provides a default copy constructor that does a shallow copy, but you can provide your own copy constructor to do a deep copy or to handle special cases.