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A copy constructor is a special type of constructor in C++ that is used to create a new object as a copy of an existing object. It is called when we initialize a new object with the help of an existing object, passing the new object by value, as a function argument to another function or return an object from a function by value.

References in C++ are aliases for variables or objects. They are used to create another name for a variable and allow to manipulate the original data through this new name. A reference is declared by using an ampersand (&) before the variable name. The main advantage of using references is that they help avoid unnecessary copying of large objects and provide a way to access the original data directly.

When passing an object to a copy constructor, using a reference has two main advantages: 1. Efficiency: If we pass the object by value instead of by reference, it would require calling the copy constructor again, resulting in an infinite loop of copy constructor calls. This would be both time and resource-consuming. By passing the object as a reference, we avoid this inefficiency. 2. Correctness: Since the purpose of a copy constructor is to create a new object as a copy of an existing one, it makes sense to ensure that the original object remains unchanged. If we pass the object by value, there is a risk of modifying the original object during the copying process, which might have unintended consequences. By passing by reference, we guarantee that the original object remains constant and unchanged. Therefore, the parameter of a copy constructor must be a reference to ensure both efficiency and correctness when creating a new object as a copy of an existing one.