A copy constructor is a special constructor in C++ that creates a new object as a copy of an existing object of the same class. It is used to initialize the data members of the new object with the values of the data members of the existing object that is being copied. Copy constructors are the standard way of copying objects in C++, as opposed to cloning. Normally, the compiler automatically creates a copy constructor for each class, known as an implicit copy constructor. However, in special cases, the programmer creates the copy constructor, known as a user-defined copy constructor. A user-defined copy constructor is generally needed when an object owns pointers or non-shareable references, such as to a file, in which case a destructor and an assignment operator should also be written. The copy constructor is used only for initializations, and does not apply to assignments where the assignment operator is used instead. A copy constructor may be called in the following cases:
- When an object of the class is returned by value.
- When an object of the class is passed (to a function) by value as an argument.
- When an object is constructed based on another object of the same class.
- When the compiler generates a temporary object.
It is not guaranteed that a copy constructor will be called in all these cases, because the C++ Standard allows the compiler to optimize the copy away in certain cases, one example is the return value optimization (sometimes referred to as RVO) .
