A friend function in object-oriented programming is a function that is granted access to the private and protected members of a class. It is declared within the class but is not a member of the class. The friend function can be either a standalone function or a member function of another class.

By declaring a function as a friend of a class, you allow that function to access the private and protected members of the class, even though it is not a member of the class itself. This provides a way to share or grant special access to certain functions without exposing the private members to the entire program.

Here are some key points about friend functions:

1. Declaration: The friend function must be declared in the class that is granting access. Typically, the declaration appears in the class definition, but the actual implementation is outside the class.
2. Access to private and protected members: The friend function can access private and protected members of the class as if it were a member of the class. This allows the function to manipulate the internal state of the class directly.
3. Scope: A friend function is not a member of the class and has no scope of the class. It can be defined in the same namespace as the class or in a different namespace.
4. Friend classes: In addition to friend functions, a class can also declare another class as a friend. This grants the friend class access to the private and protected members of the declaring class.

Friend functions can be useful in certain situations, such as when you need to provide special access to certain functions that are closely related to a class but not necessarily part of it. However, it is generally recommended to limit the use of friend functions to maintain encapsulation and minimize dependencies between classes.