In statically typed languages
In statically typed languages (such as C++ and Java), the term generic functions refers to a mechanism for compile-time polymorphism (static dispatch), specifically parametric polymorphism. These are functions defined with TypeParameters, intended to be resolved with compile time type information. The compiler uses these types to instantiate suitable versions, resolving any function overloading appropriately.
In Common Lisp Object System
In some systems for object-oriented programming such as the Common Lisp Object System (CLOS) and Dylan, a generic function is an entity made up of all methods having the same name. Typically a generic function is an instance of a class that inherits both from function and standard-object. Thus generic functions are both functions (that can be called with and applied to arguments) and ordinary objects. The book The Art of the Metaobject Protocol explains the implementation and use of CLOS generic functions in detail.
(send object :message)
With New Flavors, it was decided the message should be a real function and the usual function calling syntax should be used:
message now is a generic function, an object and function in its own right. Individual implementations of the message are called methods.
Define a generic function with two parameters object-1 and object-2. The name of the generic function is collide.
(defgeneric collide (object-1 object-2))
Methods belonging to the generic function are defined outside of classes. Here we define a method for the generic function collide which is specialized for the classes asteroid (first parameter object-1) and spaceship (second parameter object-2). The parameters are used as normal variables inside the method body. There is no special namespace that has access to class slots.
(defmethod collide ((object-1 asteroid) (object-2 spaceship)) (format t "asteroid ~a collides with spaceship ~a" object-1 object-2))
Calling the generic function:
? (collide (make-instance 'asteroid) (make-instance 'spaceship)) asteroid #<ASTEROID 4020003FD3> collides with spaceship #<SPACESHIP 40200048CB>
Common Lisp can also retrieve individual methods from the generic function. FIND-METHOD finds the method from the generic function collide specialized for the classes asteroid and spaceship.
? (find-method #'collide nil (list (find-class 'asteroid) (find-class 'spaceship))) #<STANDARD-METHOD COLLIDE NIL (ASTEROID SPACESHIP) 4150015E43>
Comparison to other languages
Generic functions correspond roughly to what Smalltalk terms methods, with the notable exception that, in Smalltalk, the receiver's class is the sole determinant of which body of code is called: the types or values of the arguments are irrelevant (single dispatch). In a programming language with multiple dispatch when a generic function is called, method dispatch occurs on the basis of all arguments, not just one which is privileged. New Flavors also provided generic functions, but only single dispatch.