Concept
In programming languages, name binding is the association of entities (data and/or code) with identifiers. An identifier bound to an object is said to reference that object. Machine languages have no built-in notion of identifiers, but name-object bindings as a service and notation for the programmer is implemented by programming languages. Binding is intimately connected with scoping, as scope determines which names bind to which objects – at which locations in the program code (lexically) and in which one of the possible execution paths (temporally). Use of an identifier in a context that establishes a binding for is called a binding (or defining) occurrence. In all other occurrences (e.g., in expressions, assignments, and subprogram calls), an identifier stands for what it is bound to; such occurrences are called applied occurrences. Static binding (or early binding) is name binding performed before the program is run. Dynamic binding (or late binding or virtual binding) is name binding performed as the program is running. An example of a static binding is a direct C function call: the function referenced by the identifier cannot change at runtime. An example of dynamic binding is dynamic dispatch, as in a C++ virtual method call. Since the specific type of a polymorphic object is not known before runtime (in general), the executed function is dynamically bound. Take, for example, the following Java code: public void foo(java.util.List list) { list.add("bar"); } List is an interface, so list must refer to a subtype of it. list may reference a LinkedList, an ArrayList, or some other subtype of List. The method referenced by add is not known until runtime. In C, such an instance of dynamic binding may be a call to a function pointed to by a variable or expression of a function pointer type whose value is unknown until it is evaluated at run-time. Rebinding should not be confused with mutation. Rebinding is a change to the referencing identifier. Mutation is a change to the referenced entity.