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).
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Use of an identifier id in a context that establishes a binding for id 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.
Binding time
The binding of names before the program is run is called static (also "early"); bindings performed as the program runs are dynamic (also "late" or "virtual").
An example of a static binding is a direct C function call: the function referenced by the identifier cannot change at runtime.
But 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:
List is an interface, so list must refer to a subtype of it. Is it a reference to a LinkedList, an ArrayList, or some other subtype of List? The actual method referenced by add is not known until runtime. In a language like C, the actual function is known.
Rebinding and mutation
Rebinding should not be confused with mutation – "rebinding" is a change to the referencing identifier; "mutation" is a change to the referenced value. Consider the following Java code:
The identifier list initially references nothing (it is uninitialized); it is then rebound to reference an object (a linked list of strings). The linked list referenced by list is then mutated, adding a string to the list. Lastly, list is rebound to null.
Late static
Late static binding is a variant of binding somewhere between static and dynamic binding. Consider the following PHP example:
In this example, the PHP interpreter binds the keyword self inside A::hello() to class A, and so the call to B::hello() produces the string "hello". If the semantics of self::$word had been based on late static binding, then the result would have been "bye".
Beginning with PHP version 5.3, late static binding is supported. Specifically, if self::$word in the above were changed to static::$word as shown in the following block, where the keyword static would only be bound at runtime, then the result of the call to B::hello() would be "bye":