Name binding


In programming languages, name binding is the association of entities 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 and in which one of the possible execution paths.
Use of an identifier id in a context that establishes a binding for id is called a binding occurrence. In all other occurrences, an identifier stands for what it is bound to; such occurrences are called applied occurrences.

Binding time

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, the executed function is dynamically bound. Take, for example, the following Java code:

public void foo

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 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 actually gets evaluated at run-time.

Rebinding and mutation

Rebinding should not be confused with mutation.
Consider the following Java code:

LinkedList list;
list = new LinkedList;
list.add;
list = null;

The identifier list initially references nothing ; it is then rebound to reference an object. 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:

class A
class B extends A
B::hello;

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":

class A
class B extends A
B::hello;