Name binding

In programming languages, name binding is the association of entities (data and/or code) with identifiers.^[1] 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 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:

 public void foo(java.util.List<String> list) {
     list.add("bar");
 }

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:

 LinkedList<String> list;
 list = new LinkedList<String>();
 list.add("foo");
 list = null;

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:

class A {
    static $word = "hello";
    static function hello() { print self::$word; }
}

class B extends A {
    static $word = "bye";
}

B::hello();

In this example, the PHP interpreter binds the function 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.^[2] Specifically, if self::$word in the above were changed to static::$word as shown in the following block, then the result of the call to B::hello() would be "bye":

class A {
    static $word = "hello";
    static function hello() { print static::$word; }
}

class B extends A {
    static $word = "bye";
}

B::hello();

See also

• Late binding
• Branch table method of applying name binding via branch table or function pointers
• Dynamic dispatch
• Higher-order abstract syntax (HOAS)

References