Name binding

From Wikipedia, the free encyclopedia

In programming languages, name binding refers to the association of values with identifiers. An identifier bound to a value is said to reference that value. Since computers themselves have no notion of identifiers, there is no binding at the machine language level — name binding is an abstraction provided by programming languages. Binding is intimately connected with scoping, as scope determines when binding occurs.

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.

[edit] 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. 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(List<String> l) {
    l.add("bar");
}

Is l 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.

Since compiled programs are often relocatable in memory, every memory reference is ultimately a dynamic binding. Each variable or function is referenced as an offset from a memory segment, which is not known until runtime. This is a pedantic distinction, however, as binding operates at the programming-language level and not the machine level.

[edit] 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> l;
l = new LinkedList<String>();
l.add("foo");
l = null;

The identifier l at first references nothing (it is null); it is then rebound to reference an object (a linked list of strings). The linked list referenced by l is then mutated, adding a string to the list. Lastly, l is rebound to null.