In C++ In C++, late binding (also called "dynamic binding") refers to what normally happens when the virtual keyword is used in a method's declaration. Typically, a call requires a lookup via a
virtual table.
In COM In
Component Object Model (COM), a late-bound method call is performed via the
IDispatch interface. This is performed by calling to look up a method by name and then to execute the method. Some contexts provide ease-of-use support for calling this interface. For example, in
Visual Basic 6, methods are called late-bound for a variable declared as . In contrast, with early binding, the compiler fixes
types of
variables and
expressions. The primary advantage of using late binding in COM programming is that one is not required to reference the providing library at
compile time. This makes the compilation process more resistant to version conflicts, in which the class's v-table may be accidentally modified. This is not an issue in
just-in-time compilation runtimes such as
.NET and
Java, because the v-table is created at runtime.
In .NET In .NET, late binding refers to
overriding a virtual method like C++ or implementing an interface. The compiler builds virtual tables for every virtual or interface method call which is used at run-time to determine the implementation to execute. Also like COM and Java, the
Common Language Runtime provides reflection APIs that can make late binding calls. The use of these calls varies by language. With C# 4, the language also added the "dynamic" pseudo-type. This would be used in place of the Object type to indicate that late binding is desired. The specific late binding mechanism needed is determined at runtime using the
Dynamic Language Runtime as a starting point. Visual Basic uses them whenever the variable is of type Object and the compiler directive "Option Strict Off" is in force. This is the default setting for a new VB project. Prior to version 9, only .NET and COM objects could be late bound. With VB 10, this has been extended to DLR-based objects.
In Java There are three definitions for late binding in Java. Early documents on Java discussed how classes were not linked together at compile time. While types are statically checked at compile time, different implementations for classes could be swapped out just prior to runtime simply by overwriting the class file. As long as the new class definition had the same class and method names, the code would still work. In this sense it is similar to the traditional definition of late binding. Currently, it is popular to use the term late binding in Java programming as a synonym for
dynamic dispatch. Specifically, this refers to Java's
single dispatch mechanism used with virtual methods.
In dynamically-typed languages In most
dynamically-typed languages (such as
JavaScript), the list of methods on an object can be altered at runtime. This requires late binding.
In Lisp In
Lisp, late bound global function calls are efficiently looked up at runtime via a
symbol's function cell. These function bindings are mutable. Example using an interactive
Clozure Common Lisp session: ? (defun foo () (bar pi)) ; a still undefined function BAR gets called ;Compiler warnings : ; In FOO: Undefined function BAR FOO ? (defun bar (x) ; now we define it (* x 2)) BAR ? (foo) ; calling foo and it uses the recent definition of BAR 6.283185307179586D0 ? (defun bar (x) ; now we redefine BAR (* x 1000)) BAR ? (foo) ; FOO now calls the new function, there is no need to recompile/link/load FOO 3141.592653589793D0 ? (type-of 'bar) ; BAR is a symbol SYMBOL ? (symbol-function 'bar) ; the symbol BAR has a function binding •
In PL/SQL and Ada When using early binding between Ada and a database-stored procedure, a timestamp is checked to verify that the
stored procedure has not changed since the code was compiled. This allows for faster executions and prevents the application from running against the wrong version of a stored procedure. When using late binding the timestamp check is not performed, and the stored procedure is executed via an anonymous
PL/SQL block. While this can be slower, it removes the need to recompile all of the client applications when a stored procedure changes. This distinction appears to be unique to PL/SQL and Ada. Other languages that can call PL/SQL procedures, as well as other database engines, only use late binding.
Dynamic linking Dynamic linking (and related dynamic linkage) is a form of late binding for importing a code
library. ==Criticism==