Editing Late binding (section)

==Late binding implementations==
===Late binding in dynamically-typed object-oriented languages===
In most [[Type system#Dynamic type checking and runtime type information|dynamically-typed]] languages, the list of methods on an object can be altered at runtime. This requires late binding.

===Late binding in Lisp===
In [[Lisp (programming language)|Lisp]], late bound global function calls are efficiently looked up at runtime via a [[Symbol (programming)|symbol]]'s function cell. These function bindings are mutable.

Example using an interactive [[Clozure CL|Clozure Common Lisp]] session:

<syntaxhighlight lang="Lisp">
? (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
#<Compiled-function BAR #x302000D1B21F>

</syntaxhighlight>

===Late binding in C++===
In C++, late binding (also called "dynamic binding") refers to what normally happens when the <code>virtual</code> keyword is used in a method's declaration. C++ then creates a so-called [[virtual method table|virtual table]], which is a look-up table for such functions that will always be consulted when they are called.

===Late binding in COM languages===
In COM programming a late-bound method call is performed using the [[IDispatch]] interface. Some COM-based languages such as Visual Basic 6 have syntactical support for calling this interface.<ref>{{cite web|title=Using early binding and late binding in Automation|url=http://support.microsoft.com/kb/245115|publisher=Support.microsoft.com|accessdate=2011-01-15}}</ref> This is done by defining the variable's type as Object. Others such as C++ require that you explicitly call GetIDsOfNames to look up a method and Invoke to call it.

===Late binding in .NET===
In .NET, late binding refers to [[method overriding|overriding]] a <code>virtual</code> 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.

===Late binding 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 [[Dynamic dispatch#Single and multiple dispatch|single dispatch]] mechanism used with virtual methods.

Finally, Java can use late binding using its reflection APIs and [[type introspection]] much in the same way it is done in COM and .NET programming. Generally speaking those who only program in Java do not call this late binding. Likewise the use of "duck typing" techniques is frowned upon in Java programming, with abstract interfaces used instead.

Oracle, the current owner of Java, has been known to use the term late binding in the "duck typing" sense when discussing both Java and other languages in the same documentation.<ref>{{cite web|url=http://download.oracle.com/docs/cd/E12840_01/wls/docs103/jcom/comtowls.html |title=Calling into WebLogic Server from a COM Client Application |publisher=Download.oracle.com |date= |accessdate=2013-08-16}}</ref>

===Early vs. late 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.<ref>{{cite web|title=Early and Late Binding, Oracle SQL *Module for Ada Programmer's Guide|url=http://download.oracle.com/docs/cd/B19306_01/appdev.102/a58231/ch3.htm#i1007434|publisher=Download.oracle.com|accessdate=2011-01-15}}</ref>

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.