This (computer science)

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In many object-oriented programming languages, this (or self) is a keyword which can be used in instance methods to refer to the object on which the currently executing method has been invoked. C++, and programming languages which derive from it (such as Java and C#) generally use the keyword this, whereas Smalltalk and its progeny prefer the keyword self. The Self programming language is named after this use of "self".

In all of the above languages, this/self is an immutable reference (a pointer in C++) which refers to the current object. It is generally guaranteed to be a valid reference (it is never the null pointer). Some languages require its use to refer to the current object; other languages uses lexical scoping to make symbols defined within the corresponding class definition visible, making use of this unnecessary in some instances.

Behind the scenes, this/self becomes an extra implicit argument to the instance method. For example, the following instance method in C++

int foo::print (bar x)

is essentially equivalent to the following function in non-object-oriented programming:

int foo_print (foo *const this, bar x)

In some languages without special object-oriented syntax, for example Perl 5; this is made explicit, in that the first argument of an instance method is a reference to the instance, and needs to be handled explicitly. In some compilers (e.g. GCC), pointers to C++ instance methods can be directly cast into a pointer to the appropriate non-object-oriented function with an explicit this pointer argument.[1]

Class methods (a.k.a. static methods) are not associated with an object, and therefore cannot use a this/self keyword.

In Dylan, an OO language which supports multimethods and thus doesn't have a concept of this, the notion of sending a message to an object is nonetheless retained in the syntax. The following two forms are equivalent; the former is deemed to be syntactic sugar for the latter:

object.method(argument1, argument2)

and

method (object, argument1, argument2)

Contents 1 this and C++ 2 this and Java 3 this and C# 4 See also 5 External links

[edit] this and C++

Early dialects of C++ were unusual in that they permitted the this pointer to be modified; in this way a method could "switch" which object it was working on. This feature was eventually deprecated, and this in C++ may not be rebound. (Early versions of C++ did not include references; had references been included in the language it is likely that this would have been of reference rather than pointer type--as the semantics of this in C++ far more resemble references than pointers).

C++ does permit objects to "commit suicide" with the construct delete this; this operation, when performed, invalidates the object (and the this pointer). Some C++ authors now consider "delete this" to be a harmful construct and advise against its use.

Some implementations of C++ allow a non-virtual instance member function to be called on a NULL pointer. Some examples can be found in MFC:

HWND CWnd::GetSafeHwnd()
{ return ( (this==NULL) ? NULL : this->m_hwnd); }

The ISO C++ Standard states that the behaviour of such code is undefined, though.

[edit] this and Java

A Java language keyword that represents the current instance of the class in which it appears. It is used to access class variables and methods.

Since all instance methods are virtual in Java, this can never be null.

[edit] this and C#

this in C# works the same way as in Java.

[edit] See also

• Self-reference

[edit] External links

• The Design and Evolution of C++ by Bjarne Stroustrup - Addison-Wesley Pub Co; 1st edition (March 29, 1994); ISBN 0-201-54330-3
• More Effective C++: 35 New Ways to Improve Your Programs and Designs by Scott Meyers -- (1995) ISBN 0-201-63371-X