this, self, and Me are keywords used in some computer programming languages to refer to the object, class, or other entity of which the currently running code is a part. The entity referred to by these keywords thus depends on the execution context (such as which object is having its method called). Different programming languages use these keywords in slightly different ways. In languages where a keyword like "this" is mandatory, the keyword is the only way to access data and methods stored in the current object. Where optional, they can disambiguate variables and functions with the same name.
In many object-oriented programming languages, this
(also called self
or Me
) is a variable that is used in instance methods to refer to the object on which they are working. The first OO language, SIMULA 67, used this
to explicitly reference the local object.[1]:4.3.2.3C++ and languages which derive in style from it (such as Java, C#, D, and PHP) also generally use this
. Smalltalk and others, such as Object Pascal, Perl, Python, Ruby, Rust, Objective-C, DataFlex and Swift, use self
. Microsoft's Visual Basic uses Me
.
The concept is similar in all languages: this
is usually an immutable reference or pointer which refers to the current object; the current object often being the code that acts as 'parent' to the property, method, sub-routine or function that contains the this
keyword. After an object is properly constructed, or instantiated, this
is always a valid reference. Some languages require it explicitly; others use lexical scoping to use it implicitly to make symbols within their class visible. Or alternatively, the current object referred to by this
may be an independent code object that has called the function or method containing the keyword this
. Such a thing happens, for example, when a JavaScript event handler attached to an HTML tag in a web page calls a function containing the keyword this
stored in the global space outside the document object; in that context, this
will refer to the page element within the document object, not the enclosing window object.[2]
In some languages, for example C++ and Java, this
or self
is a keyword, and the variable automatically exists in instance methods. In others, for example Python, Rust, and Perl 5, the first parameter of an instance method is such a reference. It needs to be specified explicitly. In Python and Perl, the parameter need not necessarily be named this
or self
; it can be named freely by the programmer like any other parameter. However, by informal convention, the first parameter of an instance method in Perl or Python is named self
. Rust requires the self object to be called &self
or self
, depending on whether the invoked function borrows the invocant, or moves it in, respectively.
Static methods in C++ or Java are not associated with instances but classes, and so cannot use this
, because there is no object. In other languages, such as Ruby, Smalltalk, Objective-C, or Swift, the method is associated with a class object that is passed as this
, and they are called class methods. For class methods, Python uses cls
to access to the class object.
When lexical scoping is used to infer this
, the use of this
in code, while not illegal, may raise warning bells to a maintenance programmer, although there are still legitimate uses of this
in this case, such as referring to instance variables hidden by local variables of the same name, or if the method wants to return a reference to the current object, i.e. this
, itself.
In some compilers (for example GCC), pointers to C++ instance methods can be directly cast to a pointer of another type, with an explicit this
pointer parameter.[3]
The dispatch semantics of this
, namely that method calls on this
are dynamically dispatched, is known as open recursion, and means that these methods can be overridden by derived classes or objects. By contrast, direct named recursion or anonymous recursion of a function uses closed recursion, with early binding. For example, in the following Perl code for the factorial, the token __SUB__
is a reference to the current function:
use feature ":5.16";
sub {
my $x = shift;
$x == 0 ? 1 : $x * __SUB__->( $x - 1 );
}
By contrast, in C++ (using an explicit this
for clarity, though not necessary) the this
binds to the object itself, but if the class method was declared "virtual" i.e. polymorphic in the base, it's resolved via dynamic dispatch (late binding) so that derived classes can override it.
unsigned int factorial(unsigned int n)
{
if (n == 0)
return 1;
else
return n * this->factorial(n - 1);
}
This example is artificial, since this is direct recursion, so overriding the factorial
method would override this function; more natural examples are when a method in a derived class calls the same method in a base class, or in cases of mutual recursion.[4][5]
The fragile base class problem has been blamed on open recursion, with the suggestion that invoking methods on this
default to closed recursion (static dispatch, early binding) rather than open recursion (dynamic dispatch, late binding), only using open recursion when it is specifically requested; external calls (not using this
) would be dynamically dispatched as usual.[6][7] The way this is solved in practice in the JDK is through a certain programmer discipline; this discipline has been formalized by C. Ruby and G. T. Leavens; it basically consists of the following rules:[8]
public
methods on this
.protected
or private
method; if it needs to be exposed directly to the users as well, then a wrapper public
method calls the internal method.Early versions of C++ would let the this
pointer be changed; by doing so a programmer could change which object a method was working on. This feature was eventually removed, and now this
in C++ is an r-value.[9]
Early versions of C++ did not include references and it has been suggested that had they been so in C++ from the beginning, this
would have been a reference, not a pointer.[10]
C++ lets objects destroy themselves with the source code statement: delete this
.
The keyword this
in C# works the same way as in Java, for reference types. However, within C# value types, this
has quite different semantics, being similar to an ordinary mutable variable reference, and can even occur on the left side of an assignment.
One use of this
in C# is to allow reference to an outer field variable within a method that contains a local variable that has the same name. In such a situation, for example, the statement var n = localAndFieldname;
within the method will assign the type and value of the local variable localAndFieldname
to n
, whereas the statement var n = this.localAndFieldname;
will assign the type and value of the outer field variable to n
.[11]
In D this
in a class, struct or union method refers to an immutable reference of the instance of the enclosing aggregate. Classes are reference types, structs and unions are value types. In the first version of D, the keyword this
is used as a pointer to the instance of the object the method is bound to, while in D2 it has the character of an implicit ref
function argument.
In the programming language Dylan, which is an object-oriented language that supports multimethods and doesn't have a concept of this
, sending a message to an object is still kept in the syntax. The two forms below work in the same way; the differences are just syntactic sugar.
object.method(param1, param2)
and
method (object, param1, param2)
Within a class text, the current type is the type obtained from the current class. Within features (routines, commands and queries) of a class, one may use the keyword Current
to reference the current class and its features. The use of the keyword Current
is optional as the keyword Current
is implied by simply referring to the name of the current class feature openly. For example: One might have a feature `foo' in a class MY_CLASS and refer to it by:
class
MY_CLASS
feature -- Access
foo: INTEGER
my_function: INTEGER
do
Result := foo
end
end
Line #10 (above) has the implied reference to Current
by the call to simple `foo'.
Line #10 (below) has the explicit reference to Current
by the call to `Current.foo'.
class
MY_CLASS
feature -- Access
foo: INTEGER
my_function: INTEGER
do
Result := Current.foo
end
end
Either approach is acceptable to the compiler, but the implied version (e.g. x := foo
) is preferred as it is less verbose.
As with other languages, there are times when the use of the keyword Current
is mandated, such as:
class
MY_CLASS
feature -- Access
my_command
-- Create MY_OTHER_CLASS with `Current'
local
x: MY_OTHER_CLASS
do
create x.make_with_something (Current)
end
end
In the case of the code above, the call on line #11 to make_with_something is passing the current class by explicitly passing the keyword Current
.
The keyword this
is 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.[13]
In JavaScript, which is a programming or scripting language used extensively in web browsers, this
is an important keyword, although what it evaluates to depends on where it is used.
this
refers to the enclosing object, which in this case is the enclosing browser window, the window
object.this
refers to depends on how the function is called. When such a function is called directly (e.g. f(x)
), this
will refer back to the global space in which the function is defined, and in which other global functions and variables may exist as well (or in strict mode, it is undefined
). If a global function containing this
is called as part of the event handler of an element in the document object, however, this
will refer to the calling HTML element.new
keyword (e.g. var c = new Thing()
) then within Thing this
refers to the Thing object itself.obj.f(x)
), this
will refer to the object that the function is contained within.[14][15] It is even possible to manually specify this
when calling a function, by using the .call()
or .apply()
methods of the function object.[16] For example, the method call obj.f(x)
could also be written as obj.f.call(obj, x)
.To work around the different meaning of this
in nested functions such as DOM event handlers, it is a common idiom in JavaScript to save the this
reference of the calling object in a variable (commonly called that
or self
), and then use the variable to refer to the calling object in nested functions.
For example:
// In this example $ is a reference to the jQuery library
$(".element").hover(function() {
// Here, both this and that point to the element under the mouse cursor.
var that = this;
$(this).find('.elements').each(function() {
// Here, this points to the DOM element being iterated.
// However, that still points to the element under the mouse cursor.
$(this).addClass("highlight");
});
});
In Lua, self
is created as syntactic sugar when functions are defined using the :
operator.[17] When invoking a method using :
, the object being indexed will be implicitly given as the first argument to the function being invoked.
For example, the following two functions are equivalent:
local obj = {}
function obj.foo(arg1, arg2)
print(arg1, arg2) -- cannot use "self" here
end
function obj:bar(arg)
print(self, arg) -- "self" is an implicit first argument before arg
end
-- All functions can be invoked both ways, with "." or with ":"
obj:foo("Foo") -- equivalent to obj.foo(obj, "Foo")
obj.bar(obj, "Bar") -- equivalent to obj:bar("Bar")
Lua itself is not object-oriented, but when combined with another feature called metatables, the use of self
lets programmers define functions in a manner resembling object-oriented programming.
In PowerShell the special automatic variable $_
contains the current object in the pipeline object. You can use this variable in commands that perform an action on every object or on selected objects in a pipeline.[18]
"one", "two", "three" | % { write $_ }
Also starting with PowerShell 5.0, which adds a formal syntax to define classes and other user-defined types,[19]$this
variable describes the current instance of the object.
In Python, there is no keyword for this
. When a member function is called on an object, it invokes the member function with the same name on the object's class object, with the object automatically bound to the first argument of the function. Thus, the obligatory first parameter of instance methods serves as this
; this parameter is conventionally named self
, but can be named anything.
In class methods (created with the classmethod
decorator), the first argument refers to the class object itself, and is conventionally called cls
; these are primarily used for inheritable constructors,[20] where the use of the class as a parameter allows subclassing the constructor. In static methods (created with the staticmethod
decorator), no special first argument exists.
In Rust, types are declared separately from the functions associated with them. Functions designed to be analogous to instance methods in more traditionally object-oriented languages must explicitly take self
as their first parameter. These functions can then be called using instance.method()
syntax sugar. For example:
struct Foo {
bar: i32,
}
impl Foo {
fn new() -> Foo {
Foo { bar: 0, }
}
fn refer(&self) {
println!("{}", self.bar);
}
fn mutate(&mut self, baz: i32) {
self.bar = baz;
}
fn consume(self) {
self.refer();
}
}
This defines a type, Foo
, which has four associated functions. The first, Foo::new()
, is not an instance function and must be specified with the type prefix. The remaining three all take a self
parameter in a variety of ways, and can be called on a Foo
instance using the dot-notation syntax sugar, which is equivalent to calling the type-qualified function name with an explicit self
first parameter.
let foo = Foo::new(); // must called as a type-specified function
foo.refer(); // prints "0". Foo::refer() has read-only access to the foo instance
foo.mutate(5); // mutates foo in place, permitted by the &mut specification
foo.consume(); // prints "5" and destroys foo, as Foo::consume() takes full ownership of self
// equivalent to foo.refer()
Foo::refer(foo); // compilation error: foo is out of scope
The Self language is named after this use of "self".
Self
is strictly used within methods of a class.
Another way to refer to Self
is to use ::
.
By: Wikipedia.org
Edited: 2021-06-18 19:24:36
Source: Wikipedia.org