this, self, and Me are keywords used in some computer programming languages to refer to the object, class, or other entity which the currently running code is a part of. The entity referred to thus depends on the execution context (such as which object has 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, these keywords can disambiguate variables and functions with the same name.
Object-oriented programming 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. C++ 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' or 'invocant' 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. In some languages, for example C++, Java, and Raku 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.
Subtleties and difficulties 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.
Open recursion 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 static dispatch. For example, in the following Perl code for the factorial, the token __SUB__ is a reference to the current function:
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 is resolved via dynamic dispatch so that derived classes can override it.
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. 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) rather than open recursion (dynamic dispatch), only using open recursion when it is specifically requested; external calls (not using this) would be dynamically dispatched as usual. 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 consists of the following rules:
No code invokes public methods on this. Code that can be reused internally (by invocation from other methods of the same class) is encapsulated in a 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. The previous recommendation can be relaxed for pure methods.
Implementations
C++
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. 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. C++ lets objects destroy themselves with the source code statement: delete this;. However, this is valid only for objects which are heap-allocated.
Explicit object parameters In C++23, explicit object parameters (colloquially known as "deducing this") is a feature which allows explicitly naming the implicit this parameter in a member function, allowing templates to automatically deduce the object type, qualifiers, and value category.
This has also been used to create recursive lambdas:
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