In object-oriented programming, a destructor is a method which is invoked mechanically just before the memory of the object is released. It can happen when its lifetime is bound to scope and the execution leaves the scope, when it is embedded in another object whose lifetime ends, or when it was allocated dynamically and is released explicitly. Its main purpose is to free the resources which were acquired by the object during its life and/or deregister from other entities which may keep references to it. Use of destructors is needed for the process of Resource Acquisition Is Initialization. With most kinds of automatic garbage collection algorithms, the releasing of memory may happen a long time after the object becomes unreachable, making destructors unsuitable for most purposes. In such languages, the freeing of resources is done either through a lexical construct, which is the equivalent to RAII, or explicitly by calling a function ; in particular, many object-oriented languages use the Dispose pattern.
The destructor has the same name as the class, but with a tilde before it. For example, a class called foo will have the destructor. Additionally, destructors have neither parameters nor return types. As stated above, a destructor for an object is called whenever the object's lifetime ends. If the object was created as an automatic variable, its lifetime ends and the destructor is called automatically when the object goes out of scope. Because C++ does not have garbage collection, if the object was created with a new and delete | statement, then its destructor is called when the delete | operator is applied to a pointer to the object. Usually that operation occurs within another destructor, typically the destructor of a smart pointer object. In inheritance hierarchies, the declaration of a virtual destructor in the base class ensures that the destructors of derived classes are invoked properly when an object is deleted through a pointer-to-base-class. Objects that may be deleted in this way need to inherit a virtual destructor. A destructor should never throw an exception.
Example
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class Foo ; int main
Objects which cannot be safely copied and/or assigned should be disabled from such semantics by declaring their corresponding functions as deleted within a public encapsulation level. A detailed description of this method can be found in Scott Meyers' popular book, Effective Modern C++.
The GNU Compiler Collection's C compiler comes with 2 extensions that allow implementing destructors:
The destructor function attribute allows defining global prioritized destructor functions: when main returns, these functions are called in priority order before the process terminates. See also: Hacking the art of exploitation.
The cleanup variable attribute allows attaching a destructor function to a variable: the function is called when the variable goes out of scope.
Destructors in Xojo can be in one of two forms. Each form uses a regular method declaration with a special name. The older form uses the same name as the Class with a ~ prefix. The newer form uses the name Destructor. The newer form is preferred because it makes refactoring the class easier. Class Foobar // Old form Sub ~Foobar End Sub // New form Sub Destructor End Sub End Class
