Destructor (computer programming)

In object-oriented programming, a destructor (dtor) is a method which is automatically invoked when the object is destroyed. It can happen when its lifetime is bound to scope and the execution leaves the scope, when it is embedded in another object which lifetime ends, or when it was allocated dynamically and is released explicitly. Its main purpose is to free the resources (memory allocations, open files or sockets, database connections, resource locks, etc.) 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 (RAII).

In a language with an automatic garbage collection mechanism, it would be difficult to deterministically ensure the invocation of a destructor, and hence these languages are generally considered unsuitable for RAII. In such languages, unlinking an object from existing resources must be done by an explicit call of an appropriate function (usually called Dispose()). This method is also recommended for freeing resources, rather than using finalizers for that.

Destructor syntax

• In C++, destructors have the same name as the class with which they are associated, but with a tilde (~) prefix.
• In D, destructors are declared with name ~this() (whereas constructors are declared with this()).
• In Object Pascal, destructors have the keyword destructor and can have user-defined names, but are mostly named Destroy.
• In Objective-C, the destructor method is named dealloc.
• In Perl, the destructor method is named DESTROY; in the Moose object system extension, it is named DEMOLISH.
• In PHP 5, the destructor method is named __destruct. There were no destructors in prior versions of PHP.^[1]
• In Python, the destructor method is named __del__.^[2]
• In Swift, the destructor method is named deinit.

In C++

The destructor has the same name as the class, but with a tilde (~) before it. If the object was created as an automatic variable, its destructor is automatically called when it goes out of scope. If the object was created with a new expression, 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.^[3]

Example

#include <cstring>
#include <iostream>

class foo_t
{
	friend std::ostream & operator << ( std::ostream & os, foo_t const & foo )
	{
		os << foo.data;
		return os;
	}

private:
	char * data;
	foo_t( foo_t const & other );                // disable copy construction
	foo_t& operator = ( foo_t const & other );   // disable assignment

public:
	foo_t( void ) : data( new char[ 14 ] ) { std::strcpy( data, "Hello, World!" ); } 
	~foo_t( void ) { delete [] data; }
};
 
int main()
{
	foo_t foo;
	std::cout << foo << '\n';
}

Objects which cannot be safely copied and/or assigned should be disabled from such semantics by declaring their corresponding functions within a non-public encapsulation level (in the above example, private). A detailed description of this method can be found in Scott Meyers' popular book, Effective C++ (Item 6: "Explicitly disallow the use of compiler-generated functions you do not want."^[4]).

In C with GCC extensions

The GNU Compiler Collection's C compiler comes with 2 extensions that allow implementing destructors:

• The destructor function attribute^[5] 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.^[6]
• The cleanup variable attribute allows attaching a destructor function to a variable: the function is called when the variable goes out of scope.

Xojo

Destructors in Xojo (REALbasic) can be in one of two forms. Each form uses a regular method declaration with a special name (with no parameters and no return value). The older form uses the same name as the Class with a ~ (tilde) 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

See also

• Finalizer
• Constructor (computer science)
• Object lifetime
• Resource Acquisition Is Initialization

References

1. Constructors and Destructors, from PHP online documentation
2. https://docs.python.org/2/reference/datamodel.html#object.__del__
3. GotW #47: Uncaught exceptions Accessed 31 July 2011.
4. Scott Meyers: Effective C++, Addison-Wesley, ISBN 0-321-33487-6
5. C "destructor" function attribute
6. Erickson, Jon (2008). Hacking the art of exploitation. No Starch Press. ISBN 1-59327-144-1.