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In object-oriented programming, a destructor (sometimes abbreviated dtor [1]) is a method which is invoked mechanically just before the memory of the object is released. [2] 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.
The rule of three (also known as the law of the big three or the big three) is a rule of thumb in C++ (prior to C++11) that claims that if a class defines any of the following then it should probably explicitly define all three: [1] destructor; copy constructor; copy assignment operator; These three functions are special member functions. If ...
Aspects of object lifetime vary between programming languages and within implementations of a language. The core concepts are relatively common, but terminology varies. For example, the concepts of create and destroy are sometimes termed construct and destruct and the language elements are termed constructor (ctor) and destructor (dtor).
The C++ language does allow a program to call a destructor directly, and, since it is not possible to destroy the object using a delete expression, that is how one destroys an object that was constructed via a pointer placement new expression.
Local variables are destroyed when the local block or function that they are declared in is closed. C++ destructors for local variables are called at the end of the object lifetime, allowing a discipline for automatic resource management termed RAII, which is widely used in C++. Member variables are created when the parent object is created.
Reference counting in C++ is usually implemented using "smart pointers" [19] whose constructors, destructors, and assignment operators manage the references. A smart pointer can be passed by reference to a function, which avoids the need to copy-construct a new smart pointer (which would increase the reference count on entry into the function ...
In C++, objects are created on the stack when the constructor is invoked without the new operator, and created on the heap when the constructor is invoked with the new operator. Stack objects are deleted implicitly when they go out of scope, while heap objects must be deleted implicitly by a destructor or explicitly by using the delete operator.
Also note the virtual destructors in the base classes, B1 and B2. They are necessary to ensure delete d can free up memory not just for D, but also for B1 and B2, if d is a pointer or reference to the types B1 or B2. They were excluded from the memory layouts to keep the example simple. [nb 2]