1.The reserved word nil is a special constant that can be assigned to any pointer. When nil is assigned to a pointer, the pointer doesn抰 reference anything.
***
Destroys an object and frees its associated memory, if necessary.
procedure Free;
Description
Use Free to destroy an object. Free automatically calls the destructor if the object reference is not nil. Any object instantiated at runtime that does not have an owner should be destroyed by a call to Free so that it can be properly disposed of and its memory released. Unlike Destroy, Free is successful even if the object is nil; so if the object was never initialized, Free won抰 result in an error.
When you call Free for a component, it calls Free for all components that it owns梩hat is, all components in its component list. Since a form owns all the controls and nonvisual components that are created on it in design mode, those components are automatically freed when the form is freed. By default, all forms are owned by the Application object; when the application terminates, it frees the Application object, which frees all forms. For objects that are not components, or for components created with a nil owner, be sure to call Free after you are finished with them; otherwise the allocated memory will not be usable until after the application terminates.
Warning: Never explicitly free a component within one of its own event handlers or the event handler of a component it owns or contains. For example, don抰 free a button, or the form that owns the button, in its OnClick event handler.
To free a form, call its Release method, which destroys the form and releases the memory allocated for it after all its event handlers and those of the components it contains are through executing.
***
Destroys the form and frees its associated memory.
procedure Release;
Description
Use Release to destroy the form and free its associated memory.
Release does not destroy the form until all event handlers of the form and event handlers of components on the form have finished executing. Any event handlers of the form should use Release instead of Free. Failing to do so could lead to an access violation.
Note: Release returns immediately to the caller. It does not wait for the form to be freed before returning.
***
Destructor for the object instance.
destructor Destroy; override;
Description
Destroy is the destructor for the object instance.
Destroy is responsible for decrementing the value of gnInstanceCount prior to completion of the destructor. When gnInstanceCount reaches 0, the TIdStack instance in GStack is Freed and set to Nil.
Applications should not call the Destroy method, but use Free instead.
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