TIP:
A class usually has one destructor, called Destroy. Delphi lets you declare additional destructors, but you shouldn''''t take advantage of that feature. Declaring multiple destructors is confusing and serves no useful purpose.
Before Delphi starts the body of the destructor, it calls the virtual method, BeforeDestruction. You can override BeforeDestruction to assert program state or take care of other business that must take place before any destructor starts. This lets you write a class safely without worrying about how or whether any derived classes will call the base class destructor.
TIP:
When writing a class, you might need to override the Destroy destructor, but you must not redeclare the Free method. When freeing an object, you should call the Free method and not the destructor. The distinction is important, because Free checks whether the object reference is nil and calls Destroy only for non-nil references. In extraordinary circumstances, a class can redefine the Free method (such as TInterface in the seldom-used VirtIntf unit), which makes it that much more important to call Free, not Destroy.
If a constructor or AfterConstruction method raises an exception, Delphi automatically calls the object''''s destructor. When you write a destructor, you must remember that the object being destroyed might not have been completely constructed. Delphi ensures that all fields start out at zero, but if the exception occurs in the middle of your constructor, some fields might be initialized and some might still be zero. If the destructor just frees objects and pointers, you don''''t need to worry, because the Free method and FreeMem procedure both check for nil pointers. If the destructor calls other methods, though, always check first for a nil pointer.
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