DESTRUCTOR (Module)


Specifies execution of a procedure at program termination

Syntax:
[Public | Private] Sub identifier [Alias "external_identifier"] [()] Destructor [priority] [Static]
{ procedure body }
End Sub

Description:
Defines a procedure to be automatically called from a compiled program's end-code. End-code is generated by the compiler and is executed when the program terminates normally. Procedures defined as destructors may be used the same way as ordinary procedures, that is, they may be called from within module-level code, as well as other procedures.

The procedure must have an empty parameter list. A compile-time error will be generated if the Destructor keyword is used in a Sub definition having one or more parameters. In a set of overloaded procedures, only one (1) destructor may be defined because of the ambiguity of having multiple Subs which take no arguments.

In a single module, depending on the build and run-time environment of the target system:
  • destructors may execute in which they are defined, or reverse order
  • destructors may execute before or after global static variables having dstructors
  • destructors may execute before or after other module destructors having priority attribute
  • destructors with priority attribute may execute before or after global static variables having destructors

The priority attribute, an integer between 101 and 65535, can be used to force destructors to be executed in a certain order. The value of priority has no specific meaning, only the relationship of the number with other destructor priorities. 101 is the lowest priority and is executed last, relative to other destructors also having priority attribute.

A module may define multiple destructor procedures. Destructor procedures may also appear in more than one module. All procedures defined with the syntax shown above will be added to the list of procedures to be called during the program's termination.

The order in which destructors defined in multiple modules are executed is known only at link time. Therefore, special care should be taken when using destructors that may call on a secondary module also defining a destructors. In such a case it is advisable to use a single destructor that explicit calls termination procedures in multiple modules to ensure a graceful termination of the application.

Destructors will be called if the program terminates normally or if error-checking is enabled and the program terminates abnormally.

Public static member procedures (a Sub having an empty parameter list), of user defined type can be defined as a module destructor, by adding the Constructor keyword used in the sub procedure definition.

The module destructor feature exposes a low-level link-time feature of the build and run-time environment. Accessing global static objects having destructors from module destructors should be avoided due to variations in execution order on different build systems.

Examples:
Sub pauseonexit Destructor
   
    '' If the program reaches the end, or aborts with an error,
    '' it will run this destructor before closing
   
    Print "Press any key to end the program..."
    Sleep
   
End Sub

Dim array(0 To 10, 0 To 10) As Integer
Dim As Integer i = 0, j = 11

'' this next line will cause the program to abort with an
'' error if you compile with array bounds checking enabled (fbc -exx ...)
Print array(i, j)


Differences from QB:
See also:
Back to Procedures
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