CONSTRUCTOR (Module)


Specifies execution of a procedure before module-level code

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

Description:
The Constructor keyword is used in Sub definitions (forbidden at declaration line level) to force execution of the procedure prior to that of module-level code. Procedures defined as constructors 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 Constructor keyword is used in a Sub definition having one or more parameters. In a set of overloaded procedures, only one (1) constructor may be defined because of the ambiguity of having multiple Subs which take no arguments.

In a single module, constructors normally execute in the reverse order in which they are defined.

The priority attribute, an integer between 101 and 65535, can be used to force constructors to be executed in a certain order. The value of priority has no specific meaning, only the relationship of the number with other constructor priorities. 101 is the highest priority and is executed first. All constructors having a priority attribute are executed before constructors with no attribute. The priority value of 65535 is the same as not assigning a priority value.

A module may define multiple constructor procedures, and multiple modules may define additional constructors provided no two Public constructors share the same procedure_name.

When linking with modules that also define constructors, the order of execution is not guaranteed at link-time unless the priority attribute is used. Therefore, special care should be taken when using constructors that may call on a secondary module also defining a constructor. In such a case it is advisable to use a single constructor that explicitly calls initialization procedures in those modules.

Note: A public static member procedure (Sub having empty parameter list) can also be defined as module constructor (also with the Constructor keyword only used in Sub definition).

Examples:
'' ConDesExample.bas : An example program that defines two sets of
'' constructors and destructors. Demonstrates when and in what order
'' they are called when linking a single module.

Sub Constructor1() Constructor
    Print "Constructor1() called"
End Sub

Sub Destructor1() Destructor
    Print "Destructor1() called"
End Sub

Sub Constructor2() Constructor
    Print "Constructor2() called"
End Sub

Sub Destructor2() Destructor
    Print "Destructor2() called"
End Sub

    '' ----------------------
    Print "module-level code"

    End 0
    '' ----------------------
Output:
Constructor2() called
Constructor1() called
module-level code
Destructor1() called
Destructor2() called


Differences from QB:
See also:
Back to Procedures
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