Constructor (Module)
Specifies execution of a procedure before module-level code
Syntax
`
[Public | Private] Sub procedure_name [Alias "external_identifier"] [()] Constructor [priority]
{ 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, depending on the build and run-time environment of the target system:

- constructors may execute in which they are defined, or reverse order
- constructors may execute before or after global static variables having constructors
- constructors may execute before or after other module constructors having `priority` attribute
- constructors with `priority` attribute may execute before or after global static variables having constructors

The `priority` attribute, an integer between 101 and 65535, can be used to force constructors to be executed in a certain order, relative to other constructors also having `priority` attribute. 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, relative to other constructors also having `priority` attribute.
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.
Public static member procedures (a `Sub` having an empty parameter list), of user defined `Type` can be defined as a module constructor, by adding the `Constructor` keyword used in the sub procedure definition.
Initialization of static simple numeric type variables, that have a value that can be determined at compile time (for example, default zero, constants, pointers to static objects, pointers to functions, etc), are initialized before any code is executed. These values are part of the executable image and have an initial value when the executable is loaded in to memory. Trivial static globals where no code is needed to initialize, are guaranteed to be initialized and can be reliably used in all code, including global static object constructors and module constructors.
The module constructor feature exposes a low-level link-time feature of the build and run-time environment. Accessing global static objects having constructors from module constructors should be avoided due to variations in execution order on different build systems.
Warning for 64-bit compiler only: See the Identifier Rules page for the choice of user procedure identifier names (and specially the 'Platform Differences' paragraph).
Example
`
'' 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

- New to FreeBASIC

See also

- Constructor (Class)
- Destructor (Module)
- Sub