Type (Alias)alias

Declares an alternative name for a type

Syntax

Type typename As symbol

Parameters

Name		Description
`typename`		new alternative name.
`symbol`		symbol or data type declaration to associate with
`typename`		.

Description

symbol may refer to any declared data type including a built-in data type, Sub or Function pointer, Type declaration, Union declaration, or Enum declaration.

A Type Alias can be used to allow forward declarations of parameters in procedure declarations, but only used with pointers (whatever their passing mode), or otherwise with parameters (excluding arrays) but passed or returned only by reference. Generally the bodies of such procedures need to be implemented further in the code once the actual types are well defined (for example because of the passing of such a reference; or when such a pointer passed is then dereferenced in any form).
A Type Alias can also be used to allow forward declarations of data fields in User Defined Types, but only used with pointers.

A type alias must be used to allow declaration of pointer to a function pointer, otherwise ptr applies on return type and not on function.

A Type Alias declaration can also be nested inside a UDT structure declaration for a Type or a Union.

Remarks

Version

Since fbc 1.10.0: Nesting of a Type Alias declaration inside a structure declaring a UDT (Type or Union) is allowed.

Differences from QB

New to FreeBASIC

Example

Type ParentFwd As Parent

Type Child

    Name As ZString * 32

    ParentRef As ParentFwd Ptr

    ''...

End Type



Type Parent

    Name As ZString * 32

    ChildList(0 To 9) As Child

    ''...

End Type



Dim p As Parent

p.Name = "Foo"

With p.ChildList(0)

    .Name = "Jr."

    .ParentRef = @p

    '' ...

End With    



With p.ChildList(0)

    Print .Name; " is child of "; .parentRef->Name

End With

Function triple (ByVal i As Integer) As Integer

    Return 3 * i

End Function



Type As Function (ByVal As Integer) As Integer function_alias



'Dim As Function (Byval As Integer) As Integer f  ''this syntax works but is less readable than the next code line

Dim As function_alias f

f = @triple

Print f(123)



'Dim As Function (Byval As Integer) As Integer Ptr pf  ''this syntax does not work because Ptr applies on Integer and not on function

Dim As function_alias Ptr pf                           ''this syntax works

pf = @f

Print (*pf)(123)  ''the dereferenced pointer to procedure pointer must be enclosed in parentheses

Reference

Documented in KeyPgTypeAlias.html