Type (Alias)

• Source: https://www.freebasic.net/wiki/wikka.php?wakka=KeyPgTypeAlias
• Last revised: 2022-11-17

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Declares an alternative name for a type

Syntax

` Type typename as symbol

`

Parameters

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.

Examples

start GeSHi

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

end GeSHi

start GeSHi

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

end GeSHi

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

See also

• Type...End Type
• Type (Temporary)

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