Const (Member)
 
Specifies that a member procedure is read only.

Syntax

Type typename
Declare Const ( Sub | Function | Constructor | Destructor | Property | Operator ) membername ...
End Type

Parameters

typename
Name of a user defined data type.
membername ...
Name of the member to declare or define with parameter list or return value following.

Description

Specifies that a method does not change the object it is called on. The hidden This parameter will be considered read-only. The declaration can be read as 'invoking membername promises not to change typename', and the compiler will error if the member procedure tries to change any of the data fields, or calls a non-const member procedure.

Read-only (Const) declarations are a measure of type safety that can be read as 'promises not to change.' The compiler uses the const declarations to check operations on variables and parameters and generate an error at compile time if their data could potentially change. There is no runtime overhead for using Const qualifiers since all of the checks are made at compile time.

Member procedures can not be both Const and Static since static member procedures do not have a hidden This parameter.

Since fbc version 0.24, Const is allowed (as for keyword Static) in front of member procedure bodies, not just in the declaration (the coherence with declaration is checked by compiler). When the procedure body is far from the declaration, it may be suitable to have here the information.

Example

'' Const Member Procedures

Type foo
  x As Integer
  c As Const Integer = 0
  Declare Const Sub Inspect1()
  Declare Const Sub Inspect2()
  Declare Sub Mutate1()
  Declare Sub Mutate2()
End Type

''
Sub foo.Mutate1()
  '' we can change non-const data fields
  x = 1

  '' but we still can't change const data
  '' fields, they are promised not to change
  '' c = 1 '' Compile error

End Sub

''
Sub foo.Mutate2()
  '' we can call const members
  Inspect1()

  '' and non-const members
  Mutate1()

End Sub

''
Sub foo.Inspect1()
  '' can use data members
  Dim y As Integer
  y = c + x

  '' but not change them because Inspect1()
  '' is const and promises not to change foo
  '' x = 10 '' Compile error

End Sub

''
Sub foo.Inspect2()
  '' we can call const members
  Inspect1()

  '' but not non-const members
  '' Mutate1() '' Compile error

End Sub


Differences from QB

  • New to FreeBASIC

See also