Const (Member)

Specifies that a member procedure is read only.

Syntax
   Type typename
      Declare Const Sub|Function|Property|Operator ...
   End Type

   [Const] Sub|Function|... typename ...
      ...
   End Sub|Function|...

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 a const method promises not to change the object', 
   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.

   Constructors and destructors cannot be Const (not useful).
   Member procedures can not be both Const and Static since static member 
   procedures do not have a hidden This parameter.

   For methods with Const in their declaration, Const can also be specified 
   on the corresponding method bodies, for improved code readability.

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
   * Const
   * Const (Qualifier)
   * Dim
   * Type

