Delphi in a Nutshell

Name

$WriteableConst Compiler Directive

Syntax

{$J+}                // default
{$WriteableConst On} // default
{$J-}
{$WriteableConst Off}

Scope

Local

Description

When you declare a const with a type, Delphi allocates memory for the constant. The program can change the value of the “constant,” just as though it were a variable. The difference between a typed constant and a local variable is that a typed constant in a subroutine keeps its value across subroutine calls.

If you disable the $WriteableConst directive, you can prevent any assignments to a typed constant, thereby making the constant truly constant. There is no substitute for typed constants in a subroutine, though, so disable $WriteableConst with care. Disable only specific constants whose value you want to protect, and do not disable this directive globally in a project or an entire file.

const
  {$WriteableConst off}
  BoolNames: array[Boolean] of string = ('No way', 'As you wish');
  {$WriteableConst on}

See Also

Const Keyword