Conditional compilation is based
on the existence and evaluation of constants, the status of compiler switches,
and the definition of conditional symbols.
Conditional symbols work like
Boolean variables: They are either defined (true) or undefined (false). Any
valid conditional symbol is treated as false until it has been defined. The
$DEFINE directive sets a specified symbol to true, and the $UNDEF directive
sets it to false. You can also define a conditional symbol by using the -D
switch with the command-line compiler or by adding the symbol to the
Conditional Defines box on the Directories/Conditionals page of the
Project|Options dialog.
The conditional directives $IFDEF,
$IFNDEF, $IF, $ELSEIF, $ELSE, $ENDIF, and $IFEND allow you to compile or
suppress code based on the status of a conditional symbol. $IF and $ELSEIF
allow you to base conditional compilation on declared Pascal identifiers.
$IFOPT compiles or suppresses code depending on whether a specified compiler
switch is enabled.
For example,
{$DEFINE DEBUG}
{$IFDEF DEBUG}
Writeln('Debug is on.'); // this code executes
{$ELSE}
Writeln('Debug is off.'); // this code does not execute
{$ENDIF}
{$UNDEF DEBUG}
{$IFNDEF DEBUG}
Writeln('Debug is off.'); // this code executes
{$ENDIF}
Conditional-directive
constructions can be nested up to 32 levels deep. For every {$IFxxx}, the
corresponding {$ENDIF} or {$IFEND} must be found within the same source file.
Conditional symbols must start
with a letter, followed by any combination of letters, digits, and underscores;
they can be of any length, but only the first 255 characters are significant.
Delphi defines the following standard conditional symbols.
VER140 Always defined, indicating that this is version 14.0 of the Object
Pascal compiler. (Each compiler version has a corresponding predefined symbol.
For example, version 10.0 has VER100 defined.)
MSWINDOWS Indicates that the operating environment is Windows. Use MSWINDOWS
to test for any flavor of the Windows platform instead of WIN32. .
WIN32 Indicates that the operating environment is the Win32 API. Use WIN32
for distinguishing between specific Windows platforms, such as 32-bit versus 64-bit
Windows. In general, don't limit code to WIN32 unless you know for sure that
the code will not work in WIN64. Use MSWINDOWS instead.
LINUX Indicates that the operating environment is Linux.
CPU386 Indicates that the CPU is an Intel 386 or better.
CONSOLE Defined if an application is being compiled as a console
application.
CONDITIONALEXPRESSIONS Tests for the use of $IF directives.
Note: An important conditional symbol that is NOT defined is LINUX. This
is defined only on the Linux version of the compiler, and can be used to
protect platform-specific code in cross-platform applications.
Conditional symbols are not Pascal
identifiers and cannot be referenced in actual program code. Similarly, Pascal
identifiers cannot be referenced in any conditional directives other than $IF
and $ELSEIF.
Note: Conditional definitions are evaluated only when source code is recompiled. If you change a conditional symbol's status and then rebuild a project, source code in unchanged units may not be recompiled. Use Project|Build All Projects to ensure everything in your project reflects the current status of conditional symbols.