Conditional compilation

The C preprocessor makes it possible to conditionally compile certain blocks of code using #define and related directives. Once again, D achieves similar results using built-in compile-time statements, such as version, debug, and static if.

The version condition

A version condition is used to instruct the compiler to generate code for anything in the version block only if the specific condition is defined. Here's an example:

version(Windows)
  pragma(msg, "We are compiling on Windows.");
else version(OSX)
  pragma(msg, "We are compiling on a Mac OS X system.");
else version(Posix)
  pragma(msg, "We are compiling on a Posix system.");

This example uses the predefined versions Windows, OSX, and Posix. Swap the order of the Posix and OSX versions and the Posix block, not the OSX block, will run on Mac OS X. Remove the else statements and then both the Posix and OSX blocks will compile on Mac. Posix is defined on all POSIX systems, which...