As explained in Chapter 1, Starting with C++, when your C++ program is compiled there is a pre-compilation step that collates all the file included in a C++ source file into a single file, which is then compiled. The pre-processor also expands macros and, depending on the value of symbols, includes some code and exclude others code.
In its simplest form, conditional compilation brackets code with #ifdef and #endif (and optionally using #else), so that the code between these directives is only compiled if the specified symbol has been defined.
#ifdef TEST cout << "TEST defined" << endl; #else cout << "TEST not defined" << endl; #endif
You are guaranteed that only one of these lines will be compiled, and you are ...
