C++ supports a special embedded language that is parsed before the rest of the code is parsed. Commands (“directives”) in this embedded language always start with #. It is useful for including files, excluding code based on simple conditions, and creating little macro functions.

Preproccesor workflow

As we saw in the compiler notes, the preprocessor handles source code before it is compiled. Preprocessor directives can muck around with the file in significant ways.

Preprocessor directives

#define, #undef

#define SOME_CONSTANT 55
#define getmax(a,b) ((a) > (b) ? (a) : (b))
#define debug(x) cout << x << endl;

The key to remember about #define is whenever the new name (e.g., SOME_CONSTANT) or macro (e.g., getmax(5,2)) appears in the source, it is replaced by the right side of the #define code. So every use of SOME_CONSTANT is replaced by 55 before compilation even begins.

This replacement happens without syntax checking or type checking. It’s easy to replace with invalid code. Use #define sparingly, especially when defining macros.

#if, #else, #elif, #endif, #ifdef, #ifndef

// some code...
// some other code

You can also use #ifdef to check if something is defined (but you don’t care about the value), or #ifndef to check if it’s not defined.

#ifdef DEBUG
// some code...

Preprocessor variables can be defined on the command line, making these kind of #ifdef DEBUG directives especially useful for turning on / turning off debug code. Use the -DFOO option, where FOO is the variable to define. Use -DFOO=3 to give FOO a value.

g++ -c -DDEBUG myfile.cpp


This directive copy-pastes a specified file into the location of the #include directive. If angle-brackets are used (<somefile.h>), then the default system paths are searched for the file. If quotes are used, then the directory of the source file is searched first, then system paths. You can also give search paths on the command line with -I/some/path.

CSCI 221 material by Joshua Eckroth is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License. Source code for this website available at GitHub.