The preprocessing step occurs before the main compilation step. Historically, the preprocessor has been a separate program, but compilers are not required to implement the preprocessor in that way. Because of its history, though, the preprocessor has syntax and semantics that are quite different from the rest of C++. See Chapter 1 for information about all the steps in compiling a source file.
The preprocessor handles preprocessing directives, which can define and undefine macros, establish regions of conditional compilation, include other source files, and control the compilation process somewhat. A macro is a name that represents other text, called the macro replacement text. When the macro name is seen in the source file, the preprocessor replaces the name with the replacement text. A macro can have formal parameters, and actual arguments are substituted in the expansion.
Preprocessor directives obey different syntax rules from the rest of the language. Directives are line-oriented. Each directive starts with whitespace characters followed by # as the first non-space character on a line. After the # is more optional whitespace (no newlines are permitted) followed by the directive name. Each directive extends to the end of the line. A backslash (\) at the end of the line continues the directive onto the subsequent line.
The directive name must be one of the names listed in this chapter. Any other preprocessing token after the initial # character is an error.