preprocessor

A preprocessor is a program that processes input data before the main compilation or interpretation step, performing textual transformations. In programming, a source code preprocessor modifies code prior to compilation, typically by macro expansion, file inclusion, and conditional compilation.

In languages like C and C++, the preprocessor handles directives beginning with a hash symbol, such as #include to insert header files, #define to define macros, and #if, #ifdef, #else, #endif for conditional compilation. It expands macros, can perform token pasting and stringification, and combines content from multiple files. The preprocessor outputs a translation unit that the compiler then processes.

CSS and stylesheet ecosystems also employ preprocessors, such as SASS, LESS, and Stylus. These tools transform higher-level syntax into standard CSS, adding features like variables, nesting, mixins, and functions. While they are not preprocessor directives in the traditional sense, they perform a preparatory transformation step before the final styling is produced.

Other domains use preprocessors to generate code or data, often as part of build or deployment pipelines. In these contexts, preprocessors can simplify development, improve portability, and enable reuse, but they can also introduce complexity, reduce readability, or hide debugging complexity if the transformations are opaque.

Overall, a preprocessor serves as an intermediary stage that prepares source content for later stages of processing, whether for programming languages, stylesheets, or data pipelines. Understanding its features and limitations helps in writing maintainable and portable code.