C preprocessor
A preprocessor is a program that takes text and performs lexical conversions on it. The conversions may include macro substitution, conditional inclusion, and inclusion of other files.
The C programming language has a preprocessor that performs the following transformations:
- Replaces trigraphs with equivalents.
- Concatenates source lines.
- Replaces comments with whitespace.
- Reacts to lines starting with an octothorp (#), performing macro substitution, file inclusion, conditional inclusion, and other transformations.
The use of preprocessors has been getting less common as recent languages provide more abstract features rather than lexical-oriented ones. Indeed, the overuse of the proprecessor might yield quite chaotic code. In designing a new language based on C, Stroustrup introduced features such as inline and templates into C++ in an attempt to make the C preprocessor less relevant.
New languages proposed recently have little or no preprocessor ability. Java has no preprocessor. D, designed as a replacement of C and C++, supports features such as imports, nested functions, versioning, debug statements, etc. that help make it practical to eliminate the preprocessor entirely.
Other preprocessors include m4 and Oracle Pro*C. The m4 preprocessor is general-purpose; Oracle Pro*C converts embedded PL/SQL into C.
Preprocessing can be quite cumbersome in incremental parsing or function.
