What is the preprocessor in C++?

The preprocessor is a directive given to the compiler before the compilation phase of the program. The compiler executes these directives before compiling the code into an executable file.

Take a look at the image below:

The image above illustrates that, after the Preprocessing phase, all preprocessor directives are replaced with C++ code in the source code file. Then, the compilation begins.

Structure of a Preprocessor directive

A preprocessor directive begins with the # character – only white-space characters can come before a preprocessor directive on a line. The directives may have additional tokenscharacters separated by spaces after them on the line. For example:

#example_directive token1 token2

Common Preprocessor directives

The following are some preprocessor directives that beginner programmers encounter.

#include Preprocessor directive

The structure of the #include directive is:

#include filename

The filename is the name of a standard library (if it is enclosed in ankle brackets <>) or a custom C++ file (if it is enclosed in double quotes "").

When the compiler encounters the #include directive, it replaces the line with the contents of the file.

#define Preprocessor directive

The #define directive is used to define Macros and Symbolic Constants. The structure of the #define directive is:

#define identifier replacement-text

In the preprocessing phase, the compiler replaces any occurrences of identifier that don’t occur as part of a string literaltext enclosed in double quotes in the source code or a commenttext in the source code that is ignored by the compiler with the replacement-text.

#error Preprocessor directive

The structure of the #error directive is:

#error tokens

Here, the tokens are characters separated by white-spaces.

In the preprocessing phase, if the compiler encounters the #error preprocessor directive, it terminates the compilation and prints the tokens as error on the standard output.

Conditional inclusion Preprocessor directives

Conditional inclusion Preprocessor directives consist of the following:

• #if
• #ifdef
• #ifndef
• #elif
• #else
• #endif

These directives instruct the compiler to conditionally include parts of the source code for compilation. The following code provides an example:

The code above outputs The value of PI is: 3.142 because #ifdef PI evaluates to true. This happens since PI is defined as 3.142 on line 4.

Note: line 10 does not get compiled because the #else clause is ignored by the compiler when #ifdef PI evaluates to true.