C Preprocessors

C/C++ Preprocessors

As the name suggests Preprocessors are programs that process our source code before compilation. There are a number of steps involved between writing a program and executing a program in C / C++. Let us have a look at these steps before we actually start learning about Preprocessors.

There are 4 main types of preprocessor directives:

1. Macros
2. File Inclusion
3. Conditional Compilation
4. Other directives

Let us now learn about each of these directives in details.

• Macros: Macros are a piece of code in a program which is given some name. Whenever this name is encountered by the compiler the compiler replaces the name with the actual piece of code. The ‘#define’ directive is used to define a macro.
• File Inclusion: This type of preprocessor directive tells the compiler to include a file in the source code program. There are two types of files which can be included by the user in the program:
  1. Header File or Standard files: These files contains definition of pre-defined functions like printf(), scanf() etc. These files must be included for working with these functions. Different function are declared in different header files. For example standard I/O funuctions are in ‘iostream’ file whereas functions which perform string operations are in ‘string’ file.
     Syntax:
     

     #include< file_name >
     

     where file_name is the name of file to be included. The ‘<‘ and ‘>’ brackets tells the compiler to look for the file in standard directory.
  2. user defined files: When a program becomes very large, it is good practice to divide it into smaller files and include whenever needed. These types of files are user defined files. These files can be included as:

     #include"filename"
     

• Conditional Compilation: Conditional Compilation directives are type of directives which helps to compile a specific portion of the program or to skip compilation of some specific part of the program based on some conditions. This can be done with the help of two preprocessing commands ‘ifdef‘ and ‘endif‘.
  Syntax:

  #ifdef macro_name
      statement1;
      statement2;
      statement3;
      .
      .
      .
      statementN;
  #endif
  

  If the macro with name as ‘macroname‘ is defined then the block of statements will execute normally but if it is not defined, the compiler will simply skip this block of statements.
• Other directives: Apart from the above directives there are two more directives which are not commonly used. These are:
  1. #undef Directive: The #undef directive is used to undefine an existing macro. This directive works as:

     #undef LIMIT
     

     Using this statement will undefine the existing macro LIMIT. After this statement every “#ifdef LIMIT” statement will evaluate to false.
  2. #pragma Directive: This directive is a special purpose directive and is used to turn on or off some features. This type of directives are compiler-specific i.e., they vary from compiler to compiler. Some of the #pragma directives are discussed below: