C++ Preprocessor

Preprocessor directives are one of the unique features of C++. It provides many tools that other high-level languages do not, and programmers can use them to create efficient, easy-to-read, easy-to-modify, and portable C ++ programs.

Before the compiler compiles a C++ program, the source code is processed by the compiler. This technique is called preprocessor, and the process is called preprocessing. It is a separate program part that the C++ compiler invokes in the first part of the translation. This technique is not a part of the compiler but a separate method that comes under the compilation process. It directs the compiler that the information should be preprocessed before the actual compilation starts.

All preprocessor directives in C++ begin with #, and they do not need to end with a semicolon(;) because this is not a statement in C++.

The #define directive creates a symbolic constant, and these symbolic constants are called macro.

The general form of the directive is:

Syntax:

#define macro-name replacement-text

Simple Program of C++ Preprocessor Directive

Example:

#include<iostream> 
#define val 12

using namespace std;

int main()
{
    cout << "Value is :" << val << endl;
}

Different preprocessor directives perform different tasks. You can categorize these Preprocessor Directives as follows:

• Inclusion Directives:
  • #include: specifies the files to be included, especially header files.
• Macro Definition Directives:
  • #define: define a macro substitution.
  • #undef: It is used for undefining a macro.
• Conditional Compilation Directives:
  • #if: It tests a compile-time condition.
  • #elif
  • #endif: It specifies the end of #if.
  • #ifdef: It is used to test for macro definition.
  • #ifndef: It tests whether a macro is not defined.
  • #else: It provides an alternative option when #if fails.
• Other Directives:
  • #error
  • #line: Supplies a line number for compiler messages.
  • #pragma: It specifies implementation-defined instructions to the compiler.