C++ preprocessor

preprocessor

#include<bits/stdc++.h>
using namespace std;

#define NUM 10
#define STR "String"
#define SUM(x,y) ((x) + (y)) // macro definition by #define

//#undef NUM --> you can undef preprocessor

void main()
{
  
  cout << NUM;
  cout << STR;
  cout << SUM(2,4);
  
#if NUM <40
  cout << "under 40" <<endl;
#elif NUM > 40
  cout << "exceed 40" << endl;
#else
  cout << NUM << endl;
#endif

}

• Code Execution
  • Preprocessor > Compiler > Linker > Generate Executable file
  • the preprocessor is executed before compiling
• Preprocessing statements are defined by #.
• Do not add a ; semicolon at the end of the preprocessing statement.

define

• #define keyword : Declare content to be used as a keyword
• Used for macro
  • When declaring macro, parentheses are processed considering operator precedence

undef

• #undef : delete keywords declared with #define

#include<bits/stdc++.h>
using namespace std;

#define NUM 40

void main()
{
#ifdef NUM
  cout << NUM << "is exist" << endl;
#else 
  cout << NUM << "is not exist" << endl;
#endif
}

ifdef

• #ifdef : if keyword is defined with #define, it is true

// header.h

#ifndef _TEST_H_
#define _TEST_H_
/// test code
#endif

ifndef

• #ifndef: true if the keyword is not declared with a #define statement
  • Prevents duplicate definitions
  • avoid redefinition
  • avoid predefinition
  • EXAMPLE
  • #ifndef TEST_H : Check if the _TEST_H_ keyword is declared
  • #define TEST_H : If there is no _TEST_H_ keyword, define _TEST_H_
  • /// test code

  • #endif

  • When accessing the ///test code again later
  • Because _TEST_H_ is already defined by the condition(#ifndef TEST_H), the define statement below is not processed.

// header.h

#pragma once

pragma once

• #pragma command: It performs a special function according to the command.
• Compiler dependent
• #pragma once : Include this header file only once