C++ Preprocessor

Preprocessor is an directives which are used to defined micros statement and reliable to includes header file in our source code. that are starts with # (hash symbol) followed by directive names. This are executed before actual compilation process and provide reliable environments.

Preprocessor is an separate process they are executed just before a compilation process. this are scan every source code file in top to bottom. and if find micros statement then it will work on relative task.

#define Preprocessor

defined preprocessor is also known as micro expansion. because that are replace single statement and group of statement in symbolic constants. see the following examples.

Replace statements

#include<iostream>
using namespace std;
#define START cout<<" Learn C++";
int main(){
  START; //execute micros
  return 0;
}

Output

Learn C++

Macros with argument

Preprocessor are capable to create function with arguments. speciality of this functions there are no need to define data type of function parameter list. see an example.

/*
  Macros with arguments
*/
#include<iostream>
using namespace std;
#define MAX(a,b) (a>b)?a:b //Macros arguments
int main(){
  int max_int=MAX(21,41);
  float max_float=MAX(100.20,50.34);
  char max_char=MAX('A','a');
  cout<<"max int :"<<max_int<<endl;
  cout<<"max float :"<<max_float<<endl;
  cout<<"max char :"<<max_char;
}

Output

max int :41
max float :100.2
max char :a

Note that in given program MAX is an macros function. that are accept two arguments. that are not initial defined that are which type of data value accept. this are capable to work on all inbuilt data type. But note that in this function are not define any return type. this are returning the largest value.

this returning value are store in result variable. assume that this value are not store by any variable. so we cannot access this return value by cout object. for example.

/*
  Macros with arguments
*/
#include<iostream>
using namespace std;
#define MAX(a,b) (a>b)?a:b //Macros arguments
int main(){
  //array store intger value
  int array[]={1,6,32,75,13,56};

  int index=1,result=0;
  
  while(index<7){
    //case 1 store value
    result=MAX(array[index-1],array[index]);
    cout<<"result :"<<result<<endl;

    //case 2 not store returnning value and 
    //try to print that values
    cout<<"try access :"<<MAX(array[index-1],array[index]);
    cout<<endl;
    index=index+2;
  }
  
}

Output

result :6
try access :0
result :75
try access :0
result :56
try access :0

Note that in normal c++ program are generate both of case are same result. In this program ostream class object are not display valid result during the run time.

macros with multi line

if including are / (forward slash) on end of micros statement . That means considered next incoming line is also part of macros. so using of forward slash can be create multi line statement.

/*
  Macros with multi line
*/
#include<iostream>
using namespace std;
#define DISPLAY(a) for(int b=1;b<=a;b++){ \
    cout<<b<<endl; \
  }
int main(){
  DISPLAY(5);
}

Output

1
2
3
4
5

Macros with constant value

/*
  Macros with constant value
*/
#include<iostream>
using namespace std;
#define PI 3.14159
int main(){
  cout<<"PI :"<<PI;
}

Output

PI :3.14159

Conditional Compilation

Conditional preprocessor macros are used to check condition statements. there are as in follows.

#ifdef : This directives are used to check given macros are exist or not in our program. if exist then executes inside instruction.

#else : This is an default and optional micros. this are used in when not satisfied the condition of #ifdef micros.

#endif : This are used to terminate #ifdef condition

#ifndef : This an special micros. this are work when given micros statement are not exist.

/*
 Example conditional micros
 #ifdef,#else,#endif,#ifndef
*/
#include<iostream>
using namespace std;
#define DISPLAY(a) cout<<a;
int main(){
  #ifdef DISPLAY //start of condition
    //statements and code here
    cout<<" DISPLAY Function exist in program."<<endl;
  #endif  //end of #ifdef

  //if and else conditional  pair
  #ifdef PI //start of condition
    //if condition is true then
    cout<<" PI are exist in this program. "<<endl;
  #else
    cout<<" Opps PI not exist in this program. "<<endl;
  #endif  //end of #ifdef PI

  //check not defined 
  #ifndef PI
    #define PI 3.14159
    cout<<" Define PI"<<endl;
  #endif  
    
  cout<<" PI :"<<PI;
  return 0; 
}

Output

 DISPLAY Function exist in program.
 Opps PI not exist in this program.
 Define PI
 PI :3.14159.

Predefined Macros

C++ are provides few standard predefined macros. there are as follows.

1) __DATE__ : This will return current date of your system. Format of date is "MM : DD : YYYY".

2) __FILE__ : This is returning the name of source file. which are create .exe file in compilation time.

3) __TIME__ : Returning the current time of your system. Time Format is "HH:MM:SS".

4) __LINE__ : returning Line number. current location of macros.

5) __STDC__ : Compiler compile ASCII standards then it will returning 1.

/*
  Example of Predefined Macros.
*/
#include<iostream>
using namespace std;

int main(){
  
  cout<<"DATE : "<<__DATE__<<endl;
  cout<<"FILE : "<<__FILE__<<endl;
  cout<<"TIME : "<<__TIME__<<endl;
  cout<<"LINE : "<<__LINE__<<endl;
  cout<<"STDC : "<<__STDC__<<endl;
  return 0; 
}

Output

DATE : Jan 29 2019
FILE : example.cpp
TIME : 03:59:47
LINE : 12
STDC : 1