Preprocessor directives are lines included in the code of programs preceded by a hash sign (#). These lines are not program statements but directives for the preprocessor. The preprocessor examines the code before actual compilation of code begins and resolves all these directives before any code is actually generated by regular statements.

These preprocessor directives extend only across a single line of code. As soon as a newline character is found, the preprocessor directive is ends. No semicolon (;) is expected at the end of a preprocessor directive. The only way a preprocessor directive can extend through more than one line is by preceding the newline character at the end of the line by a backslash (\).

macro definitions (#define, #undef)

#define identifier replacement

#define TABLE_SIZE 100
int table1[TABLE_SIZE];
int table2[TABLE_SIZE];

 // function macro
#include <iostream>
using namespace std;

#define getmax(a,b) ((a)>(b)?(a):(b))

int main()
{
  int x=5, y;
  y= getmax(x,2);
  cout << y << endl;
  cout << getmax(7,x) << endl;
  return 0;
}

#define TABLE_SIZE 100
int table1[TABLE_SIZE];
#undef TABLE_SIZE
#define TABLE_SIZE 200
int table2[TABLE_SIZE];

This would generate the same code as:

int table1[100];
int table2[200];

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
The operator #, followed by a parameter name, is replaced by a string literal that contains the argument passed (as if enclosed between double quotes):

#define str(x) #x
cout << str(test);

The operator ## concatenates two arguments leaving no blank spaces between them:

#define glue(a,b) a ## b
glue(c,out) << "test";

#define AA(a,b)#a###b

int main(int argc,char *argv[])
{
    qDebug()<<AA(v1,_m);//v1_m
}

Conditional inclusions (#ifdef, #ifndef, #if, #endif, #else and #elif)

#ifdef TABLE_SIZE
int table[TABLE_SIZE];
#endif  

#ifndef TABLE_SIZE
#define TABLE_SIZE 100
#endif
int table[TABLE_SIZE];

#if TABLE_SIZE>200
#undef TABLE_SIZE
#define TABLE_SIZE 200
 
#elif TABLE_SIZE<50
#undef TABLE_SIZE
#define TABLE_SIZE 50
 
#else
#undef TABLE_SIZE
#define TABLE_SIZE 100
#endif
 
int table[TABLE_SIZE];

#if defined ARRAY_SIZE
#define TABLE_SIZE ARRAY_SIZE
#elif !defined BUFFER_SIZE
#define TABLE_SIZE 128
#else
#define TABLE_SIZE BUFFER_SIZE
#endif