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What are macros in C++ and what are the types?

Behzad Ahmad

Overview

A macro is defined as the piece of code that is replaced by the value of the macro in the program. We can define the macro by using the #define directive. Whenever a compiler encounters the macro name, it replaces it with the definition of the macro. There is no need to terminate the macro definition using a semi-colon (;).

Let’s discuss the basic example to understand this concept:

Example

#include <iostream>
using namespace std;
#define Area 10
int main()
{
  // Print the value of macro defined
  cout << "The value of AREA is :" << Area;
 
  return 0;
}
A simple macro

Explanation

  • Line 3: We define the value of the macro.

  • Lines 4–10: We write the main’s driver program. This prints the value of the defined macro.

Types of macros

There are four types of macros:

  • Chain macros

  • Object-like macros

  • Function-like macros

  • Multi-line macros

Chain macros

Chain macros are defined as the macros inside the macros. The parent macro is expanded in the first instance and then the child macro is expanded.

Example

#include <iostream>
using namespace std;
#define Vehicles CARS
#define CARS 5
int main()
{
    cout << "I have " << CARS << " Cars";
 
    return 0;
}
Chain Macros

Explanation

  • Lines 3–4: We define the parent and the child macros.

  • Line 7: Here, the vehicle is expanded to produce CARS. After this, the macro is expanded to produce the outcome.

Object-like macros

An object-like macro is defined as the simple identifier that is replaced by the code fragment. It looks like an object in code. Therefore, it is called an object-like macro.

Example

#include <iostream>
using namespace std;
#define DATE 15
int main()
{
    cout << "The deadline is " << DATE << "-MAY-2022";
    return 0;
}
Object-like macros

Explanation

  • Line 3: We define the value of the macro.

  • Lines 4–8: We are writing the main’s driver program. This prints the value of macro defined.

Function-like macros

Function-like macros work the same as the function call. It is used to replace the entire code instead of the function name.

Example

#include <iostream>
using namespace std;
#define  PI  3.1416
#define  AREA(r)  (PI*(radius)*(radius))
 
int main() {
     
    float radius = 7;    
       
    cout<<"Area of Circle with Radius " << radius <<": "<< AREA(r);
   
    return 0;
}
Function-like macros

Explanation

  • Lines 3–4: We define the function-like macros.

  • Lines 6–13: We are writing the program’s main driver. This performs the function and prints the value of the defined macro.

Multi-line macros

An object-line macro may be of multiple lines. Therefore, if we want to create a multi-line macro, we have to use the backslash-newline.

Example

#include <iostream>
using namespace std;
 #define table 2, \
            4, \
            6, \
            8, \
            10, \
            12, \
            14, \
            16, \
            18, \
            20, \
 
int main()
{
    int arr[] = { table };
     printf("The table of 2 is :\n");
 
    for (int i = 0; i < 10; i++) {
        cout << arr[i] << ' ';
    }
    return 0;
}
Multi-line macros

Explanation

  • Lines 3–12: We define the multi-line macros.

  • Lines 14–23: We make an array in which we call multi-line macros. Then, we print the elements of that array.

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