A Detailed Overview of Macros in C++

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Introduction

A macro is described as the section of code that the macro's value replaces in a program. Using the #define directive, we can define the macro. A compiler substitutes the macro name with the macro definition whenever it comes across it. There is no requirement to use a semicolon to end the macro definition (;). Let's talk about the simple illustration to comprehend this idea:

Example

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

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Explanation

We specify the macro's value. We create the driver software for the main. This displays the defined macro's value.

Types of macros

Four different types of macros exist:

  1. Chain macros
  2. Object-like macros
  3. Function-like macros
  4. Multi-line macros

Chain macros

The definition of a chain macro is a macro inside a macro. First, the child macro is expanded, and then the parent macro is expanded.

Example

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

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Explanation

The parent and child macros are defined. The vehicle is enlarged in this case to create CARS. The macro is then enlarged to create the result.

Object-like macros

A basic identifier that is substituted by a code snippet is what is referred to as an object-like macro. In programming, it resembles an object. As a result, it is referred to as an object-like macro.

Example

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

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Explanation

We specify the macro's value. The driver code for the main is being written. This displays the macro's specified value.

Function-like macros

Function calls and function-like macros have the same functionality. Instead of just changing the function name, the complete code is changed.

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);
}

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Explanation

The function-like macros are defined. The primary driver of the software is being written. This executes the function and outputs the defined macro's value.

Multi-line macros

Multiple lines can make up an object-line macro. Consequently, we must use backslash-newline to generate multi-line macros.

Example

#include <iostream>
using namespace std;
 #define table 2, \
            4, \
            6, \
            8, \
            10, \
            12, \
            14, \
            16, \

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Explanation

The multi-line macros are defined. Multi-line macros are called inside of an array that we create. Then, we print the array's elements.

Predefined Macros in C++

There are several predefined macros in C++, including the following:

  1. __DATE__: Current date in the following format - MMM DD YYYY.
  2. __TIME__: The time is shown as HH:MM:SS.
  3. __FILE__ : Current filename
  4. __LINE__: Deconotes current line number

Conclusion

In this article you understood macros, its 4 types and some predefined macros provided by the C++ libraries along with examples and code snippets.