Macros

A macro is a piece of code that is replace by its value before getting compiled to machine code.

Types of macros:

1. Object like macros

2. Chained Macros

3. Multi-line Macros

4. Function-like Macros

Object like Macros

Used to replace symbolic names with its value.

#include <bits/stdc++.h>
#define PI 3.141
using namespace std;

int main() {
    int r;
    cin >> r;
    cout << "Area of circle: " << PI * r * r;
    return 0;
}

Chained Macros

A macro definition that contains another macro

#include <bits/stdc++.h>
#define PI 3.141
#define PIINVERSE (1 / PI)
using namespace std;

int main() {
    cout << "PI inverse is: " << PIINVERSE;
    return 0;
}

Multi-Line Macro

A macro definiton that spans multiple lines. A \ is used before ending a line.

#include <bits/stdc++.h>
#define elements 1,\
                2,\
                3
using namespace std;

int main() {
    vector<int> arr = {elements};
    for(int x: arr) 
        cout << x << ' ';
    return 0;
}

Funciton-like Macros

Basic single line funcitons can be implemented using a macro. This will be faster than creating a funciton as there will be no funciton call and save time and space creating entries in the call stack.

#include <bits/stdc++.h>
#define min(a, b) (a < b ? a : b)

using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << min(a, b) << " is smaller";
    return 0;
}

Advantages of using a Macro

• Reduces code length

  • use #define PB push_back and v.PB()

  • #define sort(v) sort(v.begin(), v.end()) reduces sort to sort(arr)

• Avoid magic number

  • which are random constants that programs use that only make sense to them

    • Loop runs for 25 times. Why? Is it because alphabets are from 0 - 26? some other reason? Who knows.

      • #define alphabets 25.better. more explicit.

• Faster than varibale as no extra memory is being created. No read time, write time or access time.

• Saves spaces.

References

• Memory leak in C++ and How to avoid it?. (2019). Retrieved 24 May 2022, from https://iq.opengenus.org/memory-leak-in-cpp-and-how-to-avoid-it/

• Macros and its types in C/C++ - GeeksforGeeks. (2020). Retrieved 24 May 2022, from https://www.geeksforgeeks.org/macros-and-its-types-in-c-cpp/