A macro is a piece of code in a program that is replaced by the value of the macro. Macro is defined by #define directive. Whenever a macro name is encountered by the compiler, it replaces the name with the definition of the macro. Macro definitions need not be terminated by a semi-colon(;).
Below are the programs to illustrate the use of macros in C/C++:
Program 1:
C
#include <stdio.h>
#define LIMIT 5
int main()
{
printf("The value of LIMIT"
" is %d",
LIMIT);
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define LIMIT 5
int main()
{
cout << "The value of LIMIT"
<< " is " << LIMIT;
return 0;
}
|
OutputThe value of LIMIT is 5
Program 2:
C
#include <stdio.h>
#define AREA(l, b) (l * b)
int main()
{
int l1 = 10, l2 = 5, area;
area = AREA(l1, l2);
printf("Area of rectangle"
" is: %d",
area);
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define AREA(l, b) (l * b)
int main()
{
int l1 = 10, l2 = 5, area;
area = AREA(l1, l2);
cout << "Area of rectangle"
<< " is: "<<
area;
return 0;
}
|
OutputArea of rectangle is: 50
Explanation:
From the above program, we can see that whenever the compiler finds AREA(l, b) in the program it replaces it with the macros definition i.e., (l*b). The values passed to the macro template AREA(l, b) will also be replaced by the statement (l*b). Therefore, AREA(10, 5) will be equal to 10*5.
Types Of Macros
- Object-like Macros: An object-like macro is a simple identifier that will be replaced by a code fragment. It is called object-like because it looks like an object in code that uses it. It is popularly used to replace a symbolic name with numerical/variable represented as constant.
Below is the illustration of a simple macro:
C
#include <stdio.h>
#define DATE 31
int main()
{
printf("Lockdown will be extended"
" upto %d-MAY-2020",
DATE);
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define DATE 31
int main()
{
cout << "Lockdown will be extended"
<< " upto " << DATE
<< "-MAY-2020";
return 0;
}
|
OutputLockdown will be extended upto 31-MAY-2020
2. Chain Macros: Macros inside macros are termed as chain macros. In chain macros first of all parent macro is expanded then the child macro is expanded.
Below is the illustration of a Chain Macro:
C
#include <stdio.h>
#define INSTAGRAM FOLLOWERS
#define FOLLOWERS 138
int main()
{
printf("Geeks for Geeks have %dK"
" followers on Instagram",
INSTAGRAM);
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define INSTAGRAM FOLLOWERS
#define FOLLOWERS 138
int main()
{
cout << "Geeks for Geeks have "
<< INSTAGRAM << "K followers on Instagram!";
return 0;
}
|
OutputGeeks for Geeks have 138K followers on Instagram
Explanation:
INSTAGRAM is expanded first to produce FOLLOWERS. Then the expanded macro is expanded to produce the outcome as 138K. This is called the chaining of macros.
3. Multi-line Macros: An object-like macro could have a multi-line. So to create a multi-line macro you have to use backslash-newline.
Below is the illustration of multiline macros:
C
#include <stdio.h>
#define ELE 1, \
2, \
3
int main()
{
int arr[] = { ELE };
printf("Elements of Array are:\n");
for (int i = 0; i < 3; i++) {
printf("%d ", arr[i]);
}
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define ELE 1, \
2, \
3
int main()
{
int arr[] = { ELE };
printf("Elements of Array are:\n");
for (int i = 0; i < 3; i++) {
cout << arr[i] << ' ';
}
return 0;
}
|
OutputElements of Array are:
1 2 3
4. Function-like Macro: These macros are the same as a function call. It replaces the entire code instead of a function name. Pair of parentheses immediately after the macro name is necessary. If we put a space between the macro name and the parentheses in the macro definition then the macro will not work.
A function-like macro is only lengthened if an only if its name appears with a pair of parentheses after it. If we don’t do this, the function pointer will get the address of the real function and lead to a syntax error.
Below is the illustration of function-like macros:
C
#include <stdio.h>
#define min(a, b) (((a) < (b)) ? (a) : (b))
int main()
{
int a = 18;
int b = 76;
printf("Minimum value between"
" %d and %d is %d\n",
a, b, min(a, b));
return 0;
}
|
C++
#include <iostream>
using namespace std;
#define min(a, b) (((a) < (b)) ? (a) : (b))
int main()
{
int a = 18;
int b = 76;
cout << "Minimum value between"
<< a << " and " << b
<< " is: " << min(a, b);
return 0;
}
|
OutputMinimum value between 18 and 76 is 18
Here is one example that will give more understanding of Macros:
Problem: We need to find the area of a circle by defining AREA(r) Macros.
C++
#include <iostream>
using namespace std;
#define PI 3.1416
#define AREA(r) (PI*(r)*(r))
int main() {
float r = 7;
cout<<"Area of Circle with radius " << r <<": "<< AREA(r);
return 0;
}
|
OutputArea of Circle with radius 7: 153.938
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