- Write a program in C to implement your own sizeof operator macro.
The sizeof is a compile time operator not a standard library function. The sizeof is a unary operator which returns the size of passed variable or data type in bytes. As we know, that size of basic data types in C is system dependent, hence we use sizeof operator to dynamically determine the size of variable at run time.
Algorithm to implement your own sizeof operator.
Here we are going to do a pointer arithmetic trick to get the size of a variable. Here is the generic approach to find size of any variable without using sizeof operator:
Here we are going to do a pointer arithmetic trick to get the size of a variable. Here is the generic approach to find size of any variable without using sizeof operator:
- Suppose we want to find the size of variable 'Var' of data type D(for example an integer variable 'Var').
- Get the base address of the variable 'Var' using address of(&) operator.
- When we increment a pointer variable, it jumps K bytes ahead where K is equal to the size of the variable's data type.
- Now, ((&Var + 1) - &Var) will give the size of the variable Var of data type D.
C program to implement you own sizeof operator
#include <stdio.h>
#define new_sizeof(var) (char *)(&var+1) - (char *)(&var)
int main() {
int i;
double f;
printf("Integer size from sizeof : %d bytes\n", sizeof(i));
printf("Integer size from my_sizeof : %d bytes\n", new_sizeof(i));
printf("Float size from sizeof : %d bytes\n", sizeof(f));
printf("Float size from my_sizeof : %d bytes\n", new_sizeof(f));
return 0;
}
Output
Integer size from sizeof : 4 bytes Integer size from my_sizeof : 4 bytes Float size from sizeof : 8 bytes Float size from my_sizeof : 8 bytes