What is Call by Value method?

In this parameter passing method, values of actual parameters are copied to function's formal parameters, and the parameters are stored in different memory locations. So any changes made inside functions are not reflected in actual parameters of the caller.

What is Call by Reference method?

In this method, the memory allocation is the same as the actual parameters. All the operation in the function are performed on the value stored at the address of the actual parameter, and the modified value will be stored at the same address.

Example of call by value method

#include <stdio.h>
int increment(int var)
{
    var = var+1;
    return var;
}

int main()
{
   int num1=20;
   int num2 = increment(num1);
   printf("num1 value is: %d", num1);
   printf("\nnum2 value is: %d", num2);

   return 0;
}

Output

num1 value is: 20
num2 value is: 21

Example of a Call by Reference method

#include <stdio.h>
void increment(int  *var)
{
    /* Although we are performing the increment on variable
     * var, however the var is a pointer that holds the address
     * of variable num, which means the increment is actually done
     * on the address where value of num is stored.
     */
    *var = *var+1;
}
int main()
{
     int num=20;
     /* This way of calling the function is known as call by
      * reference. Instead of passing the variable num, we are
      * passing the address of variable num
      */
     increment(&num);
     printf("Value of num is: %d", num);
   return 0;
}

Output

Value of num is: 21

Call by Value vs. Call by Reference

Advantages of using Call by value method

1. The method doesn't change the original variable, so it is preserving data.
2. Whenever a function is called it, never affect the actual contents of the actual arguments.
3. Value of actual arguments passed to the formal arguments, so any changes made in the formal argument does not affect the real cases.

Advantages of using Call by reference method

1. The function can change the value of the argument, which is quite useful.
2. It does not create duplicate data for holding only one value which helps you to save memory space.
3. In this method, there is no copy of the argument made. Therefore it is processed very fast.
4. A person reading the code never knows that the value can be modified in the function.

Disadvantages of using Call by value method

1. Changes to actual parameters can also modify corresponding argument variables
2. In this method, arguments must be variables.
3. You can't directly change a variable in a function body.
4. Sometime argument can be complex expressions
5. There are two copies created for the same variable which is not memory efficient.

Disadvantages of using Call by reference method

1. Strong non-null guarantee. A function taking in a reference need to make sure that the input is non-null. Therefore, null check need not be made.
2. Passing by reference makes the function not pure theoretically.
3. A lifetime guarantee is a big issue with references. This is specifically dangerous when working with lambdas and multi-threaded programs.