Explains what is the difference between Call by Value and Call by Reference, also specifies what is Double Pointer or multiple references?

1. Double Pointer
Call by Value and Call by reference
Pointers taking Multiple References

2. Content to be covered
• Double Pointer Concept
• Call by value
• Call by reference
• Example program to understand

3. Double Pointer Concept
• When a pointer start keeping address of another pointer.
• This is called either Double Pointer or Pointer to Pointer.
• To fetch value from a double pointer we have to use
multiple reference by using ** or ***.
• To obtain address of a pointer we can use & operator
itself.

4. Now check the following declarations:
int *a, **b, ***c;
And normal integer variable
int d=10;
The assignments
a=&d;
b=&a;
c=&b;
In above statements a is pointing to d’s address, b is pointing to a’s address
and c is pointing to b’s address.
*a gives 10
**b gives 10
***c gives 10
So a will be called single pointer, b, c is called Double Pointer.
10 1000 2000 3000
1000 2000 3000 4000
d a b c

5. Call by Value & Call by Reference
• Call by value: This is normal calling mechanism which
we have studied in functions in which the value of actual
parameters gets copied into formal parameters.
• Call by Reference: In this calling mechanism reference
(address) of actual parameters are copied to formal
parameters which are actually pointers of same data
type so that they are capable to keep addresses.

6. Difference between Call by Value & Call by Reference
• Difference between above two: The major difference between above two
is; in call by value, values are copied to local variables therefore whatever
changes you are making in them they are temporary till the execution of
that function as you come out of function they disappears while in call by
reference they remains permanent because addresses are copied to the
formal parameters and changes are made to them by using pointers at
actual memory location original data gets changed.
• Let’s have a program to be clear about call by value and call by reference.

7. #include<stdio.h>
#include<conio.h>
void change1 (int, int);
void change2 (int *, int *);
void change1 (int a, int b)
{
a=0;
b=0;
printf(“%d%d”,a,b);
}
void change2 ( int *a, int *b)
{
*a=0;
*b=0;
printf(“%d%d”,*a, *b);
}
Changes are performed in memory of a and b of change1() which are
treated as local variables so changes are temporary which will disappear as
execution of function definition will be over i.e. changes in a and b of this
function will not affect the a and b inside main() function.
Changes are performed in memory of a and b of change2() which are
treated as local variables but they are pointers and changing the values
on addresses of a and b of main() function so changes are permanent
which will remain in memory even after execution of function definition
i.e. changes in a and b of this function will affect the a and b inside
main() function.

8. void main()
{
int a,b;
a=10;
b=20;
printf(“%d%d”,a,b);
change1(a,b);
printf(“%d%d”, a,b);
change2(&a, &b);
printf(“%d%d”,a,b);
getch();
}
Output:
10 20
0 0
10 20
0 0
0 0
Passing values of actual parameters to
formal parameters therefore mechanism
is called “call by value”.
Passing address of actual parameters which
will be further copied to formal parameters
(pointer variables) in function definition so
it is call by reference (call by address).