Why use double indirection? or Why use pointers to pointers?

Question

When should a double indirection be used in C? Can anyone explain with a example?

What I know is that a double indirection is a pointer to a pointer. Why would I ne

Answer 1

1. Basic Concept -

When you declare as follows : -

1. char *ch - (called character pointer)
- ch contains the address of a single character.
- (*ch) will dereference to the value of the character..

2. char **ch -
'ch' contains the address of an Array of character pointers. (as in 1)
'*ch' contains the address of a single character. (Note that it's different from 1, due to difference in declaration).
(**ch) will dereference to the exact value of the character..

Adding more pointers expand the dimension of a datatype, from character to string, to array of strings, and so on... You can relate it to a 1d, 2d, 3d matrix..

So, the usage of pointer depends upon how you declare it.

Here is a simple code..

int main()
{
    char **p;
    p = (char **)malloc(100);
    p[0] = (char *)"Apple";      // or write *p, points to location of 'A'
    p[1] = (char *)"Banana";     // or write *(p+1), points to location of 'B'

    cout << *p << endl;          //Prints the first pointer location until it finds '\0'
    cout << **p << endl;         //Prints the exact character which is being pointed
    *p++;                        //Increments for the next string
    cout << *p;
}

2. Another Application of Double Pointers -
(this would also cover pass by reference)

Suppose you want to update a character from a function. If you try the following : -

void func(char ch)
{
    ch = 'B';
}

int main()
{
    char ptr;
    ptr = 'A';
    printf("%c", ptr);

    func(ptr);
    printf("%c\n", ptr);
}

The output will be AA. This doesn't work, as you have "Passed By Value" to the function.

The correct way to do that would be -

void func( char *ptr)        //Passed by Reference
{
    *ptr = 'B';
}

int main()
{
    char *ptr;
    ptr = (char *)malloc(sizeof(char) * 1);
    *ptr = 'A';
    printf("%c\n", *ptr);

    func(ptr);
    printf("%c\n", *ptr);
}

Now extend this requirement for updating a string instead of character.
For this, you need to receive the parameter in the function as a double pointer.

void func(char **str)
{
    strcpy(str, "Second");
}

int main()
{
    char **str;
    // printf("%d\n", sizeof(char));
    *str = (char **)malloc(sizeof(char) * 10);          //Can hold 10 character pointers
    int i = 0;
    for(i=0;i<10;i++)
    {
        str = (char *)malloc(sizeof(char) * 1);         //Each pointer can point to a memory of 1 character.
    }

    strcpy(str, "First");
    printf("%s\n", str);
    func(str);
    printf("%s\n", str);
}

In this example, method expects a double pointer as a parameter to update the value of a string.