Why do we require double and triple pointers?

[KushShukla]

Can anyone please help me with understanding double and triple pointers?

[weaknessforcats]

Check this out:

int x;

The variable x is an int.

int x, y;

The variable x is an int, the variable y is an int.

int x, y, *z;

The variable x is an int, the variable y is an int, the variable *z is an int.

int x, y, *z, **p;

The variable x is an int, the variable y is an int, the variable *z is an int, the variable **p is an int.

int x, y, *z, **p, ***q;

The variable x is an int, the variable y is an int, the variable *z is an int, the variable **p is an int, the variable *** q is an int.

So far so good?

OK. Let's take the variable z. In order for *z to be an int, the variable z itself ,must contain the address of that int. z is called a pointer. The * is called the dereference operator. So, you put the address of the int in z, then code *z to tell the compiler to go to the address in z and use the int at that address. Hence, *z is an int.

Let's suppose you have a poem in an array. Each line of a poem is a string expressed as a char array. Since a char array for a line is seen as a char* (remember the name of an array is the address of element 0), the poem becomes an array of pointers to char, or char*

char*  poem[5];   //a poem of five pointers 

Here is the important thing: The name of an array is the address of element 0. Therefore, the name "poem" is the address of a char*. Therefore, poem is a char**.

Hence, *poem is a char*. Hence *poem is line 1 of the poem. Hence *(poem +1) is line 2 of the poem, *(poem +2) is line 3 of the poem, etc...

If the poem is put into a book of poems you have an array of pointers-to-pointers-to-char, or char**:

char** book[5];    //a book of 5 poems.
 

To add the poem to the book, remember the name book is the address of book[0]. Therefore, *book is book[0]. Therefore:

*book = poem;  //add poem ADDRESS to book[0].
 

You can use the book array to display a line of the poem. Since book is the address of element 0, then *book is element 0 (the poem) and **book is line 1 of the poem.. The ADDRESS of second line of the poem would be *book +1 making the second line itself *(*book + 1).

This can be expanded to a array of poetry books:

char*** bookset[5];    //array of 5 books.
 

I will leave the rest to you to access a line of the poem using the bookset array.

[KushShukla]

bookset is the address of element 0,
*bookset = 1st book,
**bookset = 1st poem
***bookset = 1st line of the poem

Hence, **(*bookset+1) = 2nd line of the poem.
and so on....

Have I got right?

[weaknessforcats]

Looks OK by eyeball.

Here's some starter code. Run this through the compiler and see what happens.

int main()
{
 char line1[] = "Mary had a little lamb";
 char line2[] = "it's fleece was whte as snow";
 char line3[] = "She took it to Pittsburgh";
 char line4[] = "And now look at the poor thing.";
 
 //Put the lines in a poem
 
 char* poem[4];     //array of 4 char pointers
 *poem=      line1;
 *(poem+1) = line2;
*( poem +2) = line3;
 *(poem +3) = line4;
 
 //put the poem in a book
 
 char** book[5];    //an array 
 
 *book = poem;
 
 cout << **book << endl;
 cout << *(*book + 1) << endl;
}