c++ - What does a pointer to a pointer mean? -

i have seen c++ programs pointer pointer variable i.e **i. mean , why used. cant use single pointer instead of that. difference between single pointer , pointer pointer. please explain each step. thank you.

variables take space store. space taken memory. suppose stack (memory) starts @ 0x12 34 56 78and have integer a value 4:

int = 4; 

your memory might like:

0x12 34 56 78: 0x00 00 00 04 (a) 

now suppose have pointer a:

int = 4; int* p = &a; 

your memory like:

0x12 34 56 78: 0x00 00 00 04 (a) 0x12 34 56 7c: 0x12 34 56 78 (p) 

now suppose have pointer p:

int = 4; int* p = &a; int** q = &p; 

your memory like:

0x12 34 56 78: 0x00 00 00 04 (a) 0x12 34 56 7c: 0x12 34 56 78 (p) 0x12 34 56 80: 0x12 34 56 7c (q) 

you can q p a following addresses. pointers layer of indirection: specify is, not is.