Why do you use double star '' in initializing a variable in class?

Answer 1

The double star (**) notation is not specific to initializing a variable in a class. It is simply a double indirect reference to an object.

float myFloat; // an object
float *myFloatPtr; // a pointer to an object
float **myFloatPtrPtr; // a pointer to a pointer to an object

myFloat = 123.456; // initialize an object
myFloatPtr = &myFloat; // initialize a pointer to an object
myFloatPtrPtr = myFloatPtr; // initialize a pointer to a pointer to an object

myFloat; // refer to an object

*myFloatPtr; // refer to an object through a pointer

**myFloatPtrPtr; // refer to an object through a pointer to a pointer

*myFloatPtrPtr; // refer to the value of the pointer to the object

Double pointer notation is used where the caller intends that one of its own pointers need to be modified by a function call, so the address of the pointer, instead of the address of the object, is passed to the function.

An example might be the use of a linked list. The caller maintains a pointer to the first node. The caller invokes functions to search, add, and remove. If those operations involve adding or deleting the first node, then the caller's pointer has to change, not the .next pointer in any of the nodes, and you need the address of the pointer to do that.