What scenarios would ** be useful?

Question

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Uses for multiple levels of pointer dereferences?

I saw a question about ** (pointer to a pointer) in C here. I'd like to know whats the point of this? When should I use it?

Answer 1

This is called double indirection, meaning you have a pointer to another pointer, which in turn points to some useful data (or in rare cases yet another pointer).

There's a good article at Wikipedia that covers pointers in general and has a good section on double indirection.

Answer 2

Whenever You need a pointer to a pointer :).

For example, if You want to dynamically allocate an array of pointers, operator new or malloc will return a pointer pointing to the first pointer in the array.

Other use might be, to pass a pointer to pointer, to a function, so the function can modify the original pointer. In C++, You can pass by reference, but not in C.

Answer 3

When you have a function which wants to increment a pointer to a c-string. This is needed for things like recursive-descent parsers where each rule is responsible for incrementing things...

Answer 4

It's commonly used for out parameters that are pointers, e.g.:

bool GetSomeObject(SomeObject** object) { 
    *object = new SomeObject();
    // ... initialize object or whatever ...
}

And you would call it like thus:

SomeObject* object;
if (GetSomeObject(&object)) {
    // ... use object ...
    delete object;
}

This is a common pattern where the callee allocates and the caller frees...

Answer 5

In some variation of this:

void PutNewObjectHere(Class **dp) {
 *dp = new Class;
}

Class *p;
PutNewObjectHere(&p);
delete p;

(Note, this is a silly example for illustration. It would normally return the new pointer. The concept, however, does occasionally come up in practice)

Answer 6

In particular I've used these kind of pointers in the context of an array. For instance in my 8086 emulator project I had an array of function pointers. So it ended up looking like this:

instruction_functions[(int)opcode]();

This is also known as a jump table(sorta) and is used in a lot of places for optimization purposes.

Answer 7

You want a C function to mutate a pointer passed as argument, and it's C so there's no pass-by-reference. Therefore you pass a pointer to the pointer. Here's a favorite example adapted from Dave Hanson's C Interfaces and Implementations:

void Bit_free(struct Bit_T **set) {
    assert(set && *set);
    free(*set);
    *set = NULL;
}

By writing NULL into the freed pointer, you prevent it from dangling.