What is ** in C++?

I've seen some code, as well as some errors generated from my compiler that have a '**' token before the variable (eg **variablename unreferenced-- or something, I can't recall exactly offhand). I'm fairly certain this is related to pointers, if I had to guess it looks like it's trying to dereference twice. '**' is fairly ungoogleable. Can someone point me to a good website/documentation or would someone care to explain it here?

Thanks.

Great responses. If I can add, what would be some situations where it is useful to have a pointer to a pointer? Shouldn't you just be using the original pointer instead of creating yet another pointer to the original pointer?

Solution 1:

** is not actually only pointer to pointer (as in declaration), but is also the dereference of a dereference (in a statement).

It is used often in C which does not have the & notation for references, e.g. to update a return value which is a pointer type:

int alloc_foo(struct foo **foo_ret)
{
    *foo_ret = malloc(sizeof(struct foo));
    return 1; /* to indicate success; return value in foo_ret */
}

Solution 2:

You may recognize the signature for main():

int main(int argc, char* argv[])

The following is equivalent:

int main(int argc, char** argv)

In this case, argv is a pointer to an array of char*.

In C, the index operator [] is just another way of performing pointer arithmetic. For example,

foo[i]

produces the same code as

*(foo + i)

Solution 3:

It's not a ** token. It's simply a * token followed by another * token. In your case, you have a pointer to a pointer, and it's being dereferenced twice to get whatever's really being pointed to.