What is ** in C++?

Question

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 someth

Answer 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 */
}

Answer 2

You can interpret it literally -- pointer to a pointer

Answer 3

I just wanted to underscore some of the uses for a pointer to a pointer. Most of these are touched on by other posts, but I thought reiteration might help.

• It allows a callee to modify a pointer owned by the caller. For example, one could pass a pointer to a pointer to the beginning of a string, and the callee could modify the pointed-to pointer to now point to a position within the string where a particular character occurs.

• Because arrays degrade to pointers (and pointers can be treated as arrays), you will often see a pointer to a pointer if you have:

  • A pointer to an array. This is a generalization of the above case, since a "string" (a C-style string, anyway) is really just an array of chars.

  • An array of pointers. You might, for example, have an array of pointers to objects, allowing for polymorphism, or an array of pointers to select objects stored in another collection.

  • An array of arrays. Again, arrays degrade to pointers, so this is a specific case of the above. This is often used for so called "jagged" arrays (as opposed to rectangular).

Answer 4

One common use is that it allows a function to set the pointer to null.
So free(pointer) frees up the memory allocated to pointer but leaves the pointer dangerously pointing at the free memory.
Instead declare a my_free(**pointer) and call my_free(&pointer) so my_free() can set the pointer to null after freeing it.

Answer 5

It's one of the allures of C++ Sigils. From my own personal experience, I can vouch faster and more efficient read-access performance using dereference operators on STL's Arrays & Vectors. I've also adopted habitual shared pointer methods if you're curious. :)

Answer 6

** is a pointer to a pointer.

It might be a matrix (an array of arrays) or an array of strings (a char array), etc.