Tutorial :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 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.


Solution:4

** is a pointer to a pointer.

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


Solution:5

It's a double dereference.

int i = 3;  int* ptr_to_i = &i;  int** ptr_to_ptr_to_i = &ptr_to_i;    std::cout << **ptr_to_ptr_to_i << std::endl;  

Prints 3.


Solution:6

You can interpret it literally -- pointer to a pointer


Solution:7

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).


Solution:8

  • int **var declares a pointer to a pointer
  • **var references the content of a pointer, which in itself points to a pointer


Solution:9

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.


Solution:10

See http://www.c2.com/cgi/wiki?ThreeStarProgrammer


Solution:11

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. :)


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