What is ** in C++?
C++PointersC++ Problem Overview
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?
C++ Solutions
Solution 1 - C++
**
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 - C++
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 - C++
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 - C++
**
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 - C++
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 - C++
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
char
s. -
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 7 - C++
You can interpret it literally -- pointer to a pointer
Solution 8 - C++
int **var
declares a pointer to a pointer**var
references the content of a pointer, which in itself points to a pointer
Solution 9 - C++
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 - C++
Solution 11 - C++
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. :)