C++ * const *const

Question

I tried to search answer for this but I found it very hard to find 'exact' example of this kind. I understand very little about pointers to pointers and I feel there is something more laying under the skin of this than just pointer to something.

So how would you guys translate this?

void free(shame_1 * const * const group_1);

Am I right if I translate it as const group_1 pointer to const pointer to shame_1?

Thanks.

Answer 1

shame_1 * const * const group_1;

declares a variable named group_1, whose type is a const pointer (you cannot change where it points at) to another const pointer (same) to a shame_1-type object, whose value you can actually change.

So, for instance, you cannot compile:

group_1 = nullptr;
*group_1 = nullptr;

You can, however, do:

void f(shame_1& group) {
    //stuff that modifies group
    ...
}
f(**group1);

As zakinster commented, having a look at the spiral rule should help you understand this kind of notation.

Answer 2

Generally, just read from right to left:

void free(shame_1 * const * const group_1);

free takes a parameter called group_1 that's a const pointer to a const pointer to a shame_1 object.

• It's not relevant here, but the main hassle reading right-to-left is that some people use T const * and others use const T* and they mean the same thing, so whether you see either T const or const T you should read it as const T. In your code, only the pointers are const anyway.

For example:

shame_1 a_shame;
const shame_1* p_a_shame = &a_shame;
free(&p_a_shame);

There's very little utility in this... free() could have accepted a const pointer to a shame_1 without an extra level of indirection, and would still have been able to do all the same things to the shame_1, so it's only useful if free() has to pass the const-pointer-to-const-pointer to some other function (with ultimately there being no good reason for the whole mess).

You can visualise the memory usage / relationships as:

[a_shame]  <------ [p_a_shame]  <------  [group_1]*

• this is likely only in a CPU register, though the others may be too after optimisation.

Am I right if I translate it as const group_1 pointer to const pointer to shame_1?

So, to me the "const group_1 pointer" bit is a bit suspect... I think it reads better as "group_1 is a const pointer to..." or "const pointer (called group_1) to...".