Example why someone should use triple-pointers in C/C++?

Question

I'm searching for an example or explanation why someone should use OR not use triple-pointers in C/C++. Are there any source-code where triple-pointer arise?

Thanks in advance.

Edit: Especially i'm looking for a source-code which uses triple-pointers.

Answer 1

The best example that comes to mind is a sparse multi-level table. For instance one way to implement properties for Unicode characters might be:

prop_type ***proptable;
...
prop_type prop = proptable[c>>14][c>>7&0x7f][c&0x7f];

In this case proptable would need to have a triple-pointer type (and possibly quadruple pointer if the final resulting type is a pointer type). The reason for doing this as multiple levels rather than one flat table is that, at the first and second levels, multiple entries can point to the same subtable when the contents are all the same (e.g. huge CJK ranges).

Here's another example of a multi-level table that I implemented; I can't say I'm terribly proud of the design but given the constraints the code has to satisfy, it's one of the least-bad implementation choices:

http://git.musl-libc.org/cgit/musl/tree/src/aio/aio.c?id=56fbaa3bbe73f12af2bfbbcf2adb196e6f9fe264

Answer 2

If you need to return an array of pointers to variable length strings via a function parameter:

int array_of_strings(int *num_strings, char ***string_data)
{
    int n = 32;
    char **pointers = malloc(n * sizeof(*pointers));
    if (pointers == 0)
        return -1;  // Failure
    char line[256];
    int i;
    for (i = 0; i < n && fgets(line, sizeof(line), stdin) != 0; i++)
    {
        size_t len = strlen(line);
        if (line[len-1] == '\n')
            line[len-1] = '\0';
        pointers[i] = strdup(line);
        if (pointers[i] == 0)
        {
            // Release already allocated resources
            for (int j = 0; j < i; j++)
                free(pointers[j]);
            free(pointers);
            return -1;  // Failure
        }
    }
    *num_strings = i;
    *string_data = pointers;
    return 0;  // Success
}

^{Compiled code.}

Answer 3

If you use a linked list you have to store the address of the first element of the list ( first pointer ) .

If you need to change in that list you need another pointer ( two pointer)

If you need to pass your list that you are changing in two pointers and change it in another function you need another pointer ( three pointer )...

They are a lots of examples