To what use is multiple indirection in C++?

Question

Under what circumstances might you want to use multiple indirection (that is, a chain of pointers as in Foo **) in C++?

Answer 1

Most common usage as @aku pointed out is to allow a change to a pointer parameter to be visible after the function returns.

#include <iostream>

using namespace std;

struct Foo {
    int a;
};

void CreateFoo(Foo** p) {
    *p = new Foo();
    (*p)->a = 12;
}

int main(int argc, char* argv[])
{
    Foo* p = NULL;
    CreateFoo(&p);
    cout << p->a << endl;
    delete p;
    return 0;
}

This will print

12

But there are several other useful usages as in the following example to iterate an array of strings and print them to the standard output.

#include <iostream>

using namespace std;

int main(int argc, char* argv[])
{
    const char* words[] = { "first", "second", NULL };
    for (const char** p = words; *p != NULL; ++p) {
        cout << *p << endl;
    }

    return 0;
}