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; }