When to use Pointer-to-Pointer in C++?

Question

I was wondering when we use Pointer to Pointer in C++ and why we need to point to a pointer? I know that when we point to a pointer it means we are saving the memory address of a variable into the memory but I don't know why we need it? Also I have seen some examples that always the use Pointer-to-pointer in creating a Matrix! But why a Matrix may need Pointer to Pointer?

Answer 1

When you want to change the value of variable passed to a function as the function argument, and preserve updated value outside of that function, you require pointer(single pointer) to that variable.

void modify(int* p)
{
  *p = 10;
}

int main()
{
  int a = 5;
  modify(&a);
  cout << a << endl;
}

Now when you want to change the value of the pointer passed to a function as the function argument, you require pointer to a pointer.

In simple words, Use ** when you want to preserve (OR retain change in) the Memory-Allocation or Assignment even outside of a function call. (So, Pass such function with double pointer arg.)

This may not be a very good example, but will show you the basic use:

void safe_free(int** p) 
{ 
  free(*p); 
  *p = 0; 
}

int main()
{
  int* p = (int*)malloc(sizeof(int));
  cout << "p:" << p << endl;
  *p = 42;
  safe_free(&p);
  cout << "p:" << p << endl;
}

Answer 2

When to use Pointer-to-Pointer in C++?

I'd say it is better to never use it in C++. Ideally, you will only have to use it when dealing with C APIs or some legacy stuff, still related to or designed with C APIs in mind.

Pointer to pointer has pretty much been made obsolete by the C++ language features and the accompanying standard library. You have references for when you want to pass a pointer and edit the original pointer in a function, and for stuff like a pointer to an array of strings you are better off using a std::vector<std::string>. The same applies for multidimensional arrays, matrices and whatnot, C++ has a better way of dealing with those things than cryptic pointers to pointers.