Linked list head double pointer passing

Question

I have seen this in some book/ tutorial.

When you pass in the head pointer (of linked list) into a function, you need to pass it as a double pointer.

For eg:

Answer 1

This is very C-like code, not C++.

Basically, when something is passed by-value the function operates on a copy of the data:

void foo(int i)
{
    i = 5; // copy is set to 5
}

int x = 7;
foo(x);
// x is still 7

In C, you instead pass a pointer to the variable, and can change it that way:

void foo(int* i)
{
    *i = 5; // whatever i points to is set to 5
}

int x = 7;
foo(&x);
// x is 5

For you, instead of an int it's a digit*. (Resulting in a pointer to pointer.)

---

In C++, references were introduced. A reference is an alias to another object. So you'd do something like this:

void foo(int& i) // i is an alias to another value
{
    i = 5; // x is set to 5
}

int x = 7;
foo(x); // pass x as alias, not address of x.
// x is 5

A reference is generally preferred, since it enforces that you actually refer to an object, and simplifies both calling and operating code.

Of course in C++ you wouldn't implement a list yourself, you'd use std::list.