pass a pointer by reference in c++?

Question

#include <iostream>

void foo(int *&ptr) // pass pointer by reference
{
    ptr = nullptr; // this changes the actual ptr argument passed in, not a copy
}

int main()
{
    int x = 5;
    int *ptr = &x;     // create a pointer variable ptr, which is initialize with the memory address of x; that is, ptr is a pointer which is pointing to int variable x
    std::cout << "ptr is: " << (ptr ? "non-null" : "null") << '\n'; // prints non-null
    foo(ptr);
    std::cout << "ptr is: " << (ptr ? "non-null" : "null") << '\n'; // prints null

    return 0;
}

Here is how I understand it in the above code.

In the main function, firstly a local variable x is defined.
Then, a pointer variable with name ptr is defined, which is initialized with the memory address of x; i.e., ptr is a pointer variable which is pointing to the int variable x.
After that, check to see if ptr is null or not. Since it is initialized with a value, it is not-null?
After that, the function foo is called. Here, the parameter of the function int *&ptr can be understood as int* &ptr, i.e., this function foo accepts an int* (a pointer argument), and it is pass-by-reference because of the & in int* &ptr. Since it is pass-by-reference, the content of the pointer ptr is updated. So after the function call, the pointer variable ptr now has a value nullptr. That is why the very next std::cout would print null on the screen.

I hope I understand it correctly. An unrelated question: null is like nothing in C++, right? So nullptr is like a pointer which points to nothing?

`"null"` is just a string and your understanding of the code is correct. — George, Nov 19 '19 at 08:56
@George the OP is not refering to a string but certainly asks about `null` and `nullptr` — jps, Nov 19 '19 at 09:05
You may be looking for this `c` declaration type: `void foo(int **ptr){}`. — PhoenixBlue, Nov 19 '19 at 09:05

eike · Answer 1 · 2019-11-19T09:22:49.357

Your understanding of the code is correct. Sometimes it is easier to understand pointers to pointers and references of pointers, when you use aliases:

using int_ptr = int*;
void foo(int_ptr& ptr) // pass int_ptr by reference
{
    ptr = nullptr; // change the int_ptr that is referenced
}

This kind of an alias usually shouldn't be used in real code.

Regarding

"null" is like nothing in C++, right? So nullptr is like a pointer which points to nothing?

Yes, nullptr by definition does not point to an object or a function (and therefore must not be dereferenced). null as a keyword does not exist in C++. More info on null pointers in C++ here

perhaps mention that such an alias would rather add confusion in real code (in this example it is a nice way to illustrate that pointers are no special) — 463035818_is_not_an_ai, Nov 19 '19 at 09:15

Theodor Badea · Answer 2 · 2019-11-19T09:14:23.103

0

Yes, your understanding of the code is correct. Whenever you can, use analogy to simpler situations(like integers in your case) to understand things. A pointer is a variable which keeps a memory address. The null pointer concept means that a pointer points to nothing. You can find more about null concept here.

edited Nov 19 '19 at 09:14

answered Nov 19 '19 at 09:09

Theodor Badea

465
2
9

The linked SO thread does not actually explain what a null pointer is in C++. It just explains the C++11 definition of the `nullptr` keyword. – eike Nov 19 '19 at 09:11
also there is no "null" only `nullptr`. As the question is about just that, your "You can find more about null here" can be confusing – 463035818_is_not_an_ai Nov 19 '19 at 09:13
@formerlyknownas_463035818 talking about null as a concept is confusing to you? – Theodor Badea Nov 19 '19 at 09:15
your answer is confusing me, because null (value) does not exist as concept in c++ and your answer suggests the opposite – 463035818_is_not_an_ai Nov 19 '19 at 09:16

pass a pointer by reference in c++?

2 Answers2

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