CS 70

Pointers Aren't Just for Array Elements

So far, we've used pointers as a way to refer to array elements.

But array elements aren't the only things in memory. We already know that our regular variables have addresses on the stack, so it shouldn't be a surprise that C++ provides us with a way to find out where they are.

C and C++ actually provide a way to find the memory address of any variable, via the address-of operator, &. In many ways & is the opposite of *.

  • The * operator takes a pointer (i.e., the address of a thing) and gives you the thing it points to.
  • The & operator takes a thing and gives you a pointer to it (i.e., the thing's address).
  • Duck speaking

    So if I wrote *&x they'd cancel out and it'd be the same as writing x?

  • LHS Cow speaking

    Yes.

  • Hedgehog speaking

    But please don't do that.

  • LHS Cow speaking

    Yeah, don't.

  • RHS Cow speaking

    In fact, in general, you'll almost never use the address-of operator (i.e., &var) in modern C++ code or in code you write for CS 70.

  • Bjarne speaking

    And modern C++ code uses std::address_of instead.

  • LHS Cow speaking

    Okay, whatever.

Address-Of Example

void printElephant(const Elephant* ePtr) {
    cout << "Elephant named " << ePtr->name_ << endl;
}

int main() {
    Elephant charlie{"Charlemagne", 19};

    printElephant(&charlie);
}

Notice how we wrote &charlie to get a pointer to an Elephant (i.e., an Elephant*) to pass to printElephant.

  • Goat speaking

    Wait, this is kinda clunky. Why not just use references?

  • LHS Cow speaking

    Exactly! Sure, we can do it this way, but why would we?

  • Pig speaking

    MORE ways to do things!!!

  • Bjarne speaking

    Mostly it's just C++'s C heritage. C doesn't have references so it has to use address-of to get pointers to pass into functions. But C++ has better options.

In C++ code, it makes more sense to use reference types and not use address-of to pass a pointer.

void printElephant(const Elephant& e) {
    cout << "Elephant named " << e.name_ << endl
}

int main() {
    Elephant charlie{"Charlemagne", 19};

    printElephant(charlie);
}
  • Duck speaking

    But pointers can do what references do?

  • Pig speaking

    And MORE!!

  • LHS Cow speaking

    Yes.

  • Goat speaking

    So why have references? They're more limited and we don't actually need them.

  • LHS Cow speaking

    That's what the C programmers argue.

  • Bjarne speaking

    In 1979, I created the first incarnation of C++ (called “C with Classes“), and it didn't have references. It wasn't until 1985 that I added references (and called the language C++).

References make some use cases clearer, and the code easier to read.

Two Different Meanings of &

  • Hedgehog speaking

    I'm getting a bit muddled. Does & for references have anything to do with & for address-of?

  • LHS Cow speaking

    No. Unlike * where the * represents different aspects of “pointerness”, for & the two uses only have a very loose connection to each other.

  • Bjarne speaking

    We could say that using reference types (&) helps us avoid using the address-of operator (&) on variables, and that's the connection.

  • Goat speaking

    Yeah, uh, thanks.

In sum,

In a Type Context In a Value Context
& reference to
(used a lot)		 address-of operator
(rarely used in C++)
* pointer to indirection operator

Pointers vs. References Summary

References are for a very specific use case. One where we'll never change what's being referred to.

  • Pointers are more general.
  • Pointers have their own storage in our memory model.
  • A pointer is a memory address—to get what is actually at that address you need to follow the pointer using the indirection operator (*).
  • Pointers support math, like being added to (but only do so if the pointer is pointing into an array!).

If you print out a pointer, it shows the memory address. Using the address-of operator, you can print out the memory addresses of any of your variables!

Can you print a reference?

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