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7.4 — Passing arguments by address

There is one more way to pass variables to functions, and that is by address. Passing an argument by address involves passing the address of the argument variable rather than the argument variable itself. Because the argument is an address, the function parameter must be a pointer. The function can then dereference the pointer to access or change the value being pointed to.

Here is an example of a function that takes a parameter passed by address:

The above snippet prints:

value = 5
value = 6

As you can see, the function foo() changed the value of the argument (variable value) through pointer parameter ptr.

Pass by address is typically used with pointers, which most often are used to point to built-in arrays. For example, the following function will print all the values in an array:

Here is an example program that calls this function:

This program prints the following:

6 5 4 3 2 1

Remember that fixed arrays decay into pointers when passed to a function, so we have to pass the length as a separate parameter.

It is always a good idea to ensure parameters passed by address are not null pointers before dereferencing them. Dereferencing a null pointer will typically cause the program to crash. Here is our printArray() function with a null pointer check:

Passing by const address

Because printArray() doesn’t modify any of its arguments, it’s good form to make the array parameter const:

This allows us to tell at a glance that printArray() won’t modify the array argument passed in, and will ensure we don’t do so by accident.

Addresses are actually passed by value

When you pass a pointer to a function by address, the pointer’s value (the address it points to) is copied from the argument to the function’s parameter. In other words, it’s passed by value! If you change the function parameter’s value, you are only changing a copy. Consequently, the original pointer argument will not be changed.

Here’s a sample program that illustrates this.

tempPtr receives a copy of the address that ptr is holding. Even though we change tempPtr to point at something else (nullptr), this does not change the value that ptr points to. Consequently, this program prints:

55

Note that even though the address itself is passed by value, you can still dereference that address to change the argument’s value. This is a common point of confusion, so let’s clarify:

The following program illustrates the point:

This prints:

56

Passing addresses by reference

The next logical question is, “What if we want to change the address an argument points to from within the function?”. Turns out, this is surprisingly easy. You can simply pass the address by reference. The syntax for doing a reference to a pointer is a little strange (and easy to get backwards). However, if you do get it backwards, the compiler will give you an error.

The following program illustrates using a reference to a pointer:

When we run the program again with this version of the function, we get:

5 ptr is null

Which shows that calling setToNull() did indeed change the value of ptr from &five to nullptr!

There is only pass by value

Now that you understand the basic differences between passing by reference, address, and value, let’s get reductionist for a moment. ๐Ÿ™‚

In the lesson on references [1], we briefly mentioned that references are typically implemented by the compiler as pointers. This means that behind the scenes, pass by reference is essentially just a pass by address (with access to the reference doing an implicit dereference).

And just above, we showed that pass by address is actually just passing an address by value!

Therefore, we can conclude that C++ really passes everything by value! The properties of pass by address (and reference) comes solely from the fact that we can dereference the passed address to change the argument, which we can not do with a normal value parameter!

Pros and cons of pass by address

Advantages of passing by address:

Disadvantages of passing by address:

When to use pass by address:

When not to use pass by address:

As you can see, pass by address and pass by reference have almost identical advantages and disadvantages. Because pass by reference is generally safer than pass by address, pass by reference should be preferred in most cases.

Rule: Prefer pass by reference to pass by address whenever applicable.

7.4a -- Returning values by value, reference, and address [2]
Index [3]
7.3 -- Passing arguments by reference [4]