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7.3 — Passing arguments by reference

While pass by value is suitable in many cases, it has a couple of limitations. First, when passing a large struct or class to a function, pass by value will make a copy of the argument into the function parameter. In many cases, this is a needless performance hit, as the original argument would have sufficed. Second, when passing arguments by value, the only way to return a value back to the caller is via the function’s return value. While this is often suitable, there are cases where it would be more clear and efficient to have the function modify the argument passed in. Pass by reference solves both of these issues.

Pass by reference

To pass a variable by reference, we simply declare the function parameters as references rather than as normal variables:

When the function is called, y will become a reference to the argument. Since a reference to a variable is treated exactly the same as the variable itself, any changes made to the reference are passed through to the argument!

The following example shows this in action:

This program is the same as the one we used for the pass by value example, except foo’s parameter is now a reference instead of a normal variable. When we call foo(x), y becomes a reference to x. This snippet produces the output:

value = 5
value = 6

As you can see, the function changed the value of the argument from 5 to 6!

Here’s another example:

This produces the output:

x = 5
x = 6

Note that the value of argument x was changed by the function.

Returning multiple values via out parameters

Sometimes we need a function to return multiple values. However, functions can only have one return value. One way to return multiple values is using reference parameters:

This function takes one parameter (by value) as input, and “returns” two parameters (by reference) as output. Parameters that are only used for returning values back to the caller are called out parameters. We’ve named these out parameters with the suffix “out” to denote that they’re out parameters. This helps remind the caller that the initial value passed to these parameters doesn’t matter, and that we should expect them to be rewritten.

Let’s explore how this works in more detail. First, the main function creates local variables sin and cos. Those are passed into function getSinCos() by reference (rather than by value). This means function getSinCos() has access to the actual sin and cos variables, not just copies. getSinCos() accordingly assigns new values to sin and cos (through references sinOut and cosOut respectively), which overwrites the old values in sin and cos. Main then prints these updated values.

If sin and cos had been passed by value instead of reference, getSinCos() would have changed copies of sin and cos, leading to any changes being discarded at the end of the function. But because sin and cos were passed by reference, any changes made to sin or cos (through the references) are persisted beyond the function. We can therefore use this mechanism to return values back to the caller.

This method, while functional, has a few minor downsides. First, the syntax is a bit unnatural, with both the input and output parameters being put together in the function call. Second, the caller must pass in variables to hold the updated values, which means it must have (or create) parameters to hold these output values even if it doesn’t intend to use them. Finally, it’s not obvious from the caller’s end that sin and cos are out parameters and will be changed. This is probably the most dangerous part of this method (as it can lead to mistakes being made). Some programmers and companies feel this is a big enough problem to advise not passing by reference this way, and using pass by address instead when mixing in and out parameters (which has a clearer syntax indicating whether a parameter is modifiable or not).

Personally, we recommend avoiding mixing input and output parameters for this reason, but if you do so, good documentation on the caller’s side can help.

Limitations of pass by reference

Non-const references can only reference non-const l-values (e.g. non-const variables), so a reference parameter cannot accept an argument that is a const l-value or an r-value (e.g. literals and the results of expressions).

Pass by const reference

As mentioned in the introduction, one of the major disadvantages of pass by value is that all arguments passed by value are copied into the function parameters. When the arguments are large structs or classes, this can take a lot of time. References provide a way to avoid this penalty. When an argument is passed by reference, a reference is created to the actual argument (which takes minimal time) and no copying of values takes place. This allows us to pass large structs and classes with a minimum performance penalty.

However, this also opens us up to potential trouble. References allow the function to change the value of the argument, which is undesirable when we want an argument be read-only. If we know that a function should not change the value of an argument, but don’t want to pass by value, the best solution is to pass by const reference.

You already know that a const reference is a reference that does not allow the variable being referenced to be changed through the reference. Consequently, if we use a const reference as a parameter, we guarantee to the caller that the function will not change the argument!

The following function will produce a compiler error:

Using const is useful for several reasons:

Rule: When passing an argument by reference, always use a const references unless you need to change the value of the argument

References to pointers

It’s possible to pass a pointer by reference, and have the function change the address of the pointer entirely:

As a reminder, you can pass a C-style array by address by reference. This is useful if you need the ability for the function to change the array (e.g. for a sort function) or you need access to the array’s type information of a fixed array (to do sizeof() or a for-each loop). However, note that in order for this to work, you explicitly need to define the array size in the parameter:

This means this only works with fixed arrays of one particular length. If you want this to work with fixed arrays of any length, you can make the array length a template parameter (see chapter 13).

Pros and cons of pass by reference

Advantages of passing by reference:

Disadvantages of passing by reference:

When to use pass by reference:

When not to use pass by reference:

7.4 -- Passing arguments by address [1]
Index [2]
7.2 -- Passing arguments by value [3]