Mixing pointers and references in function definition in C++

Question

I have a function that has two instances of classes as arguments:

void cookPasta(const Tomato& tomato, const Meat* meat)
{
    if (meat != nullptr)
        cookPastaWithMeat(tomato, *meat);
    else
        cookPastaWithoutMeat(tomato);
}

As the function shows, an instance of Tomato is always required, whereas Meat is optional and a nullptr can be passed instead. I do this to allow the cookPasta function to be called even if the user has never declared an instance of the Meat class.

Is it bad practice to mix references and pointers in the function signature?

It might be a bit easier on the user of your function if both arguments were pointers but then the compiler wouldn't enforce for you that the first argument is non-null. — Petr Skocik, Aug 04 '15 at 11:19
@PSkocik. That would include an additional error check in my function. Do you think that is preferable over mixing pointers and references? — Chiel, Aug 04 '15 at 11:21
_"I have a function that has two classes as arguments"_ No, you don't. — Lightness Races in Orbit, Aug 04 '15 at 12:04
@Chiel: I think it's preferable not to mix them, and not to have this function at all. Have two `cookPasta` overloads, one taking tomato and meat and the other just taking tomato. Don't use nullability for flow control. — Lightness Races in Orbit, Aug 04 '15 at 12:05
@LightnessRacesinOrbit. Thanks, what is the danger of using the null pointer for flow control? — Chiel, Aug 04 '15 at 12:08
@LightnessRacesinOrbit Thanks, that was a very helpful answer to my question. — Chiel, Aug 04 '15 at 12:51

score 30 · Accepted Answer · answered Aug 04 '15 at 11:23

30

The one thing you lose with this approach is the possibility to pass in a temporary Meat, as its address can't be taken.

Why not use overloading, by simply renaming cookPastaWithMeat and cookPastaWithoutMeat ?

void cookPasta(const Tomato& tomato, const Meat& meat);
void cookPasta(const Tomato& tomato);

answered Aug 04 '15 at 11:23

Quentin

62,093
7
131
191

14

I never trust anonymous meat: too much like a 3am kebab. But a good point. – Bathsheba Aug 04 '15 at 11:32
@Bathsheba You're a difficult cat :p – Quentin Aug 04 '15 at 11:34
What is in this case the advantage of overloading over the optional argument? – Chiel Aug 04 '15 at 11:49
@Chiel `cookPasta(tomato, Meat{})` will compile. I'd say it also looks a bit more idiomatic, but that may be personal taste. – Quentin Aug 04 '15 at 11:52
2

This is the good practice approach. – Lightness Races in Orbit Aug 04 '15 at 12:06

score 8 · Answer 2 · edited May 23 '17 at 11:59

8

Your Practice is good

You've used const keyword.
Passing reference

But, 2nd parameter pointer can be little better using optional parameter feature of C++. check out here.

void cookPasta(const Tomato& tomato, Meat* meat = nullptr)
{
    if (meat != nullptr)
        cookPastaWithMeat(tomato, *meat);
    else
        cookPastaWithoutMeat(tomato);
}

Now, Call the same function in both way.

cookPasta(tomato); // meat will default to nullptr
cookPasta(tomato, meat);

edited May 23 '17 at 11:59

Community

1
1

answered Aug 04 '15 at 11:26

Kasim Rangwala

1,765
2
23
44

8

Except you _can't_ call the same function "in both way" without breaking symmetry. You probably meant `cookPasta(tomato, &meat);`, assuming sensible types at the callsite. Do you see now why this is _not_ good practice? – Lightness Races in Orbit Aug 04 '15 at 12:06

score 4 · Answer 3 · answered Aug 04 '15 at 11:17

4

It's good practice since you have a good reason for doing so: a pointer can be nullptr whereas a reference must always be passed. You are exploiting this elegantly.

Using const means that the function cannot modify the parameters passed from the caller; that's good too.

answered Aug 04 '15 at 11:17

Bathsheba

231,907
34
361
483

3

It's hardly elegant. You're mixing levels of indirection for objects that, semantically, are both to be treated _as_ their base objects rather than as handles. Symmetry is broken and surprise is generated. – Lightness Races in Orbit Aug 04 '15 at 12:07
Symmetry is little more than an idealism irrationally desired by physicists. – Bathsheba Aug 12 '15 at 12:36

Mixing pointers and references in function definition in C++

3 Answers3

Linked