The Music in Noise

In- and Out-Parameters - 2023-03-14

So at work I have to deal with a codebase that works with C and C++. In the part of the code that is C++ specifically I'll often find lots of reference parameters in functions. This is something great and good for objects, since passing lots of information by copy is extremely inefficient, but there is one thing that is extremely bothersome: not knowing which parameters are in-parameters and which are out-parameters.

In case you're not familiar with this concept of in- and out-parameters, this is something very common in the world of C and C++, but more from C. If you've ever done any programming (or mathematics) you probably already know what an in-parameter is: it's the parameter that serves as input for the function to do what it needs to do. But there are cases where you may want a parameter not to input something, but to output something. The idea is if you need to return an object from your function, but you also want to return an integer code if there's an error - or whatever other example you can think of, this just came to my mind. In such a case you may have something like this:

int init_obj(struct object *obj) { /* init object */ }
struct object obj;
int res = init_obj(&obj);
if (res < 0) { /* handle error */ }

In this example, we want to initialize our structure object statically, so we pass a pointer to it (&obj) so it can be modified. Therefore we cannot simply return the object, or if we did it might be costly since the compiler might try to make a copy - leave arguments about compiler optimization for another time. But also we want to know if this operation failed, and to have an error code in the case that it does (in this case the code being less than zero). Therefore, in this example, the obj parameter of the init_obj function is an out-parameter.

These are extremely useful in C and even C++. What's more, in C++ they have these wonderful things called reference parameters, which are great because you don't have to deal with the pesky pointer syntax, and you basically treat them like any other variable - the obvious downside being you may forget you're actually dealing with memory that isn't yours. I use them a lot at work, as will any good C/C++ programmer. But there's one problem: when pointers & references are used, but nowhere is it clarified whether they are in- or out-parameters.

Imagine you're reading through some code and you come across a function with a few different references in its parameters - or pointers, same thing. You look at the parameter names, but they seem completely undescriptive. You check to see if maybe you can discern something from the function name or the type definition of the parameters... No, there's nothing. You think: "Okay, maybe I can just read the function and figure this out." Only then you realize the function is some 1000+ lines long. You check to see if someone wrote a comment at the head of the function or its declaration, but no cigar, because if there's one thing programmers hate it's documenting their code (or someone else's for that matter).

There are technically three solutions to this problem... Well, three solutions you can implement in good practice to avoid someone (like yourself) falling into this problem. And they aren't mutually exclusive, you could do all three.

The first is the most obvious: document your code! Yes, I know, it's hard. I probably don't do it as often as I should either. (Who does?) But it's really the easiest thing to do. Just write a small header to the function saying what the function does, what each parameter is for, and what are the possible return values. Should only take you less than a minute.

The second is perhaps easier than the first, and if you're into the philosophy of "The Code Should Document Itself" then this is probably your way to go: prepend the keywords in_ and out_ to each reference parameter. It's simple, it's descriptive, and the next developer (or you in a week) knows what they're doing with the function.

The third and final solution, which is truly my personal favorite because it also gives optimization benefits: make all in-parameters constant. Seriously, it's C/C++ efficiency 101 that when you have a parameter that is a pointer/reference that you won't be modifying you make it const. You may think this is less effective than the previous two strategies - and at least compared to the first it is -, but considering how modern editors/IDEs work, they generally show you the kind of parameter which is required as you're filling in the function call, and even if you mess up, the linter will notice you're trying to modify a constant parameter and it will yell at you.

Well, there you go. No more excuses. Now you can continue to write good C/C++ code like a boss, but without creating headaches for some poor future developer.

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