C++: To Reference or Not

Constant Something Reference or Something constant reference... I ask myself this in a similar manner as Prince Hamlet at the start of the nunnery scene... For as a C++ programmer and English speaker I've always found myself most comfortable using the phraseology thus:

const int X (0);

And to pass this to a function:

void foo (const int& p_X);

I find this most useful, we're passing a constant integer reference to the function... However, I was recently challenged that this meant something else to a reader, her input was that it meant "constant integer" reference, that is we would ONLY be able to pass "const int" instances, not "int" instances.   The thinking being:

const int X(0);

void foo(const int& p_X);

foo(X);

Would compile, whilst:

int Y(42)

void bar (const int& p_Y);

bar (Y);

Would fail to compile or at least spit out a warning because "int" was not able to be passed into the function as "constant integer" and "integer" to this reader were different types.

They're not really of course, constant is a decorator (and one which we can remove with const_cast) the aim of using "const int reference" as the type is not all of our purpose in declaring the function like so, the purpose is to communicate what we do with the value going into the function.

It is 100% not about stopping Y being passable into the function as above.

No, we want to tell any user of our code that within the functions "foo" and "bar" we do not change the value, if they pass Y to us with 42, when the function is complete Y will stil contain 42.  If the value could potentially change we would not be able to use "const" that's the purpose of passing const in this case.

Passing by reference is just to save us the time, effort and delay in allocating memory and taking a copy of the parameter before moving into the body of the function, so:

void bar (const int p_Y)

Would be the same operation as above, we tell the user we don't change the value of the parameter, but we do because we take a copy of the value being passed in and operate upon it.

The communication we get with this is very useful.

But of course, if we're using threaded programming and we pass a reference to a value at time point A, then sometime later elsewhere edit the referenced value, we may run into unexpected behaviour, so there is an argument sometimes to take a copy at the point of instantiating the child function, in the most part however, passing by reference is considered the norm.

Software Development : Get Constant in C++

What does "const" mean to you?  Does it just mean a value can not be changed?  If so, you may want to read on!

Const is one of those things, which though not unique to C++ is given more meaning when you leverage C++ fully, const allows you to not only define a value as invariant, but also to instruct other programmers coming to your code how a value should be used, how when it is passed as a parameter it should be treated and ultimately how to protect data from needless alteration, de-synchronisation or simple corruption.

Const is therefore your friend, and if you've come to C++ from C, Java, C#, Python or one of the other myriad of languages which don't treat const with as much relevance as C++ you may want to read more than I can say on the topic. 

Bjarne Stroustrop (the inventor of C++) and other authors on the topic (notably Scott Meyers & Herb Sutter) explain in much more detail than I ever could, but for brevity here are two examples of const from my own coding standard which I implore you to digest.

1. Initialise Const Values in the constructor ONLY....

Any instance of the "Ex" class can now access it's "Pi" value, and we communicate to all viewers that the value is a constant and only edited in one place.

2. Where-ever possible pass values as constant references....

We know for certain that the "p_Radius" parameter being passed is NOT to be changed by the function, this is important when you are thinking about letting code document itself, and easily accomplished with "const", and especially annoying when using a language lacking const!

Whatever you do, use const consistently, this needs no specific explanation here, however if you begin following my second rule, you must stick to it!  Don't change half way through a program, if you import a third party API which is not using const in the same way, abstract that third party interface away with a series of your own shims, to make it correct for your usage (the compiler will optimise all these layers away) to leave you with consistent and more easily maintainable code.

You can find must much more about const-correctness elsewhere, I'd start here.