What guarantees does D 'const' provide, compared to C++?

[Jonathan M Davis]

On Friday, August 17, 2012 01:35:46 Mehrdad wrote:
> > The main thing given is transitivity.
> 
> Sure, but what kind of an advantage does that provide compared to
> C++?
> (As in, a code sample would be awesome -- it's so much easier to
> explain with an example to compare, rather than with words.)

In C++, if you have

const vector<T*>& getStuff() const;

which returns a member variable, then as long as const isn't cast away, you 
know that the container itself won't have any elements added or removed from 
it, but you have _zero_ guarantees about the elements themselves. In contrast, 
in D,

const ref Array!(T*) getStuff() const;

you would _know_ that not only is the container not altered, but you know that 
the elements aren't altered either - or anything which the elements point to. 
And since casting away const and mutating a variable is undefined behavior, and 
there is no mutable, you don't have to worry about the state of the object 
changing at all unless it's shared (since another thread could alter it 
through a non-const reference) or you can somehow get another, non-const 
reference to it in the current thread. So, it becomes trivial for a class or 
struct to guarantee that nothing can change the member variables that it 
returns from const functions.

> > The compiler will guarantee you aren't changing data that is
> > const.
> 
> Right, but it's the same with C++, right? Where's the difference?

C++ makes no such guarantees, because you're free to cast away const and 
modifiy objects and because objects can have members marked with the mutable 
keyword. All const really does in C++ is make it so that you don't 
accidentally, directly modify a variable which is const. It makes _zero_ 
guarantees about the state of the object beyond that.

> > On the note of casting away const, I don't believe that is the
> > operation which is undefined, however modifying const is
> > undefined as it could be pointing to immutable data.
> 
> Oh, then that's not what I'd understood. Seems just like C++ then.

No. Casting away const and modifying an object is well-defined in C++. The 
casting is well-defined in D but not the modification. That's undefined behavior 
- both because it will do nasty things if the object is actually immutable and 
because D allows the compiler to assume that anything which is const is not 
modified by another thread if it's not shared (the _only_ thing which could 
alter that variable is another, mutable reference to the same data on the same 
thread).

> > I believe there is an article which speaks to const, but the
> > inference benefits come from immutability and pure functions.
> 
> Ahh, right. You need immutability and purity for const to be any
> more advantageous than in C++. It seems like it's a point worth
> emphasizing in the const article (and perhaps when explaining
> const to newbies), since people might not understand why const is
> powerful unless they actually start using immutability, too.
> (Also kind of prevents the situation where C++ users try to use
> const as logical const, without realizing that it's meant to work
> with immutability/purity instead.)

const is useful even without purity, because you _know_ that nothing which is 
const can change unless you access it through a non-const reference, and 
everything being thread-local by default makes it that much harder for that to 
happen, allowing the compiler to more easily guarantee that a const object 
isn't changed and do optimizations based on that.

Now, it _does_ get far better once purity is added into the mix, because then 
neither you or the compiler has to worry about function calls altering a 
variable through global variables, since they can't be accessed, making it 
easier for you to understand the code and easier for the compiler to make 
optimizations. But D's const by itself still allows for more optimizations 
than C++'s const does.

And of course, immutable offers even better guarantees, since neither you nor 
the compiler has to worry about there being mutable references to the same 
data anywhere. So, it enables even better optimizations (and combined with 
pure, it can even lead to whole function calls being elided).

While D's const can be too restrictive at times, I'm increasingly annoyed by 
how flimsy C++'s const is in comparison. In particular, without transitivity, 
putting const on a getter frequently means next to nothing.

- Jonathan M Davis