struct vs. class

[James Dennett]

Bill Baxter wrote:
> Martin wrote:
>> Gregor Richards Wrote:
>>
>>> Martin wrote:
>>>> Inspired by the recent discussion about iterator structs/classes I
>>>> wanted to ask - what's the design rationale behind having both
>>>> structs and classes? In C++ the necessity was given by backwards
>>>> compatibility but that doesn't apply to D of course. The only
>>>> principal differences I can see (I'm not a D programmer (yet), so
>>>> please correct me if I'm wrong) are a) that structs don't contain
>>>> run-time meta information (a pointer to a vtable or however that
>>>> works) and b) that struct variables are statically allocated as
>>>> opposed to class variables which always seem to be pointers into
>>>> dynamic memory.
>>>> Somehow this looks really unorthogonal to me. Especially b) I find
>>>> strange, can somebody explain the reason for this?
>>>>
>>>> cheers
>>>> Martin
>>> Firstly, your assertion about C++ is incorrect. C++ doesn't have
>>> both, classes are just structs that are private by default, structs
>>> are just classes that are public by default.
>>
>> Yes, I know that - still, formally they are two different things
>> (albeit very similar).
> 
> And you can tell they're considered different things by the annoying
> fact that if you forward declare a type with the wrong one, the C++
> compiler will generate an error.

(No, but a non-C++ compiler might...)

> -----
>    class Foobulator;
> 
>    inline void function(Foobulator& fref) {
>       . . .
>    }
> -----
>    struct Foobulator {
>       . . .
>    };
> -----
> ==> Error Foobulator was declared as a class now it's a struct.
> 
> grrr.  I don't care which it is Mr. silly compiler!  And you shouldn't
> either!  All you need to know is that I can make a pointer to the darned
> thing!

I know of only one compiler which fails to compile that, and
it's a bug.  It's perfectly valid C++ code, and has been for
a long, long time.

class Foobulator;

is *exactly* equivalent to

struct Foobulator;

in standard C++.  (Note the semi-colons on there, making
these non-definitions.)

-- James