Why are structs and classes so different?

[Mike Parker]

On Sunday, 15 May 2022 at 20:05:05 UTC, Kevin Bailey wrote:

>
> One question is, how should we pass objects - by value or by 
> reference? In C++, you can do either, of course, but you take 
> your chances if you pass by value - both in safety AND 
> PERFORMANCE. The bottom line is that no one passes by value, 
> even for PODs (although we may return even large objects.)

Pass struct instances by ref or by value as needed, just as you 
do in C++.

For classes, you never have direct access to the instance. Your 
class reference is a handle (a pointer) that is always passed by 
value.

>
> But I asked a different question: Why can't I put a class 
> object on the stack? What's the danger?

I answered that one. You can put a class on the stack with 
`scope`. There is no danger in that.

If you're wanting direct access to the class instance, like you 
would have with a struct, you don't have that in D. Classes are 
modeled on Java, not C++.

>
> Note that operating on that object hasn't changed. If I pass by 
> reference, it's no different than if I had created a reference.

Again, you never have direct access to the object instance. You 
always access it through the handle.

>
> One might say, Well, if D creates by value, then it has to pass 
> by value. But it doesn't; it has the 'ref' keyword.

Everything is passed by value unless the `ref` keyword is present.

>
> One might want to avoid passing by value accidentally. Ok, one 
> could have D pass class objects by reference implicitly.

How do you pass by value accidentally? By forgetting the `ref` 
keyword?

>
> I don't like things silently changing like that, so one might 
> have D forbid all but pass by 'ref' or pointer for class 
> objects.

I don't understand where you're coming from here. How can things 
silently change?

>
> I hope Ali's answer isn't the real reason. I would be sad if D 
> risked seg faults just to make class behavior "consistent".
>

Where is the risk of seg faults? Are you referring to the fact 
that class references are default initialized to null?