dsimcha wrote:
== Quote from grauzone ([email protected])'s article
Andrei Alexandrescu wrote:
I'd really like to know why "scope x = new X();" is "unsafe", while
encouraging doing exactly the same with structs seems to be a perfectly
fine idea. Allocating structs on the stack is obviously not any safer
than with classes. I don't remember the exact reasons why you wanted to
turn "scope" into a library feature, but I think I remember something
about discouraging it for safety reasons; please forgive me is this is
wrong.
Because classes in D are always passed by pointer. (Technically references, but
really they're just pointers under the hood.) Returning a scope
(stack-allocated)
class from a function is equivalent to escaping a pointer to a stack variable.
Returning a struct is done by value, just like returning an int.
But you can't return scope classes from a function. You can't pass them
as ref parameters either. They're designed to be safe.
On the other hand, you can pass struct pointers all the way you want
around, and it's damn unsafe.
I don't get this "structs are safe because they are value types"
argument anyway, because the this pointer for structs is a
pointer/reference anyway. If it's trivial to break that "safety", can
you really call it "safety"?
Classes can't be value types because they are polymorphic, meaning their size
isn't known at compile time. C++ tries to make them value types but really,
there
is no *good* way to make a polymorphic type with size not known at compile time
a
value type.
Why do you want to add class functionality to structs to enable "RAII"
like features, when you could just use scope classes?
To me, this makes perfect sense. Classes are polymorphic, structs are value
types. Except in the "here be dragons" world of C++, the two are mutually
exclusive, which is the reason for the dichotomy in the first place. Therefore,
structs should do everything they can w/o being polymorphic and classes should
do
everything they can w/o being value types. You then decide which one you want
based on whether you need value semantics or polymorphism more.
The only place in D where this logic breaks down is monitor objects on classes.
Even here, while structs technically *could* be given monitors, this is
inefficient because they are value types, whereas the efficiency loss from
storing
a few extra bytes in a reference type is minimal in most cases.
Why do all objects have monitor pointers anyway? The idea, that every
object can act as lock, is a really bad one. But this is off-topic...
Scope is really a dangerous hack to allocate a *reference type* on the stack.
It's dangerous and kludgey, but in a performance-oriented language it's a
necessary evil.
You could say the same about structs.
