Re: Limits of implicit conversion of class arrays
On Thursday, 28 March 2024 at 01:53:52 UTC, Steven Schveighoffer
wrote:
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
Yes, it's unsafe, as you can replace an element of `ds` with
something that has no relation to `Derived`.
This is a suggested change that when _applied_ will make the code
unsafe yes. But the code in its current form is safe and the
compiler could be extended to prove it.
Re: Limits of implicit conversion of class arrays
On Monday, 25 March 2024 at 07:16:35 UTC, Per Nordlöw wrote:
On Saturday, 23 March 2024 at 11:04:04 UTC, Dmitry Olshansky
wrote:
The first and second is unsound (infamously allowed in Java).
In the general case, yes. But, do you see any errors with the
code
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
Yes, it's unsafe, as you can replace an element of `ds` with
something that has no relation to `Derived`.
Once you cast the slice you can populate it with Derived2
objects that are not Derived, hence breaking type safety of
the ds slice.
Again, in the general case, yes.
So what is different in this code example compared to the
general case? Hint: this has overlaps with a missing compiler
optimization in dmd (and many other statically typed languages)
enabled by a specific kind of data flow analysis. Which one?
If there is a way to end up with a `Derived` reference to point
at something that is not a `Derived` *without a cast* in system
code, or *even with a cast* in safe code, then it is an error. It
doesn't matter if you aren't actually doing it.
If you know you are not making that mistake, change it to system,
and cast to inform the compiler that you "know what you are
doing".
-Steve
Re: Limits of implicit conversion of class arrays
On Saturday, 23 March 2024 at 11:04:04 UTC, Dmitry Olshansky
wrote:
The first and second is unsound (infamously allowed in Java).
In the general case, yes. But, do you see any errors with the code
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
Once you cast the slice you can populate it with Derived2
objects that are not Derived, hence breaking type safety of the
ds slice.
Again, in the general case, yes.
So what is different in this code example compared to the general
case? Hint: this has overlaps with a missing compiler
optimization in dmd (and many other statically typed languages)
enabled by a specific kind of data flow analysis. Which one?
Re: Limits of implicit conversion of class arrays
On Saturday, 23 March 2024 at 11:04:04 UTC, Dmitry Olshansky
wrote:
On Saturday, 23 March 2024 at 09:08:45 UTC, Per Nordlöw wrote:
Is there a reason why
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
fails as
[1]: cannot implicitly convert expression `ds` of type
`Derived[]` to `Base[]`
[2]: cast from `Derived[]` to `Base[]` not allowed in safe code
?
The first and second is unsound (infamously allowed in Java).
Once you cast the slice you can populate it with Derived2
objects that are not Derived, hence breaking type safety of the
ds slice.
—
Dmitry Olshansky
CEO @ Glow labs
https://olshansky.me
Note that it works if the classes are const:
```d
const(Base)[] bs;
const(Derived)[] ds;
bs ~= ds; // pass
bs = ds; // pass
bs = cast(const(Base)[])ds; // pass
```
Exactly because you can't replace existing entries.
Re: Limits of implicit conversion of class arrays
On Saturday, 23 March 2024 at 09:08:45 UTC, Per Nordlöw wrote:
Is there a reason why
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
fails as
[1]: cannot implicitly convert expression `ds` of type
`Derived[]` to `Base[]`
[2]: cast from `Derived[]` to `Base[]` not allowed in safe code
?
The first and second is unsound (infamously allowed in Java).
Once you cast the slice you can populate it with Derived2 objects
that are not Derived, hence breaking type safety of the ds slice.
—
Dmitry Olshansky
CEO @ Glow labs
https://olshansky.me
Limits of implicit conversion of class arrays
Is there a reason why
```d
class Base {}
class Derived : Base {}
@safe pure nothrow unittest {
Base b;
Derived d;
b = d; // pass
Base[] bs;
Derived[] ds;
bs ~= ds; // pass
bs = ds; // fail [1], should pass
bs = cast(Base[])ds; // fail [2], should pass
}
```
fails as
[1]: cannot implicitly convert expression `ds` of type
`Derived[]` to `Base[]`
[2]: cast from `Derived[]` to `Base[]` not allowed in safe code
?
