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digitalmars.D.learn - Casting a class variable to void*

reply Manuel Maier <mjmaier gmx.de> writes:
I'm not sure whether I've found a bug or if I found some very 
strange but intended behavior. If it is indeed intended behavior, 
I'd like to know the rationale behind it because this one 
surprised me a lot when I found it out after 16+ hours of 
debugging...

Have a look at this code snippet:

```
import std.stdio;

struct Data {}

class Foo {
   Data data;
   alias data this;
}

class Bar {}

class Baz {
   int data = 0xDeadBeef;
   alias data this;
}

void main() {
   auto foo = new Foo();
   //auto rawFoo = cast(void*)foo; // Doesn't compile
   auto rawFoo = *cast(void**)&foo; // This works...
   auto bar = new Bar();
   auto rawBar = cast(void*)bar; // This works, as expected.
   auto baz = new Baz();
   auto rawBaz = cast(void*)baz; // Compiles, but...

   writefln("foo: 0x%0.8x", rawFoo);
   writefln("bar: 0x%0.8x", rawBar);
   writefln("baz: 0x%0.8x", rawBaz);
}
```

Here's the output:
```
foo: 0x02341010
bar: 0x02341020
baz: 0xdeadbeef
```

So, when I cast a class variable to void* I expect to get the 
pointer to the class' content, no matter what the class' content 
may be. However, when the class is implicitly convertible to 
something else via `alias this` it tries to cast that instead of 
trying the class itself first.

I understand that a class variable is supposed to act as the 
class type itself, not a pointer/reference to it, so when I ask 
that class variable for something that the underlying class 
doesn't have, the `alias this` is consulted. But in the case 
above, the class variable itself is perfectly convertible to a 
void* already, so why would it have to go through `alias this` 
first? Seems like precedence is messed up for classes and `alias 
this`.

So, is this a bug, or is it intended behavior? In case of the 
latter, I'd gladly add it to the documentation.

By the way, I'm using DMD v2.070.2
Apr 10 2016
parent ag0aep6g <anonymous example.com> writes:
On 11.04.2016 00:06, Manuel Maier wrote:

 So, is this a bug, or is it intended behavior? In case of the latter,
 I'd gladly add it to the documentation.


Looks like a bug to me. And it's already been filed:

https://issues.dlang.org/show_bug.cgi?id=13392
Apr 10 2016