On Wednesday, 22 November 2017 at 15:27:27 UTC, Dukc wrote:
> It's worth noting that you will still be passing different addresses to foo(void*) because classes are reference types in D (structs are not). In the constructor you're passing the address of the class object itself, but in the main function you're passing the address of the reference.

No, in both cases, if you do as I say, you will be passing the same address.

You cast the reference itself. Do NOT use the & operator at any point with class objects in D (unless you legit know what you are doing). Just plain cast it:

`cast(void*) this` // ok. gets address of class object itself
`Object o; foo(cast(void*) o);` // also ok, does same thing

`cast(void*) &this` // NOT OK! DO NOT DO THIS address of a local
`cast(void*) &o`    // NOT OK! DO NOT DO THIS address of a local

On Wednesday, 22 November 2017 at 15:31:36 UTC, Tim Hsu wrote:
> It seems in D, reference has its own address, am I right? unlike c++

The local variable does have its own address. Do not take its address - avoid &this or &object.

Just cast the ref itself.


In D, a class this or Object variable is already like a C++ Foo*. If you & that, you get a Foo** - not what you want in most cases.

On Wednesday, 22 November 2017 at 15:23:58 UTC, Tim Hsu wrote:
>    m_window = glfwCreateWindow();
>    glfwSetWindowUserPointer(m_window, cast(void *)(&this));

That that & out of there!

glfwSetWindowUserPointer(m_window, cast(void *)(this));

without the &, you are fine.


Then, on the other side, you cast back. Again, do NOT dereference it, just cast it:

App app = cast(App) user_ptr;

On Wednesday, 22 November 2017 at 15:31:36 UTC, Tim Hsu wrote:
> It seems in D, reference has its own address, am I right? unlike c++

In case of classes, yes. But I think function parameter references do not (or rather, of course they have since they exist in memory but it's hidden). Not sure though.

On Wednesday, 22 November 2017 at 15:36:22 UTC, Adam D. Ruppe wrote:
> On Wednesday, 22 November 2017 at 15:31:36 UTC, Tim Hsu wrote:
>> It seems in D, reference has its own address, am I right? unlike c++
>
> The local variable does have its own address. Do not take its address - avoid &this or &object.
>
> Just cast the ref itself.
>
>
> In D, a class this or Object variable is already like a C++ Foo*. If you & that, you get a Foo** - not what you want in most cases.

Thanks after doing some experiment I guess I finally understand
class App {
    @property void *ptr() {
        return cast(void *)(this);
    }
}
void printaddr(void *ptr)
{
  writeln(ptr);
}
void main()
{
    Pizza pza = new Pizza("XD");
    Pizza pza2 = pza;
    printaddr(pza.ptr);
    printaddr(pza2.ptr);
    printaddr(&pza);
    printaddr(&pza2);
}

Result:
A32000
A32000
19FDB0
19FDB4


Conclusion:
pza and pza2 is two different reference variable refer to same new-ed object.

On Wednesday, 22 November 2017 at 15:34:53 UTC, Adam D. Ruppe wrote:
> No, in both cases, if you do as I say, you will be passing the same address.

I was referring to his version of the main function that used &. But yes, if you do a cast instead it works just as you say.

On Wednesday, 22 November 2017 at 15:07:08 UTC, Tim Hsu wrote:
> I am a C++ game developer and I want to give it a try.
>
> It seems "this" in Dlang is a reference instead of pointer.
>
> How can I pass it as void *?
>
> void foo(void *);
>
> class Pizza {
> public:
>     this() {
>         Pizza newone = this;
>         // works but newone is actually not this pizza.
>         foo(&newone);
>         // this does not work..
>         foo(this);
>     }
> }
>
> void main() {
>     Pizza pizza = new Pizza();
>     // this works...
>     foo(&pizza);
> }

Note that all the examples and advice in this thread apply to _classes_, not to structs.

The 'this' in a class is a pointer, since classes are reference types (like the 'this' pointer in C++) so you can cast it to a void* directly.

The 'this' in a struct is a value, since structs are value types. So if you want the address of the struct from within one of the struct's methods you need to use the & operator (then it becomes a pointer and you can cast it to a void* .

On 11/23/17 12:57 AM, Nicholas Wilson wrote:
> On Wednesday, 22 November 2017 at 15:07:08 UTC, Tim Hsu wrote:
>> I am a C++ game developer and I want to give it a try.
>>
>> It seems "this" in Dlang is a reference instead of pointer.
>>
>> How can I pass it as void *?
>>
>> void foo(void *);
>>
>> class Pizza {
>> public:
>>     this() {
>>         Pizza newone = this;
>>         // works but newone is actually not this pizza.
>>         foo(&newone);
>>         // this does not work..
>>         foo(this);
>>     }
>> }
>>
>> void main() {
>>     Pizza pizza = new Pizza();
>>     // this works...
>>     foo(&pizza);
>> }
> 
> Note that all the examples and advice in this thread apply to _classes_, not to structs.

A further way to look at it, a class is really not the same as what you normally think as references (either ref parameter in D or & type constructor in C++). It's really simply a pointer that cannot be used with pointer math.

ref parameters:
* cannot be rebound (you can't make it point at another value)
* don't act any differently than auto-storage variables. Making a copy does not make a new reference, but rather copies the whole thing.

class references:
* CAN be rebound.
* copying to another variable does not copy the data, ever (you can't make a variable that represents the class data)

-Steve