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digitalmars.D.learn - Initializing a class pointer

reply "Tyro[a.c.edwards]" <nospam home.com> writes:
class Class{}

void main()
{
Class myClass;
Class* pClass0 = &myClass;  // OK

Class* pClass1 = new Class; // Error: cannot implicitly convert     [8]
                             // expression (new Class) of type t.Class
                             // to test.Class*
	
Class* pClass2 = &(new Class);
                             // Error: new Class is not an lvalue    [12]
	
Class mClass = &(new Class);// Error: cannot implicitly convert     [14]
	                    // expression (&new Class) of type Class*
	                    // to test.Class
}

C++ uses the process on line [8] above to initialize a class pointer. 
Obviously it does not work in D. But given the error message at [14], I 
thought [12] would have been allowed. What is the proper way to convert 
[8] to D?

Thanks
Feb 26 2011
next sibling parent reply "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Tyro[a.c.edwards] <nospam home.com> wrote:


 class Class{}

 void main()
 {
 Class myClass;
 Class* pClass0 = &myClass;  // OK

 Class* pClass1 = new Class; // Error: cannot implicitly convert     [8]
                              // expression (new Class) of type t.Class
                              // to test.Class*
 	
 Class* pClass2 = &(new Class);
                              // Error: new Class is not an lvalue    [12]
 	
 Class mClass = &(new Class);// Error: cannot implicitly convert     [14]
 	                    // expression (&new Class) of type Class*
 	                    // to test.Class
 }

 C++ uses the process on line [8] above to initialize a class pointer.  
 Obviously it does not work in D. But given the error message at [14], I  
 thought [12] would have been allowed. What is the proper way to convert  
 [8] to D?



Classes in D are already references (like Class& in C++), thus line [8]
would be a pointer to a reference to a class, something which may make
some kind of sense, but is unlikely to be what you want.

Perhaps this question is better answered if you explain why you want a
pointer to a class?


-- 
Simen
Feb 26 2011
parent reply "Tyro[a.c.edwards]" <nospam home.com> writes:
On 2/27/2011 8:10 AM, Simen kjaeraas wrote:

 Tyro[a.c.edwards] <nospam home.com> wrote:


 class Class{}

 void main()
 {
 Class myClass;
 Class* pClass0 = &myClass; // OK

 Class* pClass1 = new Class; // Error: cannot implicitly convert [8]
 // expression (new Class) of type t.Class
 // to test.Class*

 Class* pClass2 = &(new Class);
 // Error: new Class is not an lvalue [12]

 Class mClass = &(new Class);// Error: cannot implicitly convert [14]
 // expression (&new Class) of type Class*
 // to test.Class
 }

 C++ uses the process on line [8] above to initialize a class pointer.
 Obviously it does not work in D. But given the error message at [14],
 I thought [12] would have been allowed. What is the proper way to
 convert [8] to D?



 Classes in D are already references (like Class& in C++), thus line [8]
 would be a pointer to a reference to a class, something which may make
 some kind of sense, but is unlikely to be what you want.

 Perhaps this question is better answered if you explain why you want a
 pointer to a class?



I'm trying to convert some c++ code that defines

T func(par...)
{	
   Controller * pCtrl = WinGetLong<Controller *> (hwnd);
   .
   .
   .
   switch(msg)
   {
   case FirstMatch:
      pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *> 
(lParam));
      break;
   }
}

I'm not sure why I need a pointer to the class, just trying to figure it 
out.
Feb 26 2011
parent reply "Tyro[a.c.edwards]" <nospam home.com> writes:
On 2/27/2011 8:52 AM, Simen kjaeraas wrote:

 Tyro[a.c.edwards] <nospam home.com> wrote:


 I'm trying to convert some c++ code that defines

 T func(par...)
 {
 Controller * pCtrl = WinGetLong<Controller *> (hwnd);
 .
 .
 .
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *> (lParam));
 break;
 }
 }

 I'm not sure why I need a pointer to the class, just trying to figure
 it out.



 Ah. You would not need a pointer to the class in D. Instead, your function
 would look something like this:

 T funct(par...)
 {
 auto pCtrl = WinGetLong!Controller(hwnd);
 ...
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller(hWnd, cast(CREATESTRUCT*)lParam);
 break;
 }
 }

 C++ classes are in some ways more akin to D structs, in that:

 class A {};

 void foo(){
 A bar;
 }

 bar would be allocated on the stack in C++, while in D bar would be a
 pointer to a class instance on the heap. (well, it would be null, but
 when you set it to something, that something would reside on the heap)



Ok, that's essentially what I have, except that I used Controller pCtrl 
vice auto. WinGetLong however, is a template that calls 
GetWindowLongPtrA() and casts it's result (in this case) to Controller. 
  GetWindowLongPtrA() returns LONG_PTR (aka int) and therefore fails 
miserably on the cast attempt. On the reverse, there is a WinSetLong 
that attempts to cast Controller to int for use with 
SetWindowLongPtrA(). Neither of these functions complain when I use 
Controller* but I end up with the problem of trying to initialize a 
pointer with a reference to Controller.
Feb 26 2011
next sibling parent reply "Tyro[a.c.edwards]" <nospam home.com> writes:
On 2/27/2011 9:46 AM, Tyro[a.c.edwards] wrote:

 On 2/27/2011 8:52 AM, Simen kjaeraas wrote:

 Tyro[a.c.edwards] <nospam home.com> wrote:


 I'm trying to convert some c++ code that defines

 T func(par...)
 {
 Controller * pCtrl = WinGetLong<Controller *> (hwnd);
 .
 .
 .
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *>
 (lParam));
 break;
 }
 }

 I'm not sure why I need a pointer to the class, just trying to figure
 it out.



 Ah. You would not need a pointer to the class in D. Instead, your
 function
 would look something like this:

 T funct(par...)
 {
 auto pCtrl = WinGetLong!Controller(hwnd);
 ...
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller(hWnd, cast(CREATESTRUCT*)lParam);
 break;
 }
 }

 C++ classes are in some ways more akin to D structs, in that:

 class A {};

 void foo(){
 A bar;
 }

 bar would be allocated on the stack in C++, while in D bar would be a
 pointer to a class instance on the heap. (well, it would be null, but
 when you set it to something, that something would reside on the heap)



 Ok, that's essentially what I have, except that I used Controller pCtrl
 vice auto. WinGetLong however, is a template that calls
 GetWindowLongPtrA() and casts it's result (in this case) to Controller.
 GetWindowLongPtrA() returns LONG_PTR (aka int) and therefore fails
 miserably on the cast attempt. On the reverse, there is a WinSetLong
 that attempts to cast Controller to int for use with
 SetWindowLongPtrA(). Neither of these functions complain when I use
 Controller* but I end up with the problem of trying to initialize a
 pointer with a reference to Controller.



By the way, in original C++ code WinGetLong and WinSetLong are both 
using a reinterpret_cast to achieve this monkey magic. To the best of my 
knowledge, there is no reinterpret_cast facility in D. So the question 
would be, why would it have been necessary to use reinterpret_cast in 
the first place and how can similar effect be obtained in D? What was 
being reinterpreted? Was it the address of the class or the value some 
private value contained therein?
Feb 26 2011
parent Bekenn <leaveme alone.com> writes:
On 2/26/2011 5:33 PM, Tyro[a.c.edwards] wrote:

 Ok, that's essentially what I have, except that I used Controller pCtrl
 vice auto. WinGetLong however, is a template that calls
 GetWindowLongPtrA() and casts it's result (in this case) to Controller.
 GetWindowLongPtrA() returns LONG_PTR (aka int) and therefore fails
 miserably on the cast attempt. On the reverse, there is a WinSetLong
 that attempts to cast Controller to int for use with
 SetWindowLongPtrA(). Neither of these functions complain when I use
 Controller* but I end up with the problem of trying to initialize a
 pointer with a reference to Controller.



 By the way, in original C++ code WinGetLong and WinSetLong are both
 using a reinterpret_cast to achieve this monkey magic. To the best of my
 knowledge, there is no reinterpret_cast facility in D. So the question
 would be, why would it have been necessary to use reinterpret_cast in
 the first place and how can similar effect be obtained in D? What was
 being reinterpreted? Was it the address of the class or the value some
 private value contained therein?



What you need here is a double cast; class references can be cast to 
void*, and from there to LONG_PTR.  The reverse also works:

auto WinGetLong(T)(HWND hwnd)
{
	return cast(T)cast(void*)GetWindowLongPtrA(hwnd, GWLP_USERDATA);
}

void WinSetLong(T)(HWND hwnd, T o)
{
	SetWindowLongPtrA(hwnd, GWLP_USERDATA, cast(LONG_PTR)cast(void*)o;
}
Feb 26 2011
prev sibling parent "Tyro[a.c.edwards]" <nospam home.com> writes:
On 2/27/2011 10:39 PM, Steven Schveighoffer wrote:

 On Sat, 26 Feb 2011 19:46:18 -0500, Tyro[a.c.edwards] <nospam home.com>
 wrote:


 On 2/27/2011 8:52 AM, Simen kjaeraas wrote:

 Tyro[a.c.edwards] <nospam home.com> wrote:


 I'm trying to convert some c++ code that defines

 T func(par...)
 {
 Controller * pCtrl = WinGetLong<Controller *> (hwnd);
 .
 .
 .
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *>
 (lParam));
 break;
 }
 }

 I'm not sure why I need a pointer to the class, just trying to figure
 it out.



 Ah. You would not need a pointer to the class in D. Instead, your
 function
 would look something like this:

 T funct(par...)
 {
 auto pCtrl = WinGetLong!Controller(hwnd);
 ...
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller(hWnd, cast(CREATESTRUCT*)lParam);
 break;
 }
 }

 C++ classes are in some ways more akin to D structs, in that:

 class A {};

 void foo(){
 A bar;
 }

 bar would be allocated on the stack in C++, while in D bar would be a
 pointer to a class instance on the heap. (well, it would be null, but
 when you set it to something, that something would reside on the heap)



 Ok, that's essentially what I have, except that I used Controller
 pCtrl vice auto. WinGetLong however, is a template that calls
 GetWindowLongPtrA() and casts it's result (in this case) to
 Controller. GetWindowLongPtrA() returns LONG_PTR (aka int) and
 therefore fails miserably on the cast attempt. On the reverse, there
 is a WinSetLong that attempts to cast Controller to int for use with
 SetWindowLongPtrA(). Neither of these functions complain when I use
 Controller* but I end up with the problem of trying to initialize a
 pointer with a reference to Controller.



 You almost certainly do not want a pointer to a class reference. A class
 typically resides on the heap, but the reference typically does not.
 Therefore, by using a pointer to a class reference, you run very high
 risk of escaping stack data, leading to memory corruption.

 Looking at the documentation for GetWindowLongPtr, it appears to get
 data associated with a window. Likely, this information is the a pointer
 to the Controller class.

 I would recommend doing this:

 T WinGetLong(T)(HWND hwnd)
 {
 return cast(T)cast(void*)GetWindowLongPtrA(hwnd, ...);
 }

 and

 void WinSetLong(T)(HWND hwnd, T t)
 {
 SetWindowLongPtrA(hwnd, ..., cast(LONG_PTR)cast(void*)t);
 }

 where the ... is the index copied from the C++ code (guessing it's
 GWLP_USERDATA?).

 btw, reinterpret_cast<T>(x) is equivalent to (T)(void *)x;

 -Steve



Thank you all (Steve, Bekenn, and Simen) for your assistance on this.
Feb 28 2011
prev sibling next sibling parent "Simen kjaeraas" <simen.kjaras gmail.com> writes:
Tyro[a.c.edwards] <nospam home.com> wrote:


 I'm trying to convert some c++ code that defines

 T func(par...)
 {	
    Controller * pCtrl = WinGetLong<Controller *> (hwnd);
    .
    .
    .
    switch(msg)
    {
    case FirstMatch:
       pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *>  
 (lParam));
       break;
    }
 }

 I'm not sure why I need a pointer to the class, just trying to figure it  
 out.



Ah. You would not need a pointer to the class in D. Instead, your function
would look something like this:

T funct(par...)
{
     auto pCtrl = WinGetLong!Controller(hwnd);
     ...
     switch(msg)
     {
     case FirstMatch:
         pCtrl = new Controller(hWnd, cast(CREATESTRUCT*)lParam);
         break;
     }
}

C++ classes are in some ways more akin to D structs, in that:

class A {};

void foo(){
     A bar;
}

bar would be allocated on the stack in C++, while in D bar would be a
pointer to a class instance on the heap. (well, it would be null, but
when you set it to something, that something would reside on the heap)

-- 
Simen
Feb 26 2011
prev sibling parent "Steven Schveighoffer" <schveiguy yahoo.com> writes:
On Sat, 26 Feb 2011 19:46:18 -0500, Tyro[a.c.edwards] <nospam home.com>  
wrote:


 On 2/27/2011 8:52 AM, Simen kjaeraas wrote:

 Tyro[a.c.edwards] <nospam home.com> wrote:


 I'm trying to convert some c++ code that defines

 T func(par...)
 {
 Controller * pCtrl = WinGetLong<Controller *> (hwnd);
 .
 .
 .
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller (hwnd, reinterpret_cast<CREATESTRUCT *>  
 (lParam));
 break;
 }
 }

 I'm not sure why I need a pointer to the class, just trying to figure
 it out.



 Ah. You would not need a pointer to the class in D. Instead, your  
 function
 would look something like this:

 T funct(par...)
 {
 auto pCtrl = WinGetLong!Controller(hwnd);
 ...
 switch(msg)
 {
 case FirstMatch:
 pCtrl = new Controller(hWnd, cast(CREATESTRUCT*)lParam);
 break;
 }
 }

 C++ classes are in some ways more akin to D structs, in that:

 class A {};

 void foo(){
 A bar;
 }

 bar would be allocated on the stack in C++, while in D bar would be a
 pointer to a class instance on the heap. (well, it would be null, but
 when you set it to something, that something would reside on the heap)



 Ok, that's essentially what I have, except that I used Controller pCtrl  
 vice auto. WinGetLong however, is a template that calls  
 GetWindowLongPtrA() and casts it's result (in this case) to Controller.  
   GetWindowLongPtrA() returns LONG_PTR (aka int) and therefore fails  
 miserably on the cast attempt. On the reverse, there is a WinSetLong  
 that attempts to cast Controller to int for use with  
 SetWindowLongPtrA(). Neither of these functions complain when I use  
 Controller* but I end up with the problem of trying to initialize a  
 pointer with a reference to Controller.



You almost certainly do not want a pointer to a class reference.  A class  
typically resides on the heap, but the reference typically does not.   
Therefore, by using a pointer to a class reference, you run very high risk  
of escaping stack data, leading to memory corruption.

Looking at the documentation for GetWindowLongPtr, it appears to get data  
associated with a window.  Likely, this information is the a pointer to  
the Controller class.

I would recommend doing this:

T WinGetLong(T)(HWND hwnd)
{
    return cast(T)cast(void*)GetWindowLongPtrA(hwnd, ...);
}

and

void WinSetLong(T)(HWND hwnd, T t)
{
    SetWindowLongPtrA(hwnd, ..., cast(LONG_PTR)cast(void*)t);
}

where the ... is the index copied from the C++ code (guessing it's  
GWLP_USERDATA?).

btw, reinterpret_cast<T>(x) is equivalent to (T)(void *)x;

-Steve
Feb 27 2011