How to cast LPVOID to my class type??

[ppuniversal]

Hello,
I am making a thread program, in which i call :

hThread = CreateThread(    NULL,
                        0,
            (LPTHREAD_START_ROUTINE)(this->runThread),
            this,
            0,
            &ThreadId);
 

in the constructor of my class.

Here hThread is the handle to the thread created.
And runThread is my thread function.

The 4th parameter passed is the pointer to my current object, i.e. "this".

The prototype of my runThread function is:

static DWORD WINAPI runThread(LPVOID param);
 

Now the problem is:

In the runThread function when I convert the "param" to my class type i.e.

MyClass ob = (MyClass)param;
 

it gives the following error on compilation:

error C2440: 'type cast' : cannot convert from 'LPVOID' to 'MyClass'
 

Please tell either how to overcome this problem or any other way of passing my object to the runThread function.
Pawan

[AdrianH]

In the runThread function when I convert the "param" to my class type i.e.

MyClass ob = (MyClass)param;
 

it gives the following error on compilation:

error C2440: 'type cast' : cannot convert from 'LPVOID' to 'MyClass'
 

Please tell either how to overcome this problem or any other way of passing my object to the runThread function.
Pawan

The problem is that you are trying to reinterpret a type to another type, and the types have nothing in common, especially size. this is a pointer, you need to do a reinterpret_cas t from param to a pointer to the class, like one of the following:

MyClass* pOb = reinterpret_cast<MyClass*>(param);
MyClass& rOb = *reinterpret_cast<MyClass*>(param);
 

The first casts to a MyClass * and assigns it to pOb. The second does the same cast but dereferences it with the '*' operator and is then assigned to a reference variable.

Does this make sense? Hope it helps.


Adrian

[ppuniversal]

The problem is that you are trying to reinterpret a type to another type, and the types have nothing in common, especially size. this is a pointer, you need to do a reinterpret_cas t from param to a pointer to the class, like one of the following:

MyClass* pOb = reinterpret_cast<MyClass*>(param);
MyClass& rOb = *reinterpret_cast<MyClass*>(param);
 

The first casts to a MyClass * and assigns it to pOb. The second does the same cast but dereferences it with the '*' operator and is then assigned to a reference variable.

Does this make sense? Hope it helps.


Adrian

I used your method, and it compiled without errors, but when the MyClass.exe was executed, it resulted in fatal error. Then I tried simple:

MyClass *ob = (MyClass *)param;
 

and it worked for me.

But, I believe and have read on net that the your told

MyClass* pOb = reinterpret_cast<MyClass*>(param);
MyClass& rOb = *reinterpret_cast<MyClass*>(param);
 

are totally correct, it is me who is doing some wrong thing in my code.

Still, thanks for your detail answer, I will use it in future programs.
Pawan

[AdrianH]

I used your method, and it compiled without errors, but when the MyClass.exe was executed, it resulted in fatal error. Then I tried simple:

MyClass *ob = (MyClass *)param;
 

and it worked for me.

But, I believe and have read on net that the your told

MyClass* pOb = reinterpret_cast<MyClass*>(param);
MyClass& rOb = *reinterpret_cast<MyClass*>(param);
 

are totally correct, it is me who is doing some wrong thing in my code.

Still, thanks for your detail answer, I will use it in future programs.
Pawan

Odd, should work. What compiler are you using? Version number?

Also, your use of (LPTHREAD_START _ROUTINE)(this->runThread), is not necessary. Since runThread() is a static member function, you could just use (LPTHREAD_START _ROUTINE)(MyCla ss::runThread), it doesn't need the this context. In fact if you are using this and it is working it means you are calling CreateThread() from within the class so then you don't even need MyClass:: either as that static member function is implied.

Glad to help.


Adrian

[ppuniversal]

Odd, should work. What compiler are you using? Version number?

Also, your use of (LPTHREAD_START _ROUTINE)(this->runThread), is not necessary. Since runThread() is a static member function, you could just use (LPTHREAD_START _ROUTINE)(MyCla ss::runThread), it doesn't need the this context. In fact if you are using this and it is working it means you are calling CreateThread() from within the class so then you don't even need MyClass:: either as that static member function is implied.

Glad to help.


Adrian

Yes, whatever you said is correct, I did called runThread directly after some time and it worked.

For information, I am using:

Microsoft Visual Studio 2005
Version 8.0.50727.42

I dont know why the error came. But now it is working fine with the way I used,
i.e. MyClass *ob = (MyClass *)param;


Pawan