AfxBeginThread & CreateThread

Heya

I was attempting to create a thread inside of a class as so:

Code:

---

SomeClass::DoWork()
{
::AfxBeginThread(ThreadProc, NULL);
}

UINT SomeClass::ThreadProc(LPVOID pArg)
{
//work...

return TRUE;
}

---

also I tried to

Code:

---

SomeClass Instance;

::AfxBeginThread(&Instance.ThreadProc, NULL);

---

Basically I'm wondering is it possible in C++ to create a thread inside your object, or create a thread running an objects method? In C# this is no problem, I just cant seem to find any examples of doing this in C++ without making a seperate function outside of a class, or making the class's method static.

Thanks for any info.

Both AfxBeginThread() and CreateThread() expect an __stdcall calling convention function, but all class methods use __thiscall calling convention, so basically a class method can never be of the same type as AfxBeginThread() requires.

And the solution to this is to pass the "this" as the argument to the function, and then making a static member function that takes a pointer to the class itself, and you can then use the class member functions on the pointer. Something like this:

Code:

---


class SomeClass
{
...
public:
  __stdcall static ThreadProc(LPVOID pArg);

SomeClass::DoWork()
{
::AfxBeginThread(ThreadProc, this);
}

UINT SomeClass::ThreadProc(LPVOID pArg)
{
  SomeClass *myPtr = reinterpret_cast<SomeClass *>(pArg);

  myPtr->WorkToBeDoneInThread();
//work...

return TRUE;
}

---

--
Mats

Quote:

---

Originally Posted by valaris

Heya

I was attempting to create a thread inside of a class as so:

Code:

---

SomeClass::DoWork()
{
::AfxBeginThread(ThreadProc, NULL);
}

UINT SomeClass::ThreadProc(LPVOID pArg)
{
//work...

return TRUE;
}

---

also I tried to

Code:

---

SomeClass Instance;

::AfxBeginThread(&Instance.ThreadProc, NULL);

---

Basically I'm wondering is it possible in C++ to create a thread inside your object, or create a thread running an objects method? In C# this is no problem, I just cant seem to find any examples of doing this in C++ without making a seperate function outside of a class, or making the class's method static.

Thanks for any info.

---

The reason is that CreateThread is C, and not C++. AfxBeginThread merely wraps CreateThread, and thus also has a C interface.
In short, you can blame Microsoft's C compatibility for the lack of this functionality.
The problem is that these functions really should take an extra argument - the "this" pointer, as explained. That is because there are several instances of a class, so which instance's function should be called?

However, I made a quick solution that I hope works.
A C++ variant of CreateThread:

Code:

---

#include <windows.h>
#include <iostream>

template<typename ParamType, typename ClassType, typename ReturnType> struct ThreadParams
{
        ParamType* Param;
        ClassType* Instance;
        ReturnType (ClassType::* Function)(ParamType* Param);
        HANDLE hProcess;
};

template<typename ParamType, typename ClassType, typename ReturnType> DWORD WINAPI ThreadFnc(void* pParam)
{
        ThreadParams<ParamType, ClassType, ReturnType>* pParams = (ThreadParams<ParamType, ClassType, ReturnType>*)pParam;
        (pParams->Instance->*pParams->Function)(pParams->Param);
        CloseHandle(pParams->hProcess);
        delete pParams;
        return 0;
}

template<typename ParamType, typename ClassType, typename ReturnType> DWORD CPPCreateThread(ReturnType (ClassType::* Function)(ParamType* Param), ClassType* Instance, ParamType* Param)
{
        ThreadParams<ParamType, ClassType, ReturnType>* pParams = new ThreadParams<ParamType, ClassType, ReturnType>;
        pParams->Param = Param;
        pParams->Instance = Instance;
        pParams->Function = Function;
        HANDLE hProcess = CreateThread(NULL, NULL, &ThreadFnc<ParamType, ClassType, ReturnType>, pParams, CREATE_SUSPENDED, NULL);
        if (hProcess == NULL)
                return GetLastError();
        pParams->hProcess = hProcess;
        ResumeThread(hProcess);
        DWORD dwErr = GetLastError();
        if (dwErr != ERROR_SUCCESS)
        {
                CloseHandle(hProcess);
                delete pParams;
        }
        return dwErr;
}

class foo
{
public:
        foo() { CPPCreateThread<void>(&foo::ThreadFunc, this, NULL); }
        void ThreadFunc(void*) { std::cout << "Hi! This is inside the thread!"; };
};

int main() { foo f; Sleep(1000); }

---

Quote:

---

Originally Posted by Elysia

The reason is that CreateThread is C, and not C++. AfxBeginThread merely wraps CreateThread, and thus also has a C interface.
In short, you can blame Microsoft's C compatibility for the lack of this functionality.
The problem is that these functions really should take an extra argument - the "this" pointer, as explained. That is because there are several instances of a class, so which instance's function should be called?

However, I made a quick solution that I hope works.
A C++ variant of CreateThread:

Code:

---

#include <windows.h>
#include <iostream>

template<typename ParamType, typename ClassType, typename ReturnType> struct ThreadParams
{
        ParamType* Param;
        ClassType* Instance;
        ReturnType (ClassType::* Function)(ParamType* Param);
        HANDLE hProcess;
};

template<typename ParamType, typename ClassType, typename ReturnType> DWORD WINAPI ThreadFnc(void* pParam)
{
        ThreadParams<ParamType, ClassType, ReturnType>* pParams = (ThreadParams<ParamType, ClassType, ReturnType>*)pParam;
        (pParams->Instance->*pParams->Function)(pParams->Param);
        CloseHandle(pParams->hProcess);
        delete pParams;
        return 0;
}

template<typename ParamType, typename ClassType, typename ReturnType> DWORD CPPCreateThread(ReturnType (ClassType::* Function)(ParamType* Param), ClassType* Instance, ParamType* Param)
{
        ThreadParams<ParamType, ClassType, ReturnType>* pParams = new ThreadParams<ParamType, ClassType, ReturnType>;
        pParams->Param = Param;
        pParams->Instance = Instance;
        pParams->Function = Function;
        HANDLE hProcess = CreateThread(NULL, NULL, &ThreadFnc<ParamType, ClassType, ReturnType>, pParams, CREATE_SUSPENDED, NULL);
        if (hProcess == NULL)
                return GetLastError();
        pParams->hProcess = hProcess;
        ResumeThread(hProcess);
        DWORD dwErr = GetLastError();
        if (dwErr != ERROR_SUCCESS)
        {
                CloseHandle(hProcess);
                delete pParams;
        }
        return dwErr;
}

class foo
{
public:
        foo() { CPPCreateThread<void>(&foo::ThreadFunc, this, NULL); }
        void ThreadFunc(void*) { std::cout << "Hi! This is inside the thread!"; };
};

int main() { foo f; Sleep(1000); }

---

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Hey now this is very cool! And I will steal/borrow/use it ><
Thanks for everybodies help.

My last question then would be if &foo::ThreadFunc is how you take the address of a method from within a class, how do you take it (the address of the method) from outside in an object. For using it as a function pointer or some other need?

the method will have a fixed address at runtime, so just acquire a pointer to it using this->ThreadFunc not &this->ThreadFunc. C++ does not create a new copy of the function for every instance of the class, just a new copy of the data.

Quote:

---

Originally Posted by valaris

Hey now this is very cool! And I will steal/borrow/use it ><
Thanks for everybodies help.

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Feel free :)

Quote:

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My last question then would be if &foo::ThreadFunc is how you take the address of a method from within a class, how do you take it (the address of the method) from outside in an object. For using it as a function pointer or some other need?

---

It is the same from both within and outside the class. A function has a fixed address (even class functions), so it works inside as well as outside.
You just need to make sure you pass the proper instance. This varies from inside the class and the outside, of course.

Quote:

---

Originally Posted by abachler

the method will have a fixed address at runtime, so just acquire a pointer to it using this->ThreadFunc not &this->ThreadFunc. C++ does not create a new copy of the function for every instance of the class, just a new copy of the data.

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This does not work:

Code:

---

        foo() { CPPCreateThread<void>(/*&foo::ThreadFunc*/this->ThreadFunc, this, NULL); }

---

Error 1 error C3867: 'foo::ThreadFunc': function call missing argument list; use '&foo::ThreadFunc' to create a pointer to member g:\w00t\documents\visual studio 2008\projects\temp\temp2.cpp 44