_beginthreadex static member function

Question

How do I create a thread routine of a static member function

class Blah
{
    static void WINAPI Start();
};

// .. 
// ...
// ....

hThread = (HANDLE)_beginthreadex(NULL, 0, CBlah::Start, NULL, NULL, NULL);

This gives me the following error:

***error C2664: '_beginthreadex' : cannot convert parameter 3 from 'void (void)' to 'unsigned int (__stdcall *)(void *)'***

What am I doing wrong?

Answer 1

Sometimes, it is useful to read the error you're getting.

cannot convert parameter 3 from 'void (void)' to 'unsigned int (__stdcall *)(void *)'

Let's look at what it says. For parameter three, you give it a function with the signature void(void), that is, a function which takes no arguments, and returns nothing. It fails to convert this to unsigned int (__stdcall *)(void *), which is what _beginthreadex expects:

It expects a function which:

• Returns an unsigned int:
• Uses the stdcall calling convention
• Takes a void* argument.

So my suggestion would be "give it a function with the signature it's asking for".

class Blah
{
    static unsigned int __stdcall Start(void*);
};

Answer 2

Following is the compiling version:

class CBlah
{
public:
    static unsigned int WINAPI Start(void*)
    {
    return 0;
    }
};

int main()
{
    HANDLE hThread = (HANDLE)_beginthreadex(NULL, 0, &CBlah::Start, NULL, NULL, NULL);

    return 0;
}

Following are the changes required:

(1). Start() function should return unsigned int

(2). It should take a void* as the parameter.

EDIT

Deleted point (3) as per comment

Answer 3

class Blah
{
    static unsigned int __stdcall Start(void*); // void* should be here, because _beginthreadex requires it.
};

The routine passed to _beginthreadex must use the __stdcall calling convention and must return a thread exit code.

Implementation of Blah::Start:

unsigned int __stdcall Blah::Start(void*)
{
  // ... some code

  return 0; // some exit code. 0 will be OK.
}

Later in your code you could write any of the following:

hThread = (HANDLE)_beginthreadex(NULL, 0, CBlah::Start, NULL, NULL, NULL);
// or
hThread = (HANDLE)_beginthreadex(NULL, 0, &CBlah::Start, NULL, NULL, NULL);

In first case Function-to-pointer conversion will be applied according to C++ Standard 4.3/1. In second case you'll pass pointer to function implicitly.

Answer 4

class Blah
{
  public:
    static DWORD WINAPI Start(void * args);
};

Answer 5

class Blah
{
    static unsigned int __stdcall Start(void *);
};