Clever Geek Handbook

_Beginthreadex static member function

How to create a thread procedure for 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?

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5 answers


It is sometimes helpful to read the error you are getting.

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

See what he says. For parameter three, you give it a function with a signature void(void)

, that is, a function that takes no arguments and returns nothing. It cannot convert this value to unsigned int (__stdcall *)(void *)

, which means _beginthreadex

:

It expects a function that:



  • Returns unsigned int

    :
  • Uses a calling convention stdcall

  • Accepts an argument void*

    .

So my suggestion would be "give it the signature function it asks for."

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

The procedure passed to _beginthreadex

must use the calling convention __stdcall

and must return a thread exit code .

Blah :: Start implementation:

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

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


Later in the code, you can 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 the first case, it Function-to-pointer conversion

will be applied in accordance with the C ++ Standard 4.3 / 1. In the second case, you will be passing the function pointer implicitly.

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class Blah
{
  public:
    static DWORD WINAPI Start(void * args);
};
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Below is the compiled 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 required changes:

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

(2). It must take a void * value as a parameter.

EDIT

Remote point (3) as per comment

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