Error of argument type with _beginthreadex

Question

To define my thread I have in my Header file:

class HttpClient
{
public:
    ...
    unsigned int __stdcall  PerformLogin(void*);
    ...

};

Then in my cpp file I have:

unsigned int __stdcall PerformLogin(void*){
...
}

And to call this thread I use

hThread = (HANDLE)_beginthreadex( NULL, 0, &PerformLogin, NULL, 0, &threadID );

But i Have an error on the &PerformLogin saying that:

the args of type unsigned int (__stdcall HttpClient::)(void) is not compatible with the param unsigned int (__stdcall*)(void*).

I understand the error, but I really don't know how to fix this!

You're missing the `HttpClient::` at the beginning of `PerformLogin`'s definition. You'll still get an error after you add it (as @hmjd) describes, though, but at least your class will be defined correctly. — Eran, Jul 02 '12 at 08:15
@darkheir: see http://stackoverflow.com/questions/1259815/beginthreadex-static-member-function for sample code of using `_beginthreadex` with member functions. — tenfour, Jul 02 '12 at 10:27

score 2 · Accepted Answer · answered Jul 02 '12 at 08:09

2

A possible way to fix this would be to make the member function static, though this means PerformLogin() does not have a this pointer and would have no access to non-static members of HttpClient.

Another is to move PerformLogin() out of HttpClient altogether, and make it a free function.

answered Jul 02 '12 at 08:09

hmjd

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I'm going to try to move PerformLogin() out, seems to be a good solution! – darkheir Jul 02 '12 at 08:14
1

@darkheir, you should decide where to put your functions based on other criteria, like where does it belong in your model considering OOP principles. – Vsevolod Golovanov Jul 02 '12 at 10:34

Martin James · Answer 2 · 2012-07-02T08:48:59.400

2

What I usually to is to add 'this' as the void* parameter to the static function - you can then call methods on it in the static function with a bit of casting..

edited Jul 02 '12 at 08:48

answered Jul 02 '12 at 08:40

Martin James

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This is redundant, see my answer – Vsevolod Golovanov Jul 02 '12 at 10:23
It's not redundant. Casting to `this` is better than relying on calling convention hackery. – tenfour Jul 02 '12 at 10:24
@tenfour, you're right about hackiness, but I still deem the implicit `this` approach better. – Vsevolod Golovanov Jul 02 '12 at 11:08

Vsevolod Golovanov · Answer 3 · 2012-07-02T11:04:54.590

Member functions get this pointer implicitly as a first parameter. So if you want to start a thread with a class member function, you should explicitly pass a pointer to a class instance in your call to _beginthreadex.

So, remove explicit argument:

class HttpClient
{
    public:
    ...
    unsigned int __stdcall  PerformLogin();
    ...
};

And call _beginthreadex while passing this as an argument:

hThread = (HANDLE)_beginthreadex( NULL, 0, &PerformLogin, this, 0, &threadID );

It's worth mentioning that this is a bit hacky. C++ FAQ advises against it. I still prefer this approach. Of course I just use boost::thread usually.

Error of argument type with _beginthreadex

3 Answers3