Programming Forums > Application Development > C++

core8583 · Jun 14th, 2008, 2:56 PM

I am trying to workout how to assign a class function call SNcallback as a windows API callback function.

My function is defined as

(Toggle Plain Text)

	SNMPAPI_STATUS CALLBACK SNcallback
			   (HSNMP_SESSION SNsession, HWND hWnd, UINT wMsg,
				WPARAM wParam, LPARAM lParam, LPVOID lpClientData)

and i need to assign it to the second parameter of the SnmpCreateSession function for WinSNMP

(Toggle Plain Text)

		SNMPAPI_CALLBACK callback = &SNcallback;
			SNsession = SnmpCreateSession(NULL, 0, callback ,NULL);

The error i get is

error C2276: '&' : illegal operation on bound member function expression

I have tried using

(Toggle Plain Text)

			SNMPAPI_CALLBACK callback = &snmp::SNcallback;
			SNsession = SnmpCreateSession(NULL, 0, callback ,NULL);

but it comes back with

(Toggle Plain Text)

error C2440: 'initializing' : cannot convert from 'SNMPAPI_STATUS (__stdcall snmp::* )(HSNMP_SESSION,HWND,UINT,WPARAM,LPARAM,LPVOID)' to 'SNMPAPI_CALLBACK'
        There is no context in which this conversion is possible

Any ideas anyone, is it even possible to use a class function as callback function?

Cache · Jun 14th, 2008, 4:23 PM

The member function would have to be declared static

Cache · Jun 14th, 2008, 4:59 PM

Here is an untested hint to calling a non-static member function in the callback. 'SNcallback' could of course be made a static member function of 'SNMP_worker' if you'd like.

(Toggle Plain Text)

struct SNMP_worker
{
    void run(/* params */) {}
};

SNMPAPI_STATUS CALLBACK SNcallback(
          HSNMP_SESSION SNsession
        , HWND          hWnd
        , UINT          wMsg
        , WPARAM        wParam
        , LPARAM        lParam
        , LPVOID        lpClientData
        )
{
    if (SNMP_worker* p_worker = 
        static_cast<SNMP_worker*>(lpClientData))
    {
        p_worker->run(/* params */);
    }
}

// ...

SNMP_worker worker;

HSNMP_SESSION session = SnmpCreateSession(NULL, 0, &SNcallback, (void*)&worker);

core8583 · Jun 15th, 2008, 7:42 AM

Thanks alot, I couldn't get it work without defining it as a static, and so the function losed access to all the class functions and data sets. I got around this by passing a struct into the function with the this pointer of the class so i can't get to all the data i need to get to.

Seif · Jun 18th, 2008, 7:20 PM

Just an extra note. Although static members are type compatible with pointer to functions with most compilers, there are some ABI's where they may differ between C and C++.
In these cases you'd probably want a wrapper function that uses a global object to remember the function.

http://www.parashift.com/c++-faq-lit...o-members.html

33.2 for an example of this.

Jun 14th, 2008, 2:56 PM	#1
core8583 Newbie Join Date: Apr 2008 Posts: 5 Rep Power: 0	Assigning a class function as a callback function I am trying to workout how to assign a class function call SNcallback as a windows API callback function. My function is defined as (Toggle Plain Text) SNMPAPI_STATUS CALLBACK SNcallback (HSNMP_SESSION SNsession, HWND hWnd, UINT wMsg, WPARAM wParam, LPARAM lParam, LPVOID lpClientData) SNMPAPI_STATUS CALLBACK SNcallback (HSNMP_SESSION SNsession, HWND hWnd, UINT wMsg, WPARAM wParam, LPARAM lParam, LPVOID lpClientData) and i need to assign it to the second parameter of the SnmpCreateSession function for WinSNMP (Toggle Plain Text) SNMPAPI_CALLBACK callback = &SNcallback; SNsession = SnmpCreateSession(NULL, 0, callback ,NULL); SNMPAPI_CALLBACK callback = &SNcallback; SNsession = SnmpCreateSession(NULL, 0, callback ,NULL); The error i get is error C2276: '&' : illegal operation on bound member function expression I have tried using (Toggle Plain Text) SNMPAPI_CALLBACK callback = &snmp::SNcallback; SNsession = SnmpCreateSession(NULL, 0, callback ,NULL); SNMPAPI_CALLBACK callback = &snmp::SNcallback; SNsession = SnmpCreateSession(NULL, 0, callback ,NULL); but it comes back with (Toggle Plain Text) error C2440: 'initializing' : cannot convert from 'SNMPAPI_STATUS (__stdcall snmp::* )(HSNMP_SESSION,HWND,UINT,WPARAM,LPARAM,LPVOID)' to 'SNMPAPI_CALLBACK' There is no context in which this conversion is possible error C2440: 'initializing' : cannot convert from 'SNMPAPI_STATUS (__stdcall snmp::* )(HSNMP_SESSION,HWND,UINT,WPARAM,LPARAM,LPVOID)' to 'SNMPAPI_CALLBACK' There is no context in which this conversion is possible Any ideas anyone, is it even possible to use a class function as callback function?

Jun 14th, 2008, 4:23 PM	#2
Cache Hobbyist Join Date: Sep 2005 Posts: 261 Rep Power: 4	Re: Assigning a class function as a callback function The member function would have to be declared `static`

Jun 14th, 2008, 4:59 PM	#3
Cache Hobbyist Join Date: Sep 2005 Posts: 261 Rep Power: 4	Re: Assigning a class function as a callback function Here is an untested hint to calling a non-static member function in the callback. 'SNcallback' could of course be made a static member function of 'SNMP_worker' if you'd like. (Toggle Plain Text) struct SNMP_worker { void run(/* params /) {} }; SNMPAPI_STATUS CALLBACK SNcallback( HSNMP_SESSION SNsession , HWND hWnd , UINT wMsg , WPARAM wParam , LPARAM lParam , LPVOID lpClientData ) { if (SNMP_worker p_worker = static_cast<SNMP_worker>(lpClientData)) { p_worker->run(/ params /); } } // ... SNMP_worker worker; HSNMP_SESSION session = SnmpCreateSession(NULL, 0, &SNcallback, (void)&worker); struct SNMP_worker { void run(/* params /) {} }; SNMPAPI_STATUS CALLBACK SNcallback( HSNMP_SESSION SNsession , HWND hWnd , UINT wMsg , WPARAM wParam , LPARAM lParam , LPVOID lpClientData ) { if (SNMP_worker p_worker = static_cast<SNMP_worker>(lpClientData)) { p_worker->run(/ params /); } } // ... SNMP_worker worker; HSNMP_SESSION session = SnmpCreateSession(NULL, 0, &SNcallback, (void)&worker);

Jun 15th, 2008, 7:42 AM	#4
core8583 Newbie Join Date: Apr 2008 Posts: 5 Rep Power: 0	Re: Assigning a class function as a callback function Thanks alot, I couldn't get it work without defining it as a static, and so the function losed access to all the class functions and data sets. I got around this by passing a struct into the function with the `this` pointer of the class so i can't get to all the data i need to get to. Last edited by Ancient Dragon; Jun 15th, 2008 at 8:58 PM. Reason: corrected code tags

Jun 18th, 2008, 7:20 PM	#5
Seif Hobbyist Programmer Join Date: Jan 2006 Location: UK Posts: 215 Rep Power: 3	Re: Assigning a class function as a callback function Just an extra note. Although static members are type compatible with pointer to functions with most compilers, there are some ABI's where they may differ between C and C++. In these cases you'd probably want a wrapper function that uses a global object to remember the function. http://www.parashift.com/c++-faq-lit...o-members.html 33.2 for an example of this.

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Assigning a class function as a callback function