CODEWITHSUNDEEP
c++windows
Vern
Vern

Reputation: 147

c++ calling a non static function from a static function

I'm experiencing an access violation error when I try to call non static function.

my .h file looks like this.

class World
{
public:
    World();
    virtual ~World();
    static void CALLBACK DispatchCallback(
        SIMCONNECT_RECV *pData,
        DWORD cbData,
        void *pContext
    );
    void Process(SIMCONNECT_RECV *pData, DWORD cbData);
    virtual void frameEvent();
    virtual void init();
};

Now in my .cpp file the init() function I call a function that return data to my callback function.

SimConnect_CallDispatch(hSimConnect, DispatchCallback, NULL);

Which sends data to the DisPatchCallback function.

In this function the following code resides:

void CALLBACK World::DispatchCallback(
    SIMCONNECT_RECV *pData,
    DWORD cbData,
    void *pContext)
{
    World *pThis = reinterpret_cast<World*>(pContext);
    pThis->Process(pData, cbData);
}

this function is a static function which creates a World object to call the Process function.

This works but it break on the line where it wants to access the frameEvent function.

void World::Process(SIMCONNECT_RECV *pData, DWORD cbData)
{
    HRESULT hr;

    switch(pData->dwID)
    {
    case SIMCONNECT_RECV_ID_EVENT_FRAME:
        frameEvent(); //it breaks here frameEvent is a non static function
        break;
    }
}

Access violation reading location 0x00000000.

Can someone point me to the right direction as to solve this issue.?

If you wonder I'm writing a plugin for Microsoft Flight Simulator X.

I'm trying to implement simconnect.h in a oo way. Msdn shows an example that I'm trying to implement.

class CName
{
    void Dispatch();
    static void DispatchCallback(
        SIMCONNECT_RECV *pData,
        DWORD cbData,
        void *pContext
    );
    void Process(SIMCONNECT_RECV *pData, DWORD cbData);

    HANDLE hSimConnect; // use SimConnect_Open to set this value.
};

void CName::Dispatch()
{
    ::SimConnect_Dispatch(hSimConnect, &CName;::DispatchCallback, this);
}

// static function
void CName::DispatchCallback(
    SIMCONNECT_RECV *pData,
    DWORD cbData,
    void *pContext)
{
    CName *pThis = reinterpret_cast<CName*>(pContext);
    pThis->Process(pData, cbData);
}

void CName::Process(SIMCONNECT_RECV *pData, DWORD cbData)
{
    // do processing of SimConnect data
}

I hope I gave enough information.

Upvotes: 2

Views: 1009

Answers (3)

littleadv
littleadv

Reputation: 20272

It breaks because this is NULL.

Check this portion of your code:

World *pThis = reinterpret_cast<World*>(pContext);
pThis->Process(pData, cbData);

If the reinterpret_cast fails gets a NULL - it returns NULL, and you don't check for it.

Upvotes: 2

Greg Hewgill
Greg Hewgill

Reputation: 993105

Your use of:

SimConnect_CallDispatch(hSimConnect, DispatchCallback, NULL);

seems very suspicious. What's the NULL parameter mean in the call to that function? Is that pContext?

Upvotes: 2

Adam Rosenfield
Adam Rosenfield

Reputation: 400274

You're passing NULL as the context parameter to SimConnect_CallDispatch, so your callback has no idea which World object to call Process on -- how could it possibly if you don't tell it? Change the call to pass in this as the context parameter, like the example does:

SimConnect_CallDispatch(hSimConnect, DispatchCallback, this);

Upvotes: 8

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