Nrthugu

Weekend programmer, newbie to C++, battling the war of the pointers!

I am trying to code in C++ a method to retrieve data from an API using a vector that gets passed via a void * argument to a callback function.

The returned data will get passed back to lua, which is the front-end used in the application. The lua portion is not what has me baffled, it is the challenge of getting my struct filled with the data from the callback.

I had this all working using simple int pointers, but was told of a preferred way to do it using a struct, it was suggested I do the following:

1 - define a struct that contains the 6 ints

2 - create an std::vector that can contain instances of the new struct

3 - pass a pointer to the vector as refcon into XPLMGetAllMonitorBoundsGlobal

inside the callback function:

    4 - cast the refcon pointer back to the vector type

    5 - create a new instance of the struct and fill it with the 6 ints that are passed to the callback function

    6 - push that instance into the vector

7 - when XPLMGetAllMonitorBoundsGlobal returns, the vector will be filled with the bounds of all screens

8 - convert the vector into a lua-compatible thing, probably a two-dimensional array

push that array into lua

Here is the code I came up with, did lot's of Google/Stackoverflow searches to come this close, but even though the code compiles, it hangs the application, or causes seg faults, depends on what I tweaked. I think the primary problem is how I am casting the refcon pointer back to a vector type, too many references/dereferences of pointers, and there are probably other issues too.

struct  MonitorBoundsStructure    // #1

{

    int     MonitorIndex;

    int     LeftBx;

    int     TopBx;

    int     RightBx;

    int     BottomBx;

    int     RefCon;

};



static void LuaReceiveMonitorBoundsOS_t(int inMonitorIndex, int inLeftBx, int inTopBx, int inRightBx, int inBottomBx, void * refcon)

{

    vector<MonitorBoundsStructure*>& MonitorBounds = *reinterpret_cast<vector<MonitorBoundsStructure*> *>(refcon);  #4

    //vector<MonitorBoundsStructure> MonitorBounds = reinterpret_cast<vector<MonitorBoundsStructure*> *>(refcon);

    //vector<MonitorBoundsStructure> MonitorBounds = *reinterpret_cast<vector<MonitorBoundsStructure> *>(refcon);

    //vector<MonitorBoundsStructure> MonitorBounds = reinterpret_cast<vector<MonitorBoundsStructure> *>(refcon);

    //vector<MonitorBoundsStructure>* MonitorBounds = static_cast<vector<MonitorBoundsStructure> *>(refcon);



    MonitorBoundsStructure returnData; //{0,0,0,0,0,0};   #5

    returnData.MonitorIndex = inMonitorIndex;

    returnData.LeftBx = inLeftBx;

    returnData.TopBx = inTopBx;

    returnData.RightBx = inRightBx;

    returnData.BottomBx = inBottomBx;

    returnData.RefCon = *(int *)refcon;



    MonitorBounds.push_back(&returnData);   // #6

}



static int LuaXPLMGetAllMonitorBoundsOSTest()

{

    //std::vector<std::shared_ptr<MonitorBoundsStructure>> MonitorBounds;

    vector<MonitorBoundsStructure> MonitorBounds;  // #2



    XPLMGetAllMonitorBoundsOS(LuaReceiveMonitorBoundsOS_t, &MonitorBounds);  // #3



    int i = 0;

    for (vector<MonitorBoundsStructure>::iterator it = MonitorBounds.begin(); it != MonitorBounds.end(); ++it)

    {

        i++;

        logMsg(logToAll, string("MonitorBounds ").append(to_string(i)));



    }



    return 1;

}

If I comment out MonitorBounds.push_back(&returnData), The code will at least get to where the struct inside the callback is filled with the correct data, I am hitting a wall getting that struct back to the calling function, meaning my cast of the void* to vector is wrong at the very least.

I left some of my other attempts in the comments to show what I have tried.

Am I close to the solution or way off?

Solution:
Thanks to Ted Lyngmo for providing the solution, even going as far as to write up test code on his own system. My problem, as I suspected, was in the cast of pointer to vector. I have included both the push_back and emplace_back (much neater I find) answers.

-callback function:

static void LuaReceiveMonitorBoundsOS_e(int inMonitorIndex, int inLeftBx, int inTopBx, int inRightBx, int inBottomBx, void * refcon)

{

    auto& MonitorBounds = *reinterpret_cast<vector<MonitorBoundsStructure>*>(refcon);





    MonitorBoundsStructure returnData;

    returnData.MonitorIndex = inMonitorIndex;

    returnData.LeftBx = inLeftBx;

    returnData.TopBx = inTopBx;

    returnData.RightBx = inRightBx;

    returnData.BottomBx = inBottomBx;



    MonitorBounds.push_back(returnData);

}

and

static void LuaReceiveMonitorBoundsOS_e(int inMonitorIndex, int inLeftBx, int inTopBx, int inRightBx, int inBottomBx, void * refcon)

{

    auto& MonitorBounds = *reinterpret_cast<vector<MonitorBoundsStructure>*>(refcon);



    MonitorBounds.emplace_back(

         MonitorBoundsStructure{

             inMonitorIndex,

             inLeftBx,

             inTopBx,

             inRightBx,

             inBottomBx

         }

     );

}

-calling function:

static int LuaXPLMGetAllMonitorBoundsOSTestE(lua_State *L)

{

    vector<MonitorBoundsStructure> MonitorBounds;



    XPLMGetAllMonitorBoundsOS(LuaReceiveMonitorBoundsOS_e, &MonitorBounds);



    for (auto& m : MonitorBounds) {

         std::cout << m.MonitorIndex << "n";

     }



    return 1;

}

MonitorBoundsStructure returnData; //{0,0,0,0,0,0};   #5

MonitorBounds.push_back(&returnData);   // #6

After that, returnData gets out of scope and is destroyed. The pointer you now have in MonitorBounds is invalid.

It also looks like you are casting to the wrong type. If this is the vector:

vector<MonitorBoundsStructure> MonitorBounds;

You should cast to that in your calllback:

auto& MonitorBounds = *reinterpret_cast<vector<MonitorBoundsStructure>*>(refcon);

And push the whole struct and not a pointer to it:

MonitorBounds.push_back(returnData);

Edit: I made a test version with my own XPLMGetAllMonitorBoundsOS to check that it's working and it looks ok.

#include <iostream>

#include <string>

#include <vector>



typedef void (*XPLMReceiveMonitorBoundsOS_f)( int   inMonitorIndex,

                                              int   inLeftPx,

                                              int   inTopPx,

                                              int   inRightPx,

                                              int   inBottomPx,

                                              void* inRefcon);



struct  MonitorBoundsStructure    // #1

{

    int     MonitorIndex;

    int     LeftBx;

    int     TopBx;

    int     RightBx;

    int     BottomBx;

};



static void LuaReceiveMonitorBoundsOS_t(int inMonitorIndex, int inLeftBx, int inTopBx,

                                        int inRightBx, int inBottomBx, void* refcon)

{

    auto& MonitorBounds = *reinterpret_cast<std::vector<MonitorBoundsStructure>*>(refcon);



    MonitorBounds.emplace_back(

        MonitorBoundsStructure{

            inMonitorIndex,

            inLeftBx,

            inTopBx,

            inRightBx,

            inBottomBx

        }

    );

}



void XPLMGetAllMonitorBoundsOS(XPLMReceiveMonitorBoundsOS_f callback, void* inRefcon)

{

    callback(0,100,0,100,100, inRefcon);

    callback(1,99,1,99,99, inRefcon);

    callback(2,100,1,100,100, inRefcon);

}



static int LuaXPLMGetAllMonitorBoundsOSTest() {

    std::vector<MonitorBoundsStructure> MonitorBounds;  // #2



    XPLMGetAllMonitorBoundsOS(LuaReceiveMonitorBoundsOS_t, &MonitorBounds);  // #3



    for (auto& m : MonitorBounds) {

        std::cout << m.MonitorIndex << "n";

    }



    return 1;

}



int main() {

    LuaXPLMGetAllMonitorBoundsOSTest();

}

Output

0

1

2