Pointer problem using functions from non-object API in objects - c++

Question

I am writing some C++ code to get data from a SDR (software defined radio) and analyse it using an FFT library (FFTW3). The software uses an API (SDR_play_API and its DLL). I am trying to include the API functionality into an object. The library comes with a non-object based example program.
The IDE I am using is VS2105.
Three of the functions are callbacks to handle events from the hardware. The address of the callback functions that handle these events are passed to the API through a structure. I include the relevant code snippets from the example program, the header files and my own code after a description of the problem.
Though I have been on some C++ training - and taught others the basics - I am far from an expert and the problem goes beyond my understanding of pointers and objects.
The problem I have is one that has been discussed before on Stack Exchange. It is a problem of pointers to member functions.
The simple solution would seem to be to make the functions STATICs. However that causes problems because of some of the variables used in these functions. Without making them static then the compiler complains and if I make them static the linker complains because there is no matching definition for static variables in the library.
I have looked through Stack Exchange tried the solutions suggested but I can't seem to make any of them work - I get compiler errors.
So I feel ideally I just want to pass the pointers to the member functions to API through the structure ... but I have tried all sorts of variations on syntax without success. Can anyone help?
The API comes with an example program that does not use objects. So before I reveal my attempts to use the functions in an object, here is the code from the example program. First I will show where it assigns the location of the functions to the members of the structure, then I will include the structure definitions etc from the header file.
First then, the assignment:
// Assign callback functions to be passed to sdrplay_api_Init()
cbFns.StreamACbFn = StreamACallback;
cbFns.StreamBCbFn = StreamBCallback;
cbFns.EventCbFn = EventCallback;
Here, from the header file that comes with the API, is the definition of the structure
{
sdrplay_api_StreamCallback_t StreamACbFn;
sdrplay_api_StreamCallback_t StreamBCbFn;
sdrplay_api_EventCallback_t EventCbFn;
} sdrplay_api_CallbackFnsT;
and here, again from the header file that comes with the API is the definition of the data types - you can see the these are function pointers
typedef void (*sdrplay_api_StreamCallback_t)(short *xi, short *xq, sdrplay_api_StreamCbParamsT *params, unsigned int numSamples, unsigned int reset, void *cbContext);
typedef void (*sdrplay_api_EventCallback_t)(sdrplay_api_EventT eventId, sdrplay_api_TunerSelectT tuner, sdrplay_api_EventParamsT *params, void *cbContext);
here is one of the call back function declarations
void StreamACallback(short *xi, short *xq, sdrplay_api_StreamCbParamsT *params, unsigned int numSamples, unsigned int reset, void *cbContext)
{
...
}
So that all works fine and compiles - but it that works in a non-object scenario.
However when I make the functions a member of a class then I get errors. I have tried the following variations without success. Can someone please point me in the right direction - if I had had hair by now I wouldn't have anymore cause it would be on the floor all around me in clumps.
function definition:
void sdr_object::StreamACallback(short *xi, short *xq, sdrplay_api_StreamCbParamsT *params, unsigned int numSamples, unsigned int reset, void *cbContext)
{
...
}
Unsuccessful attempts to assign to structure include (including compiler error)
cbFns.StreamACbFn = this->StreamACallback;//"non standard syntax: use '&' to create a pointer to the member"
//cannot convert from 'void (__cdecl sre_object::*)(...) to sdrplay_api_StreanCallback_t
cbFns.StreamACbFn = &this->StreamACallback;//'&': illegal operation on bound member function experssion
cbFns.StreamACbFn = this->sdr_object::StreamACallback; //"non standard syntax: use '&' to create a pointer to the member"
//cannot convert from 'void (__cdecl sre_object::*)(...) to sdrplay_api_StreanCallback_t
cbFns.StreamACbFn = &this->sdr_object::StreamACallback; //'&': illegal operation on bound member function experssion
Any wisdom will e received most gratefully. (How to do it will be received even more gratefully ...!)

Answer 1

This answer comes with many thanks to Igor Tandetnik who suggested the solution in comments above.
Igor wrote "Make your callbacks static member functions. Pass this pointer for cbContext parameter to sdrplay_api_Init - you get that same pointer back in the callback. Now the callback can do static_cast<MyClass*>(cbContext)->memberFunction(params); . The actual logic can now be implemented in a non-static member function; the static one just serves as a glue between C++ implementation and C API"
My implementation of that is as follows:
Add to the class declaration in the header file an intermediary "glue" function for each event handler e.g.:
static void sdr_object::StreamACallback_static(short *xi, short *xq, sdrplay_api_StreamCbParamsT *params, unsigned int numSamples, unsigned int reset, void *cbContext)
{
static_cast<sdr_object*>(cbContext)->StreamACallback(xi, xq, params, numSamples, reset, cbContext);
};
For the function above, the pointer that is handed to the API then becomes
cbFns.StreamACbFn = this->StreamACallback_static;
In my particular example, I then had to do the same for the other two functions.
I did not have to change any other existing code.
Again, many thanks to Igor for the answer - I hope I have implemented it as he intended ... that said, what I have described works, so I imagine I have!