Callback function and void pointer C++

Question

ps3000aGetStreamingLatestValues
(
int16_t handle,
ps3000aStreamingReady lpPs3000AReady,
void * pParameter
)

lpPs3000AReady, a pointer to your ps3000aStreamingReady callback function
pParameter, a void pointer that will be passed to the ps3000aStreamingReady callback. The callback may optionally use this pointer to return information to the application.

And here is the ps3000aStreamingReady callback function:

typedef void (CALLBACK *ps3000aStreamingReady)
(
int16_t handle,
int32_t noOfSamples,
uint32_t startIndex,
int16_t overflow,
uint32_t triggerAt,
int16_t triggered,
int16_t autoStop,
void * pParameter
)

My question is how I can declare pParameter in my main function? pParameter use as a communicator between those two functions and it's a void pointer because it might have to pass any kinds of data.Somehow I have to allocate memory before pass this void pointer as a arguments on those function. But how can do that?

* pParameter, a void pointer passed from ps3000aGetStreamingLatestValues.ps3000aStreamingReady callback function can write to this location to send any data, such as a status flag, back to the application. — Anik Islam
– Anik Islam, Commented Dec 11, 2020 at 15:30
void* userData is the C-way to capture extra data, C++ way with std::function allow capturing lambda and even custom functor. — Jarod42
– Jarod42, Commented Dec 11, 2020 at 15:39
You can pass a pointer to (almost) anything as a parameter that is declared as void*. And you can add an explicit cast (in the function call) if you really want to. — Adrian Mole
– Adrian Mole, Commented Dec 11, 2020 at 15:40
@AnikIslam which parameters do you need to get from pParameter so that you can call lpPs3000AReady? — Guillaume Racicot
– Guillaume Racicot, Commented Dec 11, 2020 at 16:11

Jarod42 · Accepted Answer · 2020-12-11 17:43:20Z

Traditional C-way for callback to allow extra parameter is to have extra void* that user provides.

using callback = bool (int, int, void* userData);

struct CallBack
{
    bool operator ()(int a, int b) { /*..*/ }

    // extra members ...
};

bool run_callback(int a, int b, void* userData) {
    CallBack* c = reinterpret_cast<CallBack*>(userData);

    return (*c)(a, b);
}

with possible usage:

CallBack t(true); // Care to lifetime, should still be there when `run_callback` is called
Register(&run_callback, &t); // Would call later run_callback(2, 3, &t);

If you don't need that extra parameter, you might pass your regular function and nullptr

bool run_callback(int a, int b, void*) {
    // ...
}

and

Register(&run_callback, nullptr); // Would call later run_callback(2, 3, nullptr);

For C++ way of type erasure, you might use std::function.

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