如何通过成员函数作为参数? (PortAudio)

时间:2018-12-27 22:45:19

标签: c++ c++17 mingw-w64 portaudio

我正在尝试在portaudio中创建多个流。 这就是打开流所需要的

PaError Pa_OpenDefaultStream( PaStream** stream,
                              int numInputChannels,
                              int numOutputChannels,
                              PaSampleFormat sampleFormat,
                              double sampleRate,
                              unsigned long framesPerBuffer,
                              PaStreamCallback *streamCallback,
                              void *userData );

这是PaStreamCallback函数。

typedef int PaStreamCallback(
    const void *input, void *output,
    unsigned long frameCount,
    const PaStreamCallbackTimeInfo* timeInfo,
    PaStreamCallbackFlags statusFlags,
    void *userData );

当我尝试使用以下方法传递回调函数时,编译器不接受:

std::bind(&MyStreamClass::OutputRouting, this, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, std::placeholders::_4, std::placeholders::_5, std::placeholders::_6)

这是我得到的错误:

error: cannot convert 'std::_Bind_helper<false, int (MyStreamClass::*)(const void*, void*, long unsigned int, const PaStreamCallbackTimeInfo*, long unsigned int, void*), MyStreamClass*, const std::_Placeholder<1>&, const std::_Placeholder<2>&, const std::_Placeholder<3>&, const std::_Placeholder<4>&, const std::_Placeholder<5>&, const std::_Placeholder<6>&>::type' {aka 'std::_Bind<int (MyStreamClass::*(MyStreamClass*, std::_Placeholder<1>, std::_Placeholder<2>, std::_Placeholder<3>, std::_Placeholder<4>, std::_Placeholder<5>, std::_Placeholder<6>))(const void*, void*, long unsigned int, const PaStreamCallbackTimeInfo*, long unsigned int, void*)>'} to 'int (*)(const void*, void*, long unsigned int, const PaStreamCallbackTimeInfo*, PaStreamCallbackFlags, void*)' {aka 'int (*)(const void*, void*, long unsigned int, const PaStreamCallbackTimeInfo*, long unsigned int, void*)'}
 Pa_OpenDefaultStream(&stream, 0, 0, paFloat32, 0, 0, std::bind(&MyStreamClass::OutputRouting, this, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3, std::placeholders::_4, std::placeholders::_5, std::placeholders::_6), nullptr);

那么,有什么方法可以将我的成员函数作为参数传递?

2 个答案:

答案 0 :(得分:2)

您的回调应类似于:

int MyCallback(
    const void *input, void *output,
    unsigned long frameCount,
    const PaStreamCallbackTimeInfo* timeInfo,
    PaStreamCallbackFlags statusFlags,
    void *userData )
{
    return reinterpret_cast<MyStreamClass*>(userData)->OutputRouting(input, output, frameCount, timeInfo, statusFlags);
}

答案 1 :(得分:2)

有两种方法可以执行此操作。您可以使用void* userData参数从非成员(或静态)函数指向您的类,并从该函数访问您的类。或者,您可以使用该非成员(或静态)函数在对象上调用成员函数:

class MyClass
{
public:

    void register_callback_here()
    {
        Pa_OpenDefaultStream(stream, 2, 2, PaSampleFormat::whatever, 44100,
            1024, &MyClass::pa_callback_mapper, this); // PASS this!!!
    }

private:

    // The actual call back
    int callback(
        const void* input, void* output,
        unsigned long frameCount,
        const PaStreamCallbackTimeInfo* timeInfo,
        PaStreamCallbackFlags statusFlags)
    {
        // actual callback here
        return 0;
    }

    // a pass-through function that reinterprets the userData as the this pointer
    // then cals the member function using this.
    static int pa_callback_mapper(
        const void* input, void* output,
        unsigned long frameCount,
        const PaStreamCallbackTimeInfo* timeInfo,
        PaStreamCallbackFlags statusFlags,
        void* userData)
    {
        if(auto self = reinterpret_cast<MyClass*>(userData))
            return self->callback(input, output, frameCount, timeInfo, statusFlags);
        return 0;
    }
};

注册回叫时,您将this作为userData传递。回调在回调函数中将指针返回给您。您可以将其强制转换为您的类类型,然后通过它访问您的类,或将调用转发给成员函数(如本例所示)。