非成员函数指针作为 API 中成员函数的回调 [英] non-member function pointer as a callback in API to member function

问题描述

我使用的 API 要求我将函数指针作为回调传递.我正在尝试在我的 C++ 类中使用此 API，但出现编译错误.

I'm using an API that requires me to pass a function pointer as a callback. I'm trying to use this API from my class in C++ but I'm getting compilation errors.

API 定义为:

typedef void (__stdcall *STREAM_CALLBACK)(void *userdata);

__declspec(dllimport) int __stdcall set_stream_callback(
    STREAM_CALLBACK streamCB, void *userdata);

第三方提供的一个示例文件是:

One example file, provided by the third party, is:

void __stdcall streamCB(void *userdata)
{
  // callback implementation
}

int main(int argc, const char argv[])
{
  int mid = 0;
  set_stream_callback(streamCB, &mid);
}

而且效果很好.但是，当我尝试在课堂上使用它时，出现错误:

And that works fine. However when I try to use that in a class, I have an error:

错误 C3867:'MyClass::streamCB':函数调用缺少参数列表；使用&MyClass::streamCB"创建指向成员的指针

error C3867: 'MyClass::streamCB': function call missing argument list; use '&MyClass::streamCB' to create a pointer to member

使用建议

&MyClass::streamCB

&MyClass::streamCB

不起作用.我知道 set_stream_callback 只接受非成员函数.

doesn't work. I understood that the set_stream_callback only accepts a non-member function.

这个问题非常类似于如何将类成员函数作为回调?其中约翰内斯提出了一个简洁的建议，但我不太明白.任何人都可以扩展一点，如果我是对的，它与这个问题有关吗?

The problem is very similar to How can I pass a class member function as a callback? in which Johannes makes a concise suggestion, however I do not understand it very well. Could anyone expand a bit, if I am correct that it is relevant to this question?

我试过了:

void __stdcall MyClass::streamCB(void *userdata)
{
  // callback implementation
}

static void MyClass::Callback( void * other_arg, void * this_pointer ) {
    MyClass * self = static_cast<ri::IsiDevice*>(this_pointer);
    self->streamCB( other_arg );
}

//and in the constructor
int mid = 0;
set_stream_callback(&MyClass::Callback, &mid);

但是

错误 C2664:set_stream_callback":无法将参数 1 从'void (__cdecl *)(void *,void *)' 到 'STREAM_CALLBACK'

error C2664: 'set_stream_callback' : cannot convert parameter 1 from 'void (__cdecl *)(void *,void *)' to 'STREAM_CALLBACK'

我该如何解决这个问题?

How do I get around this?

Edit1:另外，我想在 streamCB 回调中使用 userdata.

Also, I want to use userdata inside the streamCB callback.

推荐答案

您可以通过将 userdata 转换为 MyClass 的属性来实现您想做的事情.那么您就不必将它传递给 MyClass::Callback，这是不可能的，因为您只能传递一个参数，而且它将是对象实例.这是一个例子.

You can achieve what you want to do by turning the userdata into a property of MyClass. Then you don't have to pass it to MyClass::Callback, which would be impossible, since you can only pass one parameter, and it would be the object instance. Here's an example.

void __stdcall MyClass::streamCB()
{
  // callback implementation
}

static void MyClass::Callback(void * this_pointer ) {
    MyClass * self = static_cast<MyClass>(this_pointer);
    self->streamCB();
}

MyClass::MyClass(void *userdata) {
    // do whatever you need to do with userdata
    // (...)
    // and setup the callback at C level
    set_stream_callback(&MyClass::Callback, (void *)this);
}

在您的示例中，int mid 变量将成为该类的属性，因此可以从回调实现 streamCB 访问.

In your example, the int mid variable would become a property of that class, and thus be accessible from the callback implementation streamCB.

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