37

我正在编写一个 GLFW 应用程序，其中我将函数调用包装到一个简单的类中。我在设置键回调时遇到问题。我的班级定义为：

class GAME 
{
private:
    bool running;
public:
    GAME();
    int execute();
    void events(int, int);
    int loop();
    int render();
};

执行函数为：

int GAME::execute() 
    {
        glfwOpenWindow(640, 320, 8, 8, 8, 8, 0, 0, GLFW_WINDOW);
        glfwSetWindowTitle("Viraj");
        glfwSetKeyCallback(events);
        running = true;
        while(glfwGetWindowParam(GLFW_OPENED))
        {
            glfwPollEvents();
            loop();
            render();
        }
        return 0;
    }

在 Visual Studio 2010 上编译以下代码会出现错误：

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

使用&GAME::events给出：

error C2664: 'glfwSetKeyCallback' : cannot convert parameter 1 from 'void (__thiscall GAME::* )(int,int)' to 'GLFWkeyfun' 1> There is no context in which this conversion is possible

4

7 回答 7

62

其他答案中提供的代码示例未描述如何将回调重定向到每个对象的成员函数，可能包含任意数量的对象。使您的班级成为单身人士将限制您的设计，并且不会扩展到多个 glfw 窗口。

可扩展的解决方案是将 glfw 窗口用户指针设置为您的对象，然后在回调中获取它，并调用成员函数：

class MyGlWindow
{
public:
     void mouseButtonPressed();
};

void makeWindow()
{
    GLFWwindow* glfwWindow;
    MyGlWindow* myWindow;

    /* ... Initialize everything here ... */

    glfwSetWindowUserPointer(glfwWindow, myWindow);

    auto func = [](GLFWwindow* w, int, int, int)
    {
        static_cast<MyGlWindow*>(glfwGetWindowUserPointer(w))->mouseButtonPressed( /* ... */ );
    }

    glfwSetMouseButtonCallback(glfwWindow, func);
}

此解决方案更短，适用于任意数量的窗口。

于 2015-02-22T17:09:56.050 回答
24

我也遇到了另一个 glfw 回调函数的问题，但我不想将我的类方法声明为static，因为我需要访问其中的成员变量。所以我尝试std::function并std::bind让我能够将实例方法绑定为回调函数，但不幸的是，在使用 C 回调时它不是一个选项。

这个问题的答案也在 GLFW 常见问题解答“我如何使用 C++ 方法作为回调”中说明：

您不能将常规方法用作回调，因为 GLFW 是一个 C 库并且不知道对象和 this 指针。如果您希望接收对 C++ 对象的回调，请使用静态方法或常规函数作为回调，将指向您希望调用的对象的指针存储在回调可到达的某个位置，并使用它来调用对象上的方法。

但是，这鼓励我为我的回调类应用单例模式并将其集成如下：

  • 我的类的回调方法仍然是静态的，所以它可以被指定/用作 glfw 回调
  • 此静态回调方法利用单例并将回调参数传递给实例方法
  • 这个实例方法实际上是处理回调参数，好处是可以访问成员变量

这是它的样子：

// Input.h (the actual callback class for glfwSetMouseButtonCallback)
class Input 
{
public:
    static Input& getInstance() // Singleton is accessed via getInstance()
    {
        static Input instance; // lazy singleton, instantiated on first use
        return instance;
    }

    static void mouseButtonCallback(int key, int action) // this method is specified as glfw callback
    {
        //here we access the instance via the singleton pattern and forward the callback to the instance method
        getInstance().mouseButtonCallbackImpl(key, action); 
    }

    void mouseButtonCallbackImpl(int key, int action) //this is the actual implementation of the callback method
    {
        //the callback is handled in this instance method           
        //... [CODE here]
    }

private:
    Input(void) // private constructor necessary to allow only 1 instance
    {
    }

    Input(Input const&); // prevent copies
    void operator=(Input const&); // prevent assignments
};

在我的 main.cpp 中：

Input &hexmap = Input::getInstance(); // initialize the singleton

//The glfw callback is set up as follows:   
glfwSetMouseButtonCallback( &Input::mouseButtonCallback); // specifying the static callback method, which internally forwards it to the instance method
于 2013-06-05T22:37:40.950 回答
20

有指向类成员方法的 C++ 语法，但您不能将它们传递给 C 风格的 API。C 理解函数调用和每个非静态对象方法，以你events为例，在 C 术语中看起来像这样的想法：void events(void* this, int, int);这意味着除了标准参数之外的每个方法也得到一个this静默传递的指针。

为了使您的eventsC 兼容，请制作static void events(int, int);. 这样，它将遵循 C 调用语义——它不需要this传递指针。您还必须以某种其他方式将您的对象传递给此回调（如果您在回调中需要此对象的数据）。

于 2011-10-06T16:05:54.030 回答
5

我遇到了同样的问题，在阅读了这个帖子后，我想出了一个类似的解决方案。我认为这样更干净一些。它基于静态函数，但它嵌套在我们设置所有内容的类中。

标题如下所示： 

class Application
    {
    public:
        ...
    private:
        ...
        void MousePositionCallback(GLFWwindow* window, double positionX, double positionY);
        void KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
        ...
        class GLFWCallbackWrapper
        {
        public:
            GLFWCallbackWrapper() = delete;
            GLFWCallbackWrapper(const GLFWCallbackWrapper&) = delete;
            GLFWCallbackWrapper(GLFWCallbackWrapper&&) = delete;
            ~GLFWCallbackWrapper() = delete;

            static void MousePositionCallback(GLFWwindow* window, double positionX, double positionY);
            static void KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods);
            static void SetApplication(Application *application);
        private:
            static Application* s_application;
        };
    };

和源代码：

void Application::GLFWCallbackWrapper::MousePositionCallback(GLFWwindow* window, double positionX, double positionY)
{
    s_application->MousePositionCallback(window, positionX, positionY);
}

void Application::GLFWCallbackWrapper::KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    s_application->KeyboardCallback(window, key, scancode, action, mods);
}

void Application::GLFWCallbackWrapper::SetApplication(Application* application)
{
    GLFWCallbackWrapper::s_application = application;
}

Application* Application::GLFWCallbackWrapper::s_application = nullptr;

void Application::MousePositionCallback(GLFWwindow* window, double positionX, double positionY)
{
    ...
}

void Application::KeyboardCallback(GLFWwindow* window, int key, int scancode, int action, int mods)
{
    ...
}

void Application::SetCallbackFunctions()
{
    GLFWCallbackWrapper::SetApplication(this);
    glfwSetCursorPosCallback(m_window, GLFWCallbackWrapper::MousePositionCallback);
    glfwSetKeyCallback(m_window, GLFWCallbackWrapper::KeyboardCallback);
}
于 2016-12-11T18:46:50.620 回答
4

受 N0vember 回答的启发，我向您展示了更通用和动态的解决方案：

class MyGlWindow {
public:
    std::function<void(MyGlWindow*)> onClose;
    std::function<void(MyGlWindow*, int, int, int)> onMouseClick = [](auto self, int, int, int) { /*some default behavior*/ };
};

void makeWindow() {
    GLFWwindow* glfwWindow;
    MyGlWindow* myWindow;

    /* ... Initialize everything here ... */

    glfwSetWindowUserPointer(glfwWindow, myWindow);

    #define genericCallback(functionName)\
        [](GLFWwindow* window, auto... args) {\
            auto pointer = static_cast<MyGlWindow*>(glfwGetWindowUserPointer(window));\
            if (pointer->functionName) pointer->functionName(pointer, args...);\
        }

    glfwSetWindowCloseCallback(glfwWindow, genericCallback(onClose));
    glfwSetMouseButtonCallback(glfwWindow, genericCallback(onMouseClick));

    myWindow->onMouseClick = [](auto self, int, int, int) {
        std::cout << "I'm such a rebel" << std::endl;
        self->onClose = [](auto self) {
            std::cout << "I'm such a rebellion" << std::endl;
        };
    };
}
于 2016-08-29T19:19:35.683 回答
1

在头文件中将 events(int, int) 设置为静态方法。这为我解决了这个问题。

class GAME 
{
private:
    bool running;
public:
    GAME();
    int execute();
    static void events(int, int); //Changed here to static void 
    int loop();
    int render();
};
于 2015-07-17T09:50:26.740 回答
0

这是对帮助我解决相同问题的可能解决方案的有用讨论，我正在添加我的解决方案以防它被证明有用。

问题陈述

我的方案比 BIC、L.Seionis 和 N0vember 所解决的方案更普遍。特别是，我的用例要求：

  • 通常，回调必须可以访问实例的数据
  • 可以使用一组通用的响应处理程序创建许多应用程序
  • 在一个应用程序中，可以创建任意数量的窗口
  • 附加到每个窗口的回调集合应该从某个可能的响应者库中混合和匹配。

建议的解决方案使用

简单的单例设计不再解决问题。相反，我提供了一个GLFWResponder处理所有设置复杂性的超类。为了使用该类并将响应附加到窗口，这是必需的。

// Implement custom responder
class MyResponder : public GLFWResponder {
  public:
    virtual void cursor_position_callback(GLFWwindow* w, double x, double y) {...}
    ... override relevant callbacks ...
 };

// in main ************************************************

// Assuming initialized GLFWwindow* my_window and my_other_window

MyResponder resp;
MyResponder resp2;  // Can be another subclass of GLFWResponder

// Two responders can respond to same window
resp.respond_to(my_window, GLFWResponder::CURSOR_POSITION);
resp2.respond_to(my_window, GLFWResponder::CURSOR_POSITION);

// One responder can respond to multiple windows
resp2.respond_to(my_other_window, GLFWResponder::CURSOR_POSITION);

// One window can have different handlers for different events
resp.respond_to(my_other_window, GLFWResponder::CURSOR_ENTER);

建议的解决方案实施

这是GLFWResponder实现的草图，功能齐全，但有一些 TODO。可能对性能有一些影响，我还没有调查过。

// GLFWResponder.h ************************************************
/**
 * Responder superclass that allows subclasses to handle events from multiple
 * GLFW windows (which have only C API for callbacks).
 * Callbacks are automatically cleaned up when responder goes out of scope.
 */
class GLFWResponder {
 public:
  virtual ~GLFWResponder();

  // Interface -----------------------------------
  enum GLFWEventType {
    CURSOR_POSITION = 0,
    CURSOR_ENTER = 1
    // TODO: add support for other callbacks
  };

  void respond_to(GLFWwindow* window, GLFWEventType event);

  bool does_respond_to(GLFWwindow* window, GLFWEventType event) const;

  // Subclasses implement ------------------------
  virtual void cursor_position_callback(GLFWwindow* window, double xpos, double ypos);

  virtual void cursor_enter_callback(GLFWwindow* window, int entered);

  // TODO: add support for other callbacks


  // Under the hood ------------------------------
  static std::set<GLFWResponder*> getResponders(GLFWwindow* windo, GLFWEventType event);

 private:
  // Windows and events that this instance responds to
  std::set<std::pair<GLFWwindow*, GLFWEventType> > enabled_events_;

  // Global responders keyed by events they respond to
  // (each responder knows which windows it responds to)
  static std::map<GLFWEventType, std::set<GLFWResponder*> > responders_;
}; 

// GLFWResponder.cpp **************************************************
namespace {

void cursor_position_callback_private(GLFWwindow* window, double xpos, double ypos) {
  for (GLFWResponder* r : GLFWResponder::getResponders(window, GLFWResponder::CURSOR_POSITION)) {
    r->cursor_position_callback(window, xpos, ypos);
  }
}

void cursor_enter_callback_private(GLFWwindow* window, int entered) {
  for (GLFWResponder* r : GLFWResponder::getResponders(window, GLFWResponder::CURSOR_ENTER)) {
    r->cursor_enter_callback(window, entered);
  }
}

} // namespace

std::map<GLFWResponder::GLFWEventType, std::set<GLFWResponder*> > GLFWResponder::responders_;

GLFWResponder::~GLFWResponder() {
  for (auto& pr : responders_) {
    pr.second.erase(this);
  }
  // TODO: also clean up window's callbacks
}

void GLFWResponder::respond_to(GLFWwindow* window, GLFWResponder::GLFWEventType event) {
  enabled_events_.insert(std::make_pair(window, event));
  responders_[event].insert(this);
  if (event == CURSOR_POSITION) {
    glfwSetCursorPosCallback(window, cursor_position_callback_private);
  } else if (event == CURSOR_ENTER) {
    glfwSetCursorEnterCallback(window, cursor_enter_callback_private);
  } else {
    // TODO: add support for other callbacks
    LOG(FATAL) << "Unknown GLFWResponder event: " << event;
  }
}

bool GLFWResponder::does_respond_to(GLFWwindow* window, GLFWEventType event) const {
  return enabled_events_.find(std::make_pair(window, event)) != enabled_events_.end();
}

std::set<GLFWResponder*> GLFWResponder::getResponders(
    GLFWwindow* window, GLFWEventType event) {
  std::set<GLFWResponder*> result;
  auto it = responders_.find(event);
  if (it != responders_.end()) {
    for (GLFWResponder* resp : it->second) {
      if (resp->does_respond_to(window, event)) {
        result.insert(resp);
      }
    }
  }
  return result;
}

void GLFWResponder::cursor_position_callback(
    GLFWwindow* window, double xpos, double ypos) {
  // TODO: fail with message "GLFWResponder::do_respond called on a subclass that does not implement a handler for that event"
}

void GLFWResponder::cursor_enter_callback(GLFWwindow* window, int entered) {
  // TODO: fail with message "GLFWResponder::do_respond called on a subclass that does not implement a handler for that event"
}
于 2017-05-01T18:32:46.750 回答