将 std::function 绑定到不同对象实例的相同函数

Question

是否可以重新绑定 std::function 以指向相同的函数但具有不同的对象实例？

假设我有一个对象，该对象具有绑定到另一个函数的 std::function，但如果该对象被复制到另一个实例，我想将 std::function 重新绑定到那个新实例而不是旧实例。

#include "stdafx.h"
#include <iostream>
#include <functional>

class EventHandler
{
public:
    int Num;
    std::function<int()> OnEvent;

    EventHandler (int inNum)
    {
        Num = inNum;
    }

    EventHandler (const EventHandler& other)
    {
        Num = other.Num;
        OnEvent = other.OnEvent; //TODO:  Need some way to redirect the std::function to the new instance rather than having the delegate point to the original object's handler.
    }

    int HandleEvent ()
    {
        return Num;
    }
};

int main()
{
    EventHandler a(4);
    a.OnEvent = std::bind(&EventHandler::HandleEvent, a);
    EventHandler b(a);
    b.Num = 5;
    //Uncommenting the line below is a manual way of redirecting event handler to the new instance.
    //b.OnEvent = std::bind(&EventHandler::HandleEvent, b);

    int aResult = a.OnEvent();
    int bResult = b.OnEvent();

    //This will print out 4 and 4 instead of 4 and 5 since b is still bound to a's event handler.
    std::cout << "aResult=" << aResult << "  bResult=" << bResult << '\n';

    return 0;
}

我愿意使用 std::function 的包装器来存储其他信息。

Answer 1

AFAIK 你问的是不可能的，但我认为你可以做一个解决方法：

class EventHandler
{
public:
    int Num;
    std::function<int()> OnEvent;

    template <typename Func>
    EventHandler (int inNum, Func on_event)
    {
        Num = inNum;
        OnEvent = [=]() { return (this->*on_event)(); };
    }

    EventHandler (const EventHandler& other): EventHandler(other.Num, &EventHandler::HandleEvent) {}

    int HandleEvent ()
    {
        return Num;
    }
};

int main()
{
    EventHandler a(4, &EventHandler::HandleEvent);
    EventHandler b(a);
    b.Num = 5;

    int aResult = a.OnEvent();
    int bResult = b.OnEvent();

    //This will print out 4 and 4 instead of 4 and 5 since b is still bound to a's event handler.
    std::cout << "aResult=" << aResult << "  bResult=" << bResult << '\n';

    return 0;
}

这会根据需要打印 "aResult=4 bResult=5"。另外，我认为通过使用更多的元编程魔法，我们可以尝试美化语法。

让我知道这是否适合你。

Answer 2

您的事件处理程序的作用应取决于调用它的实例。因此，从逻辑上讲，解决问题的正确方法是将实例作为参数提供给处理函数，例如

#include <iostream>
#include <functional>

class EventHandler
{
private:
  std::function<int(EventHandler &)> handlingFunction;

public:
  int Num;

  EventHandler (int inNum)
  : handlingFunction ([] (EventHandler &) -> int { throw 0; })
  , Num (inNum)
  { }

  void SetHandlingFunction (std::function<int(EventHandler &)> f) {
     handlingFunction = f;
  }

  // for convenience, if the handling function is a member
  void SetHandlingFunction (int EventHandler::*mf ()) {
      handlingFunction = 
         [mf] (EventHandler & myself) -> int { return myself.*mf (); }
      ;
  }

  int OnEvent () {
     return handlingFunction (*this);
  }

  int HandleEvent ()
  {
     return Num;
  }
};

int main()
{
   EventHandler a(4);
   a.SetHandlingFunction ( [] (EventHandler & h) -> int { return h.HandleEvent (); } );

   // or
   a.SetHandlingFunction (&EventHandler::HandleEvent);

   EventHandler b(a);
   b.Num = 5;

   int aResult = a.OnEvent();
   int bResult = b.OnEvent();

   std::cout << "aResult=" << aResult << "  bResult=" << bResult << '\n';

   return 0;
}

当然，如果你的处理函数总是是一个成员函数，你可以简单地用指向成员函数的指针替换std::function。

请注意，您应该在 EventHandler class 的构造函数中正确初始化 handlingFunction 成员，例如通过将其设置为虚拟函数。

Answer 3

下面的代码引入了一个binding_function<R(Args...)>，它的调用方式类似于function<R()>，构造后可以随时重新绑定参数（假设不是nullptr）。

#include <functional>
#include <tuple>
#include <utility>
#include <memory>
#include <iostream>

template <typename T>
class binding_function;

template <typename R, typename... Args>
class binding_function<R(Args...)> : std::function<R()>
{
  using base_function = std::function<R(Args...)>;
  using binded_function = std::function<R()>;
  base_function base;

public:
  binding_function() = default;

  template <typename BaseF, typename... TArgs>
  binding_function(BaseF&& f, TArgs&&... args)
    : base(std::forward<BaseF>(f)) {
    rebind(std::forward<TArgs>(args)...);
  }

  template <typename... TArgs>
  void rebind(TArgs&&... args)
  {
    static_cast<binded_function&>(*this) = 
      std::bind(base, std::forward<TArgs>(args)...);
  }

  using binded_function::operator();
};

class EventHandler
{
public:
    // change type of OnEvent to binding_function
    binding_function<int(EventHandler)> OnEvent;

    // others remain the same
};

int main()
{
    EventHandler a(4);

                // first binding
    a.OnEvent = {&EventHandler::HandleEvent, a};
    EventHandler b(a);
    b.Num = 5;
    b.OnEvent.rebind(b);  // rebinding

    int aResult = a.OnEvent();
    int bResult = b.OnEvent();

    //This will print out 4 and 4 instead of 4 and 5 since b is still bound to a's event handler.
    std::cout << "aResult=" << aResult << "  bResult=" << bResult << '\n';

    return 0;
}

Answer 4

我扩展了 user1887915 的回答以允许带有参数的函数：

#include <functional>
#include <tuple>
#include <utility>
#include <memory>
#include <iostream>

template <typename T>
class binding_function;

template <typename R, typename... Args, typename SelfType>
class binding_function<R(SelfType, Args...)> : std::function<R(Args...)>
{
  using base_function = std::function<R(SelfType, Args...)>;
  using binded_function = std::function<R(Args...)>;
  base_function base;

public:
  binding_function() = default;

  template <typename BaseF, typename... TArgs>
  binding_function(BaseF&& f, SelfType t, TArgs&&... args)
    : base(std::forward<BaseF>(f)) {
    rebind(std::forward<SelfType>(t), std::forward<TArgs>(args)...);
  }

  template <typename T, typename... TArgs>
  void rebind(T&& t, TArgs&&... args)
  {
    static_cast<binded_function&>(*this) =
      std::bind(base, std::forward<SelfType>(t), std::forward<TArgs>(args)...);
  }

  using binded_function::operator();
};



class EventHandler
{
public:
    int Num;
    binding_function<int(EventHandler, int)> OnEvent;


    EventHandler (int inNum)
    {
        Num = inNum;
    }

    EventHandler (const EventHandler& other)
    {
        Num = other.Num;
        OnEvent = other.OnEvent; //TODO:  Need some way to redirect the std::function to the new instance rather than having the delegate point to the original object's handler.
    }
    int HandleEvent (int value)
    {
        return Num + value;
    }
};
int main()
{
    EventHandler a(4);

                // first binding
    a.OnEvent = {&EventHandler::HandleEvent, a, std::placeholders::_1};
    EventHandler b(a);
    b.Num = 5;
    b.OnEvent.rebind(b, std::placeholders::_1);  // rebinding

    int aResult = a.OnEvent(1);
    int bResult = b.OnEvent(1);

    //This will print out 4 and 4 instead of 4 and 5 since b is still bound to a's event handler.
    std::cout << "aResult=" << aResult << "  bResult=" << bResult << '\n';

    return 0;
}

将 std::function 绑定到不同对象实例的相同函数

Binding a std::function to the same function of a different object instance

c++

c++11

std-function