如何在同一个class的函数中获取class成员的函数指针？

Question

我被这个难住了。我有一个带有函数 DoTheThing1 的 class Foo，它接受一个指向带有 0 个参数的 void 函数的指针并调用该函数。

class Foo {
public:
   Foo () {}

   void DoTheThing1 (void (*theThing)()) {
      theThing();
   }
};

我有另一个 class Bar 有一个 Foo 的实例。 Class Bar 也有自己的函数 DoTheThing2 ，它尝试在其构造中将 DoTheThing2 的指针传递给 Foo's DoTheThing1.

class Bar {
public:
   Foo* foo = new Foo();

   Bar () {
      foo->DoTheThing1(&Bar::DoTheThing2);
   }

   void DoTheThing2 () {
      // Something happens.
   }

};

我在传入函数指针的行收到此错误 error C2664: 'void Foo::DoTheThing1(void (__cdecl *)(void))': cannot convert argument 1 from 'void (__cdecl Bar::* )(void)' to 'void (__cdecl *)(void)。

Bar () {
      foo->DoTheThing1(&Bar::DoTheThing2); /// Does not like.
   }

我不确定如何解决这个问题。似乎需要一些奇怪的演员表。

编辑

实际上，我的情况比仅从自身内部的 class 成员调用函数指针要复杂一些。我的代码实际上做的是将指针设置为一个变量，然后稍后调用它。

class Foo {
public:

   void (*m_onEvent) ();

   Foo () {}

   void SetTheThing (void (*theThing)()) {
      m_onEvent = theThing;
   }

   template <typename T>
   void SetTheThing (T&& theThing) {
      m_onEvent = theThing;
   }

   void DoTheThing1 () {
      m_onEvent();
   }
};

class Bar {
public:
   Foo* foo = new Foo();

   Bar () {
      foo->SetTheThing([this](){ DoTheThing2(); });  // error C2440:  '=': cannot convert from 'T' to 'void (__cdecl *)(void)'
      foo->SetTheThing(&DoTheThing2);                // '&' illegal operation on bound member function expression.
   }

   void DoTheThing2 () {
      std::cout << "I did the thing." << std::endl;
   }

};


int main () {

   Bar* bar = new Bar();
   bar->foo->DoTheThing1();

}

编辑

所以现在我正尝试使用 class 模板破解它，但我一直被这个错误阻止：Term does not evaluate to a function taking 0 arguments.

我正在尝试弄清楚我的函数指针为何不计算任何值。

template <typename U>
class Foo {
public:


   void (U::*m_theThing) ();

   Foo () {}

   void SetTheThing (void (U::*theThing)()) {
      m_theThing = theThing;
   }

   void DoTheThing1 () {
      m_theThing(); // Term does not evaluate to a function taking 0 arguments.
   }
};

class Bar {
public:
   Foo<Bar>* foo = new Foo<Bar>();

   Bar () {
      foo->SetTheThing(&Bar::DoTheThing2);
   }

   void DoTheThing2 () {
      std::cout << "I did the thing." << std::endl;
   }
};


int main () {
   Bar* bar = new Bar();
   bar->foo->DoTheThing1();
}

Answer 1

&Bar::DoTheThing2是成员函数指针，不是普通函数指针。因此错误。这是使用 lambdas 和 std::functional:

的解决方法

#include <functional>

class Foo {
public:
   Foo () {}

   void DoTheThing1 (std::function<void()>& theThing) {
      theThing();
   }
};

class Bar {
public:
   Foo* foo = new Foo();

   Bar () {
      foo->DoTheThing1([this](){ DoTheThing2(); });
   }

   void DoTheThing2 () {
      // Something happens.
   }

};

如果 std::functional 被证明是瓶颈，您可以改用模板函数：

#include <functional>

class Foo {
public:
   Foo () {}

   template <typename Callable> 
   void DoTheThing1 (Callable&& theThing) {
      theThing();
   }
};

class Bar {
public:
   Foo* foo = new Foo();

   Bar () {
      foo->DoTheThing1([this](){ DoTheThing2(); });
   }

   void DoTheThing2 () {
      // Something happens.
   }

};

编辑：

如果要存储指向成员函数的指针，还需要该成员函数的实例 class 以便稍后调用它。以下是您可以如何修复您的示例：

#include <iostream>

template <typename U>
class Foo {
public:
   void (U::*m_theThing) ();
   U* m_u;

   Foo (U* u): m_u{u} {}

   void SetTheThing (void (U::*theThing)()) {
      m_theThing = theThing;
   }

   void DoTheThing1 () {
      (m_u->*m_theThing)(); // Works fine.
   }

};

class Bar {
public:
   Foo<Bar>* foo = new Foo<Bar>(this);

   Bar () {
      foo->SetTheThing(&Bar::DoTheThing2);
   }

   void DoTheThing2 () {
      std::cout << "I did the thing." << std::endl;
   }
};


int main () {
   Bar* bar = new Bar();
   bar->foo->DoTheThing1();
}

Answer 2

旧方法：您需要一个模板来获取 class 和函数的专业化。

工作示例：

#include <iostream>

//For member function of class C
template <typename C = void>
struct TheCaller
{
    TheCaller() : theClass(nullptr), mf(nullptr) {}
    C* theClass;
    void (C::*mf)();

    void SetTheThing(C* aClass, void (C::*memberFunction)())
    {
        theClass = aClass;
        mf = memberFunction;
    }

    void CallTheThing()
    {
        if ( theClass )
            (theClass->*mf)();
    }
};

//Specialization for any function
template <>
struct TheCaller<void>
{
    TheCaller() : mf(nullptr) {}
    void (*mf)();

    void SetTheThing(void (*memberFunction)())
    {
        mf = memberFunction;
    }

    void CallTheThing()
    {
        if ( mf )
            mf();
    }
};

struct Bar
{
    void DoTheBarThing()
    { std::cout << "DoTheBarThing called" << std::endl; }
};

void AFunction()
{ std::cout << "AFunction called" << std::endl; }

int main()
{
    TheCaller<Bar> foo;
    Bar bar;
    foo.SetTheThing(&bar, &Bar::DoTheBarThing);
    foo.CallTheThing();

    TheCaller<> foo2;
    foo2.SetTheThing(&AFunction);
    foo2.CallTheThing();
}

如何在同一个class的函数中获取class成员的函数指针？

How to get the function pointer of a class member in a function of the same class?

c++

function-pointers

class