C ++理解cocos2d-x使用函数指针 [英] C++ understanding cocos2d-x use of function pointers

问题描述

我正在试验扩展cocos2d-x CCMenuItem组件，并遇到了一些我以前没有在C ++中看到的东西。如果有人用他们的函数指针声明详细说明什么是有帮助的。

大多数cocos2d-x对象的基类是CCObject，它有以下定义

  class CC_DLL CCObject：public CCCopying 
 {
 public：
 //代码省略
}; 
 
 //我有一个关于
的问题typedef void（CCObject :: * SEL_SCHEDULE）（float）; 
 typedef void（CCObject :: * SEL_CallFunc）（）; 
 typedef void（CCObject :: * SEL_CallFuncN）（CCNode *）; 
 typedef void（CCObject :: * SEL_CallFuncND）（CCNode *，void *）; 
 typedef void（CCObject :: * SEL_CallFuncO）（CCObject *）; 
 typedef void（CCObject :: * SEL_MenuHandler）（CCObject *）; 
 typedef void（CCObject :: * SEL_EventHandler）（CCEvent *）; 
 typedef int（CCObject :: * SEL_Compare）（CCObject *）; 
 
 #define schedule_selector（_SELECTOR）（SEL_SCHEDULE）（& _SELECTOR）
 #define callfunc_selector（_SELECTOR）（SEL_CallFunc）（& _SELECTOR）
 #define callfuncN_selector （SEL_CallFuncN）（& _SELECTOR）
 #define callfuncND_selector（_SELECTOR）（SEL_CallFuncND）（& _SELECTOR）
 #define callfuncO_selector（_SELECTOR）（SEL_CallFuncO）（& _SELECTOR）
 #define menu_selector（_SELECTOR）（SEL_MenuHandler）（& _SELECTOR）
 #define event_selector（_SELECTOR）（SEL_EventHandler）（& _SELECTOR）
 #define compare_selector（_SELECTOR）（SEL_Compare）（& _SELECTOR）
  

所以在CCObject类之外，但是在cocos2d命名空间中，存在一个函数指针和帮助宏的声明使用它们。我在调用函数指针的这些声明时是否正确？

我理解typedef将一个关键字与一个类型相关联（参见 Typedef函数指针？），返回类型必须为void，函数必须有一个CCObject *的强制参数。但是，我在理解它的适当用法，范围以及C ++如何处理通过另一个函数传递一个函数的方法上迷失了。

问题1 >

我没有按照如何解释声明的函数指针的作用域。在它们的声明中，它们显示由CCObject类限定的函数指针。我应该如何解释这？这是否意味着当它被分配的功能作为成员函数属于CCObject？这是令人困惑的，因为它定义在类体之外，但范围为CCObject。

  typedef void（CCObject :: * SEL_MenuHandler）（CCObject *）; 
  

问题2

在cocos2d-x CCMenuItem类中，它有一个下面定义的静态工厂方法

 这个？是一个函数对待这里的对象吗？ 
 static CCMenuItem * create（CCObject * rec，SEL_MenuHandler selector）; 
 
 
 
 CCMenuItem * CCMenuItem :: create（CCObject * rec，SEL_MenuHandler selector）
 {
 CCMenuItem * pRet = new CCMenuItem 
 pRet-> initWithTarget（rec，selector）; 
 pRet-> autorelease（）; 
 return pRet; 
} 
 
 bool CCMenuItem :: initWithTarget（CCObject * rec，SEL_MenuHandler selector）
 {
 setAnchorPoint（ccp（0.5f，0.5f））; 
 m_pListener = rec; 
 m_pfnSelector = selector; 
 m_bEnabled = true; 
 m_bSelected = false; 
 return true; 
} 
 
 //来自CCMenuItem头的代码段
 protected：
 CCObject * m_pListener; 
 SEL_MenuHandler m_pfnSelector; //成员变量存储指向函数的指针？ 
 int m_nScriptTapHandler; 
}; 
 

所以这个typedef意味着当我传递一个函数时，我传递的值与指针？如果函数没有通过指针，C ++如何处理。是一个函数被视为具有复制构造函数的对象？

我感谢任何帮助和建议。感谢

解决方案

void（CCObject :: *）（CCObject *）是方法指针类型（指向成员的指针），而不是普通的函数指针。它是一个指向 CCObject 类的实例方法的指针，它接受 CCObject * 类型的参数。类的类型它的一个方法是指针类型的一部分（由 CCObject :: 表示），类似于参数（因为在下面，指向current对象是所有实例方法的隐藏参数， this ）。

typedef 只是定义 SEL_MenuHandler 为该方法指针类型的同义词。

一个方法指针，你需要提供一个实例作为 this 以及参数，使用这样的语法：

  CCObject * object; 
 CCObject * anotherObject; 
 SEL_MenuHandler methodPointer; 
（object-> * methodPointer）（anotherObject）; 
 //或等效地：（（* object）。* methodPointer）（anotherObject）; 
 





如果函数没有通过指针传递，C ++如何处理。
是一个类似于具有复制构造函数的对象的函数？

在C / C ++中， 函数类型或方法类型的表达式。任何时候，当你试图得到的东西函数类型，它会自动转换为指针函数类型。

I am experimenting around with extending cocos2d-x CCMenuItem components and came across something I have not seen before in C++. It would be helpful if someone would elaborate on what is going on with their function pointer declarations

The base class for most cocos2d-x objects is CCObject which has the following definition

class CC_DLL CCObject : public CCCopying
{
public:
    // Code omitted 
};

// The part in which I have a question about
typedef void (CCObject::*SEL_SCHEDULE)(float);
typedef void (CCObject::*SEL_CallFunc)();
typedef void (CCObject::*SEL_CallFuncN)(CCNode*);
typedef void (CCObject::*SEL_CallFuncND)(CCNode*, void*);
typedef void (CCObject::*SEL_CallFuncO)(CCObject*);
typedef void (CCObject::*SEL_MenuHandler)(CCObject*);
typedef void (CCObject::*SEL_EventHandler)(CCEvent*);
typedef int (CCObject::*SEL_Compare)(CCObject*);

#define schedule_selector(_SELECTOR) (SEL_SCHEDULE)(&_SELECTOR)
#define callfunc_selector(_SELECTOR) (SEL_CallFunc)(&_SELECTOR)
#define callfuncN_selector(_SELECTOR) (SEL_CallFuncN)(&_SELECTOR)
#define callfuncND_selector(_SELECTOR) (SEL_CallFuncND)(&_SELECTOR)
#define callfuncO_selector(_SELECTOR) (SEL_CallFuncO)(&_SELECTOR)
#define menu_selector(_SELECTOR) (SEL_MenuHandler)(&_SELECTOR)
#define event_selector(_SELECTOR) (SEL_EventHandler)(&_SELECTOR)
#define compare_selector(_SELECTOR) (SEL_Compare)(&_SELECTOR)

So outside of the CCObject class, but within the cocos2d namespace, there exists a declaration for function pointers and helper macros to use them. Am I correct in calling these declarations for function pointers?

I understand that typedef is associating a keyword with a type (See Typedef function pointer?) and that the return type is must be void and the function must have one mandatory argument of CCObject *. However I am lost in understanding its appropriate usage, scope, and how C++ treats passing a function through another function.

Question 1

I am not following how to interpret the scope of the declared function pointer. In their declaration, they show the function pointer being scoped by the CCObject class. How should I interpret this? Does this mean when it is assigned that function belongs as a member function to CCObject? This is confusing to me as it is defined outside of the class body but scoped with CCObject.

typedef void (CCObject::*SEL_MenuHandler)(CCObject*);

Question 2

In the cocos2d-x CCMenuItem class, it has a static factory method defined below

// How does C++ treat the this? Is a function treated like an object here?
static CCMenuItem* create(CCObject *rec, SEL_MenuHandler selector);



   CCMenuItem* CCMenuItem::create(CCObject *rec, SEL_MenuHandler selector)
{
    CCMenuItem *pRet = new CCMenuItem();
    pRet->initWithTarget(rec, selector);
    pRet->autorelease();
    return pRet;
}

bool CCMenuItem::initWithTarget(CCObject *rec, SEL_MenuHandler selector)
{
    setAnchorPoint(ccp(0.5f, 0.5f));
    m_pListener = rec;
    m_pfnSelector = selector;
    m_bEnabled = true;
    m_bSelected = false;
    return true;
}

// A snippet from CCMenuItem header
protected:
    CCObject*       m_pListener;
    SEL_MenuHandler    m_pfnSelector; // member variable which stores a pointer to a  function?
    int             m_nScriptTapHandler;
};

So does this typedef mean that when I pass a function it, I am passing by value with a pointer? How would C++ handle this if the function was not passed by pointer. Is a function treated like an object with a copy constructor?

I appreciate any help and advice. Thanks

解决方案

void (CCObject::*)(CCObject*) is a method pointer type (one type of pointer to member), not an ordinary function pointer. It's a pointer that can point to an instance method of the CCObject class that takes a parameter of CCObject* type. The type of class it's a method of is part of the pointer type (denoted by the CCObject::), similar to the parameters (because underneath, a pointer to the "current object" is a hidden parameter to all instance methods, this).

The typedef simply defines SEL_MenuHandler to be a synonym for that method pointer type.

To use a method pointer, you need to provide both an instance to act as this, as well as the arguments, using a syntax like this:

CCObject* object;
CCObject* anotherObject;
SEL_MenuHandler methodPointer;
(object->*methodPointer)(anotherObject);
// or equivalently: ((*object).*methodPointer)(anotherObject);

How would C++ handle this if the function was not passed by pointer. Is a function treated like an object with a copy constructor?

In C/C++, it is not possible to have an expression of "function type" or "method type". Any time you try to get something of "function type", it is automatically converted to a "pointer to function" type.

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