C++ understanding cocos2d-x use of function pointers
c++, cocos2d-x, function-pointers, typedef
Solution
`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.
Problem
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