प्वाइंटर

Question

मैं सदस्य कार्यों से कुछ कॉलबैक बनाने के लिए कोशिश कर रहा हूँ अभ्यावेदन और सब कुछ ठीक था जब तक मैं एक टेम्पलेट वर्ग कॉलबैक वस्तु के रूप में 2 वर्गों से प्राप्त का उपयोग करने की कोशिश की जब मैं निम्नलिखित त्रुटि मिली:

error C2440: 'reinterpret_cast' : Pointers to members have different representations; cannot cast between them 

इस बात ने मुझे संकेत दिया कि सदस्य फ़ंक्शन पॉइंटर्स के अलग-अलग प्रतिनिधित्व होते हैं (doh!)

ये प्रतिनिधित्व क्या हैं? उनके बीच क्या अंतर है?

Answer 1

Danny Kalev explains this quite nicely:

The Underlying Representation of Pointers to Members

Although pointers to members behave like ordinary pointers, behind the scenes their representation is quite different. In fact, a pointer to member usually consists of a struct containing up to four fields in certain cases. This is because pointers to members have to support not only ordinary member functions, but also virtual member functions, member functions of objects that have multiple base classes, and member functions of virtual base classes. Thus, the simplest member function can be represented as a set of two pointers: one holding the physical memory address of the member function, and a second pointer that holds the this pointer. However, in cases like a virtual member function, multiple inheritance and virtual inheritance, the pointer to member must store additional information. Therefore, you can't cast pointers to members to ordinary pointers nor can you safely cast between pointers to members of different types.

To get a notion of how your compiler represents pointers to members, use the sizeof operator. In the following example, the sizes of a pointer to data member and a pointer to a member function are taken. As you can see, they have different sizes, hence, different representations:

struct A 
{ 
int x; 
void f(); 
}; 
int A::*pmi = &A::x; 
void (A::*pmf)() = &A::f; 
int n = sizeof (pmi); // 8 byte with my compiler 
int m = sizeof (pmf); // 12 bytes with my compiler 

Note that each of these pointers may have a different representation, depending on the class in question and whether the member function is virtual.

Answer 2

यह एक Microsoft बात है: वे सदस्य कार्यों अलग अभ्यावेदन है कि करने के लिए संकेत के उत्पादन की कीमत पर कुछ मामलों में छोटे सदस्य कार्यों की ओर इशारा करते हैं,, जैसा कि आपने अभी देखा है। इसे बंद करने के लिए कहीं भी एक स्विच है, ताकि सदस्यों के सभी पॉइंटर्स का एक ही प्रतिनिधित्व हो।