使用纯虚拟成员的地址进行虚拟呼叫。这合法吗？ [英] Virtual Calls using address of pure virtual member. Is it legal?

问题描述

我读回来（可能是在c.l.c ++。版主），虚拟函数调用可以被模板化。我尝试过以下几行。

I read sometime back (probably on c.l.c++.moderated) that virtual function calls can be templatized. I tried something on the following lines.

#include <iostream>

template<class T, class FUN> 
void callVirtual(T& t, FUN f){ 
   (*t.*f)(); 
} 


struct Base{ 
   virtual ~Base(){} 
   virtual void sayHi()=0; 
}; 


struct Derived : public Base{ 
   void sayHi(){ 
      std::cout << "Hi!" << std::endl; 
   } 
}; 


void Test(){ 
   Base* ptr = new Derived; 
   callVirtual(ptr,&Base::sayHi); 
} 

int main()
{
   Test();
   return 0;
}

Output:
Hi!

虽然在编译时给出了纯虚基本成员方法的地址，但是模板化的方法调用了正确的方法运行。
标准C ++中是否合法获取纯虚拟会员的地址？

The templatized method though given the address of pure virtual base member method at compile time calls the correct method at runtime. Is it legal in standard C++ to take address of a pure virtual member?

提前致谢

EDIT-1：我删除了问题的第二部分'它是如何工作的？'。看起来这就引起了人们的注意。

EDIT-1: I removed the second part of the question 'how does it work?'. Looks like that is what is catching attention.

EDIT-2：我搜索了clc ++。版主并遇到了这个链接（ http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread / 5ddde8cf1ae59a0d ）。由于标准不限制它，所以达成共识似乎是有效的。

EDIT-2: I searched c.l.c++.moderated and came across this link (http://groups.google.com/group/comp.lang.c++.moderated/browse_thread/thread/5ddde8cf1ae59a0d). The consensus seems like since the standard does not restrict it, it is vaild.

EDIT-3：阅读代码项目文章后（感谢ovanes），我认为编译器会做一些魔术。由于虚函数是通过vtable（特定于编译器）实现的，因此获取虚函数的地址总是会给出vtable中的偏移量。根据所使用的'this'指针，调用适当的函数（其地址位于偏移量）。我不确定如何证明这一点，因为标准没有说明任何内容！

EDIT-3: After reading the codeproject article (thanks to ovanes), i am thinking that the compilers do some magic. Since virtual functions are implemented via vtable (which is compiler specific), taking address of a virtual function always gives the offset in the vtable. Depending on the 'this' pointer used, the appropriate function (whose address is at the offset) is called. I am not sure how to vindicate this though as the standard does not say anything abt it!

推荐答案

以下文章广泛讨论成员C ++中的函数指针，它们是如何实现的以及缺陷在哪里。它还处理虚拟成员函数指针等等。我想它会回答你的所有问题。

This following article extensively discusses member function pointers in C++, how they are implemented and where are the flaws. It also handles virtual member function pointers and much more. I think it will answer all your questions.

http://www.codeproject.com/KB/cpp/FastDelegate.aspx

它还展示了如何在C ++中实现委托您可能陷入的陷阱。

It also shows how to implement delegates in C++ and what pitfalls you might trap in.

有善意的问候，

Ovanes

这篇关于使用纯虚拟成员的地址进行虚拟呼叫。这合法吗？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！