通过函数指针跨类调用虚函数.doc

通过函数指针跨类调用虚函数 Dec 02 2008 经过试验，发现了一些关于在不同类的实例间通过指针调用虚函数的方法，实验记录如下： A(f,/]{ ?? UC{mD24W. ?发现以下现象： A/W:K3 ?对于： ,_0LKGKN ?class A U~PHmNy*n ?{ U-d)vy#xP ?public: L ^TI ??virtual void fun(){cout1endl;} D9}+*HR ??virtual void fun2(){cout2endl;} |6Yz# ?}; ~Z~5PNm] ?class B:public A \rvHo2nmO ?{ tMTD* P ?public: wsFS{QIJ ??void fun(){cout3endl;} T$2VE6 [# ??void fun2(){cout4endl;} TCb`,l~C ?}; #~bRW | ?*C)TQharJ ?--------------------------Experiment 1----------------------------- :S(n$ ?1. A* a = new A;? // Create a new obj of class A Mul#wRVUg ?2. void (A::*funI)();? // Declared a func ptr A::funI sZ*ZRB7X ?3. funI = A::fun;? // Assign addr of A::fun() to func ptr A::funI 0Ye#:k? ?4. (a-*funI)();? // Call func (a-*funI)() which is actually A::fun() c S1e ?8%$mc7.R ?output: 1 -lt0p,2L ?--------------------------Experiment 2----------------------------- Q+ZrvB ?1. A* a = new B;? // Create a new obj of class B Qs d?B^@/ ?2. void (A::*funI)();? // Declared a func ptr A::funI C* v| ?3. funI = A::fun;? // Assign addr of A::fun() to func ptr A::funI w / PMHfDE ?4. (a-*funI)();? // Call func (a-*funI)() which is actually A::fun() iqmaqo ?n 9ao7$B ?output:?3 Gz]Mg`. ?analysis: The 3rd line doesnt seem to work. The fact is as talked above, that A::fun is not the real addr of A::fun() but the addr of vptr pointing to vtbl. The vtbl, which contains two slots in this example, holds two addrs, the real addr of A::fun() and A::fun2(), to O8T!Rz/y ?call virtual function A::fun() by explicitly calling (a-*funI)()(from a B type), one available method is as below. e}b-^8^ ?L(8!W ?--------------------------Experiment 2----------------------------- ^{:(ah@ ?A* a = new A; ??????//?Create a new obj of class A c0ALFLG7 : ?B* b = new B; ??????//?Create a new obj of class B {]WW.N-$t ?void (A::*funI)(); ?????//?Declared a func ptr A::funI =r|OhV# ?long l_VptrAddr; ?????//?Declared a long variable storing the addr of vptr ,4kd]m ?memcpy(l_Vp