C++中的虚函数！

 1 /*
 2  * test.cpp
 3  *
 4  *  Created on: 2013-4-19
 5  *      Author: Administrator
 6  */
 7 #include <utils/RefBase.h>
 8 #include <stdio.h>
 9 namespace android {
10 
11 class A: public RefBase {
12 
13 };
14 
15 class B {
16 public:
17     virtual void p() {
18         printf("!!!!!!!!!!!A\n");
19     }
20 };
21 
22 class C: public B {
23 public :
24     
25     void p()
26     {
27         printf("!!!!!!!!!!!B\n");
28     }
29 };
30 extern "C" int main(int argc,char **argv) {
31     A* pA = new A;
32     {
33         sp<A> spA(pA);
34         wp<A> wpA(pA);
35     }
36 
37     C c;
38     c.p();
39 }
40 
41 }

正常编译，运行结果：

1 root@android:/ # t                                                             
2 !!!!!!!!!!!B

第二种情况：

如果C中不实现p函数，及：

 1 /*
 2  * test.cpp
 3  *
 4  *  Created on: 2013-4-19
 5  *      Author: Administrator
 6  */
 7 #include <utils/RefBase.h>
 8 #include <stdio.h>
 9 namespace android {
10 
11 class A: public RefBase {
12 
13 };
14 
15 class B {
16 public:
17     virtual void p() {
18         printf("!!!!!!!!!!!A\n");
19     }
20 };
21 
22 class C: public B {
23 public :
24 
25 };
26 extern "C" int main(int argc,char **argv) {
27     A* pA = new A;
28     {
29         sp<A> spA(pA);
30         wp<A> wpA(pA);
31     }
32 
33     C c;
34     c.p();
35 }
36 
37 }

正常编译，正常运行。结果是：

1 root@android:/ # t
2 !!!!!!!!!!!A

第三种情况：

如果B中不实现p函数。编译报错：

 1 #include <utils/RefBase.h>
 2 #include <stdio.h>
 3 namespace android {
 4 
 5 class A: public RefBase {
 6 
 7 };
 8 
 9 class B {
10 public:
11     virtual void p();
12 };
13 
14 class C: public B {
15 public :
16 
17 };
18 extern "C" int main(int argc,char **argv) {
19     A* pA = new A;
20     {
21         sp<A> spA(pA);
22         wp<A> wpA(pA);
23     }
24 
25     C c;
26     c.p();
27 }
28 }
29 //报错：
30 **** Build of configuration Default for project T ****
31 
32 E:\android\adt-bundle-windows-x86-20130219\android-ndk-r8e\ndk-build.cmd all 
33 "Compile++ thumb : t <= test.cpp
34 Executable     : t
35 E:/android/adt-bundle-windows-x86-20130219/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/http://www.cnblogs.com/http://www.cnblogs.com/arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi/objs/t/test.o: in function main:jni/test.cpp:32: error: undefined reference to 'android::B::p()'
36 E:/android/adt-bundle-windows-x86-20130219/android-ndk-r8e/toolchains/arm-linux-androideabi-4.6/prebuilt/windows/bin/../lib/gcc/arm-linux-androideabi/4.6/http://www.cnblogs.com/http://www.cnblogs.com/arm-linux-androideabi/bin/ld.exe: ./obj/local/armeabi/objs/t/test.o: in function vtable for android::C:test.cpp(.data.rel.ro._ZTVN7android1CE+0x8): error: undefined reference to 'android::B::p()'
37 collect2: ld returned 1 exit status
38 make: *** [obj/local/armeabi/t] Error 1
39 
40 **** Build Finished ****

第四种情况：

如果B中函数p为这样：

 1 #include <utils/RefBase.h>
 2 #include <stdio.h>
 3 namespace android {
 4 
 5 class A: public RefBase {
 6 
 7 };
 8 
 9 class B {
10 public:
11     virtual void p() = 0;
12 };
13 
14 class C: public B {
15 public :
16 
17 };
18 extern "C" int main(int argc,char **argv) {
19     A* pA = new A;
20     {
21         sp<A> spA(pA);
22         wp<A> wpA(pA);
23     }
24 
25     C c;
26     c.p();
27 }
28 }
29 //报错信息：
30 **** Build of configuration Default for project T ****
31 
32 E:\android\adt-bundle-windows-x86-20130219\android-ndk-r8e\ndk-build.cmd all 
33 "Compile++ thumb : t <= test.cpp
34 jni/test.cpp: In function 'int android::main(int, char**)':
35 jni/test.cpp:31:4: error: cannot declare variable 'c' to be of abstract type 'android::C'
36 jni/test.cpp:20:7: note:   because the following virtual functions are pure within 'android::C':
37 jni/test.cpp:17:15: note:     virtual void android::B::p()
38 make: *** [obj/local/armeabi/objs/t/test.o] Error 1
39 
40 **** Build Finished ****

说明：c++中的虚函数还是比较宽松的。只是没有想到C++中的基类虚函数居然可以自带实现。

解决 has virtual method 'p' but non-virtual destructor警告。

1 //给class B增加虚的析构函数，因为继承是从基类开始析构的。
2 //如果不添加会出现警告：Class '[C@3c9c92' has virtual method 'p' but non-virtual destructor
3 virtual ~B(){};