C++中的虚函数！

/*
 * test.cpp
 *
 *  Created on: 2013-4-19
 *      Author: Administrator
 */
#include <utils/RefBase.h>
#include <stdio.h>
namespace android {

class A: public RefBase {

};

class B {
public:
    virtual void p() {
        printf("!!!!!!!!!!!A\n");
    }
};

class C: public B {
public :
    
    void p()
    {
        printf("!!!!!!!!!!!B\n");
    }
};
extern "C" int main(int argc,char **argv) {
    A* pA = new A;
    {
        sp<A> spA(pA);
        wp<A> wpA(pA);
    }

    C c;
    c.p();
}

}

正常编译，运行结果：

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

第二种情况：

如果C中不实现p函数，及：

/*
 * test.cpp
 *
 *  Created on: 2013-4-19
 *      Author: Administrator
 */
#include <utils/RefBase.h>
#include <stdio.h>
namespace android {

class A: public RefBase {

};

class B {
public:
    virtual void p() {
        printf("!!!!!!!!!!!A\n");
    }
};

class C: public B {
public :

};
extern "C" int main(int argc,char **argv) {
    A* pA = new A;
    {
        sp<A> spA(pA);
        wp<A> wpA(pA);
    }

    C c;
    c.p();
}

}

正常编译，正常运行。结果是：

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

第三种情况：

如果B中不实现p函数。编译报错：

#include <utils/RefBase.h>
#include <stdio.h>
namespace android {

class A: public RefBase {

};

class B {
public:
    virtual void p();
};

class C: public B {
public :

};
extern "C" int main(int argc,char **argv) {
    A* pA = new A;
    {
        sp<A> spA(pA);
        wp<A> wpA(pA);
    }

    C c;
    c.p();
}
}
//报错：
**** Build of configuration Default for project T ****

E:\android\adt-bundle-windows-x86-20130219\android-ndk-r8e\ndk-build.cmd all 
"Compile++ thumb : t <= test.cpp
Executable     : t
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()'
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()'
collect2: ld returned 1 exit status
make: *** [obj/local/armeabi/t] Error 1

**** Build Finished ****

第四种情况：

如果B中函数p为这样：

#include <utils/RefBase.h>
#include <stdio.h>
namespace android {

class A: public RefBase {

};

class B {
public:
    virtual void p() = 0;
};

class C: public B {
public :

};
extern "C" int main(int argc,char **argv) {
    A* pA = new A;
    {
        sp<A> spA(pA);
        wp<A> wpA(pA);
    }

    C c;
    c.p();
}
}
//报错信息：
**** Build of configuration Default for project T ****

E:\android\adt-bundle-windows-x86-20130219\android-ndk-r8e\ndk-build.cmd all 
"Compile++ thumb : t <= test.cpp
jni/test.cpp: In function 'int android::main(int, char**)':
jni/test.cpp:31:4: error: cannot declare variable 'c' to be of abstract type 'android::C'
jni/test.cpp:20:7: note:   because the following virtual functions are pure within 'android::C':
jni/test.cpp:17:15: note:     virtual void android::B::p()
make: *** [obj/local/armeabi/objs/t/test.o] Error 1

**** 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。

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