首先，看一下这几行简单的代码：

class  A

{

int  a;

public:

A(int aa):a(aa){};

void funa(){ a++;};

};

class B:public A

{

int b;

public:

B(int aa,int bb):A(aa),b(bb){};

virtual void funb()=0;//pure virtual function

};

class C

{

int c;

public:

C(int cc):c(cc){};

virtual void func()=0;

};

A a(1);//应该正确

C c(1);//应该错误

B  b(1,2);//对不对呢？

在C++语言中，拥有纯虚函数的类成为抽象类，该类不能实例化一个对象，必须在进一步继承中实现该函数，然后用派生类定义对象。那么，现在的问题是，基类为非抽象类，从他继承来的类是不是非抽象类呢？如果该继承类有纯虚函数，又属于抽象类还是非抽象类呢？还是先测试一下上面代码，看看结果：

第一次测试：（使用VC++6.0）

int main(int argc, char* argv[])
{
 A a(1);
 C c(1);
 B b(1,2);
}

看看结果：

F:\work2000\ddd\ddd.cpp(29) : error C2259: 'C' : cannot instantiate abstract class due to following members:
        F:\work2000\ddd\ddd.cpp(20) : see declaration of 'C'
F:\work2000\ddd\ddd.cpp(29) : warning C4259: 'void __thiscall C::func(void)' : pure virtual function was not defined
        F:\work2000\ddd\ddd.cpp(24) : see declaration of 'func'
F:\work2000\ddd\ddd.cpp(29) : error C2259: 'C' : cannot instantiate abstract class due to following members:
        F:\work2000\ddd\ddd.cpp(20) : see declaration of 'C'
F:\work2000\ddd\ddd.cpp(29) : warning C4259: 'void __thiscall C::func(void)' : pure virtual function was not defined
        F:\work2000\ddd\ddd.cpp(24) : see declaration of 'func'
F:\work2000\ddd\ddd.cpp(30) : error C2259: 'B' : cannot instantiate abstract class due to following members:
        F:\work2000\ddd\ddd.cpp(12) : see declaration of 'B'
F:\work2000\ddd\ddd.cpp(30) : warning C4259: 'void __thiscall B::funb(void)' : pure virtual function was not defined
        F:\work2000\ddd\ddd.cpp(17) : see declaration of 'funb'
F:\work2000\ddd\ddd.cpp(30) : error C2259: 'B' : cannot instantiate abstract class due to following members:
        F:\work2000\ddd\ddd.cpp(12) : see declaration of 'B'
F:\work2000\ddd\ddd.cpp(30) : warning C4259: 'void __thiscall B::funb(void)' : pure virtual function was not defined
        F:\work2000\ddd\ddd.cpp(17) : see declaration of 'funb'
Error executing cl.exe.
定义抽象类C 的对象肯定是错的，去掉C对象的定义，而编译结果也显示了B为抽象类。

再用D继承B,使其成为非抽象类

class D:public B

{

int d;

public:

D(int dd,int bb,int aa):B(bb,aa),d(dd){};

void fund(){d++;};

void dunb(){d--;};

};

int main(int argc, char* argv[])
{
 A a(1);
 //C c(1);
// B b(1,2);

D d(1,2,3);

return 0;
}

这次顺利编译通过。

所以得出的结论是：继承于非抽象类的子类，也可以通过在其内部声明纯虚函数而成为抽象类。