Question

C++中的绝对难题！！！哪位高手可以解决这个问题呢？

#include<iostream>
#include<string>
using namespace std;
class Fish
{
public:
virtual Fish* Clone()=0;
virtual void Swim();
};
class Tuna:public Fish
{
public:
Fish* Clone()
{
return new Tuna(*this);
}
void Swim()
{
cout<<"Tuna swims fast in the sea"<<endl;
}
};
class Carp:public Fish
{
Fish* Clone()
{
return new Carp(*this);
}
void Swim()
{
cout<<"Carp swims slow in the lake"<<endl;
}
};
int main()
{
const int ARRAY_AIZE=4;
Fish* myFishes[ARRAY_AIZE]={NULL};
myFishes[0]=new Tuna();
myFishes[1]=new Carp();
myFishes[2]=new Tuna();
myFishes[3]=new Carp();
Fish* myNewFishes[ARRAY_AIZE];
for(int Index=0;Index<ARRAY_AIZE;++Index)
myNewFishes[Index]=myFishes[Index]->Clone();
for(int Inde=0;Inde<ARRAY_AIZE;++Inde)
myNewFishes[Inde]->Swim();
for(int Ind=0;Ind<ARRAY_AIZE;++Ind)
{
delete myFishes[Ind];
delete myNewFishes[Ind];
}

return 0;
}

以上这个函数是可以通过编译的。但是一点击运行就报错了！unresolved external symbol "public: virtual void __thiscall Fish::Swim(void)" (?Swim@Fish@@UAEXXZ
fatal error LNK1120: 1 unresolved externals
求C++专家级的人士来解答！
感激不敬啊！

Answer 1

将virtual void Swim()改成virtual void Swim()=0;，虚函数需要在基类中进行定义，而上述函数没有在基类进行定义，而是在子类中实现了，所以它也是纯虚函数，不能是虚函数。纯虚函数不能在基类中定义，她只是一个接口，要到子类中去实现。

Answer 2

把你的第8行代码改成virtual void Swim()=0;即可。

Answer 3

Fish::Swim()，要么定义了，要么设成纯虚