定義一個哺乳動物Mammal類，再由此派生出狗Dog類，二者都定義 Speak()成員函數，基類中定義爲虛函數，定義一個Dog類的對象，調用Speak函數，觀察運行結果

class Mammal { public: Mammal(){cout<<"Mammal"<<endl;} virtual void speak(){} virtual ~Mammal() { cout<<"~Mammal::speak()"<<endl; } }; class Dog:public Mammal { public: Dog(){cout<<"Dog"<<endl;} virtu

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