1 #include
     
       2 class Boat;//必须事先声明 3 class Car 4 { 5     int size; 6 public: 7     void setSize(int j){size=j;} 8     int getSize(){
   return size;} 9     friend int leisure(int time,Car& aobj,Boat& bobj);10 };11 class Boat:Car//此处的Car可要可不要12 {13     int size;14 public:15     void setSize(int j){size=j;}16     int getSize(){
   return size;}17     friend int leisure(int time,Car& aobj,Boat& bobj);18 };19 int leisure(int time,Car& aobj,Boat& bobj)20 {21     return time*aobj.size*bobj.size;22 }23 int main()24 {25     Car c1;26     c1.setSize(2);27     Boat b1;28     b1.setSize(3);29     std::cout<
      
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