c++編程練習 019:統計動物數量

北大程序設計與算法（三）測驗題彙總(2020春季)

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描述

代碼填空，使得程序能夠自動統計當前各種動物的數量

#include <iostream>
using namespace std;
// 在此處補充你的代碼
void print() {
	cout << Animal::number << " animals in the zoo, " << Dog::number << " of them are dogs, " << Cat::number << " of them are cats" << endl;
}

int main() {
	print();
	Dog d1, d2;
	Cat c1;
	print();
	Dog* d3 = new Dog();
	Animal* c2 = new Cat;
	Cat* c3 = new Cat;
	print();
	delete c3;
	delete c2;
	delete d3;
	print();
}

輸入
無

輸出
0 animals in the zoo, 0 of them are dogs, 0 of them are cats
3 animals in the zoo, 2 of them are dogs, 1 of them are cats
6 animals in the zoo, 3 of them are dogs, 3 of them are cats
3 animals in the zoo, 2 of them are dogs, 1 of them are cats

樣例輸入
None

樣例輸出
0 animals in the zoo, 0 of them are dogs, 0 of them are cats
3 animals in the zoo, 2 of them are dogs, 1 of them are cats
6 animals in the zoo, 3 of them are dogs, 3 of them are cats
3 animals in the zoo, 2 of them are dogs, 1 of them are cats

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分析

此處利用構造函數與析構函數引起數據變化；
發現每定義一個類的對象，就會產生數量加1，那麼知道在構造函數內部有一個累加變量，然後每次銷燬一個對象的時候，數量會減1，那麼可知在析構函數內部有減1的設計。

此處需要注意的是Animal* c2 = new Cat;delete c2;由於此處是基類指針指向的派生類，那麼在調用析構函數的時候，基類的析構函數必須是虛函數，因爲這樣才能保證先調用派生類的析構函數，然後調用基類的析構函數，兩者不缺。

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解決方案

#include <iostream>
using namespace std;
class Animal{
public:
	static int total_Animal;
	Animal(){ total_Animal ++;}
	virtual ~Animal(){ total_Animal--;}		
}; 
class Dog:public Animal{
public:
	static int Dog_num;
	Dog(){Dog_num++;}
	~Dog(){Dog_num--;}		
};
class Cat:public Animal{
public:
	static int Cat_num;
	Cat(){Cat_num++;}		
	~Cat(){Cat_num--;}
};
int Animal::total_Animal = 0,Dog::Dog_num = 0,Cat::Cat_num = 0;
void print() {
	cout << Animal::total_Animal << " animals in the zoo, " << Dog::Dog_num << " of them are dogs, " << Cat::Cat_num << " of them are cats" << endl;
}

int main() {
	print();
	Dog d1, d2;
	Cat c1;
	print();
	Dog* d3 = new Dog();
	Animal* c2 = new Cat;
	Cat* c3 = new Cat;
	print();
	delete c3;
	delete c2;
	delete d3;
	print();
}

執行效果