多态--遗失的子类析构函数(重要)
通过阅读下面的代码以及将其置于编译器上编译运行:
#include<iostream>
using namespace std;class Father {
public:Father(const char* addr="中国") {cout << "执行Father类构造函数" << endl;int len = strlen(addr) + 1;this->addr = new char[len];strcpy_s(this->addr, len, addr);}~Father() {cout << "执行Father类析构函数" << endl;delete addr;addr = NULL;}
private:char* addr;
};class Son :public Father {
public:Son(const char* game="联盟",const char*addr="中国"):Father(addr) {cout << "执行Son类构造函数" << endl;int len = strlen(game) + 1;this->game = new char[len];strcpy_s(this->game, len, game);}~Son() {cout << "执行Son类析构函数" << endl;delete game;game = NULL;}
private:char* game;
};int main(void) {cout << "----case 1----" << endl;Father* father = new Father();delete father;cout << endl;cout << "----case 2----" << endl;Son* son = new Son();delete son;cout << endl;system("pause");return 0;
}运行结果:
结果完全符合我们的预期,但是就此可以说明我们的代码就完全正确或者说不存在致命错误???
我们不妨在源代码中加入这样一种情况:用父类指针指向子类对象
Father* father = new Son();
delete father;完整代码:
#include<iostream>
using namespace std;class Father {
public:Father(const char* addr="中国") {cout << "执行Father类构造函数" << endl;int len = strlen(addr) + 1;this->addr = new char[len];strcpy_s(this->addr, len, addr);}~Father() {cout << "执行Father类析构函数" << endl;delete addr;addr = NULL;}
private:char* addr;
};class Son :public Father {
public:Son(const char* game="联盟",const char*addr="中国"):Father(addr) {cout << "执行Son类构造函数" << endl;int len = strlen(game) + 1;this->game = new char[len];strcpy_s(this->game, len, game);}~Son() {cout << "执行Son类析构函数" << endl;delete game;game = NULL;}
private:char* game;
};int main(void) {Father* father = new Son();delete father;cout << endl;system("pause");return 0;
}运行结果:
我们会惊奇的发现:子类的析构函数没有被执行 ,这样将会导致内存泄漏,引发严重后果;
重要:
为了防止内存泄漏,最好是在基类(父类)析构函数上添加virtual关键字,使基类析构函数为虚函数
目的在于:
当使用delete释放基类指针时,会实现动态的析构;
如果基类指针指向的是基类对象,那么只调用基类的析构函数;
如果基类指针指向的是子类对象,那么先调用子类的虚构函数,再调用父类的析构函数.
正确代码:
#include<iostream>
using namespace std;class Father {
public:Father(const char* addr="中国") {cout << "执行Father类构造函数" << endl;int len = strlen(addr) + 1;this->addr = new char[len];strcpy_s(this->addr, len, addr);}virtual ~Father() {cout << "执行Father类析构函数" << endl;delete addr;addr = NULL;}
private:char* addr;
};class Son :public Father {
public:Son(const char* game="联盟",const char*addr="中国"):Father(addr) {cout << "执行Son类构造函数" << endl;int len = strlen(game) + 1;this->game = new char[len];strcpy_s(this->game, len, game);}~Son() {cout << "执行Son类析构函数" << endl;delete game;game = NULL;}
private:char* game;
};int main(void) {cout << "----case 1----" << endl;Father* father = new Father();delete father;cout << endl;cout << "----case 2----" << endl;Son* son = new Son();delete son;cout << endl;cout << "----case 3----" << endl;father = new Son();delete father;cout << endl;system("pause");return 0;
}运行结果:
