条款07：为多态基类声明virtual析构函数

1.工厂方法举例：多态基类析构函数不声明为virtual会发生什么

#include <iostream>
using namespace std;

class Base
{
public:
    ~Base(){}
};

class Box :public Base
{
public:
    const static int s_i = 0;
};

class Box1 :public Base
{
public:
    const static int s_i = 0;
};

int main() {
    Base* factory1 = new Box();
    Base* factory2 = new Box1();
    delete factory1;
    delete factory2;
    return 0;
}

结果：内存泄漏，一共泄露sizeof(int)*2 = 8个字节 = 64bit（即派生类的内容没被释放）

2.避免内存泄漏：多态基类析构函数声明为virtual

#include <iostream>
using namespace std;

class Base
{
public:
   virtual ~Base(){}
};

class Box :public Base
{
public:
    const static int s_i = 0;
};

class Box1 :public Base
{
public:
    const static int s_i = 0;
};

int main() {
    Base* factory1 = new Box();
    Base* factory2 = new Box1();
    delete factory1;
    delete factory2;
    return 0;
}

3.C++常规情况：类占用内存情况

#include <iostream>
using namespace std;

class Base
{
public:
   ~Base(){}
};

class Box :public Base{};

class Box1 :public Base
{
public:
    double s_idd = 0.0;
};

int main() {
    Base base;
    Box b;
    Box1 b1;
    cout << sizeof(base) << endl;
    cout << sizeof(b) << endl;
    cout << sizeof(b1) << endl;
    return 0;
}

输出（没有成员就占用1个字节标识自己，有成员变量就是实际成员变量的大小）

1
1
8

4.C++类无端加上虚析构函数（非多态场景）

#include <iostream>
using namespace std;

class Base
{
public:
   virtual ~Base(){}
};

int main() {
    Base base;
    cout << sizeof(base) << endl;
    return 0;
}