[C++知识库]C++ delete，静态变量，问题

delete释放的指针，再访问

例1

#include <iostream>
using namespace std;

class Box
{
public:

    Box(int,int);

    ~Box();

    void volume();

    static int height;

    int width;

    int length;
};

Box::Box(int wi, int le)
{
    width = wi;
    length = le;
}

Box::~Box(){cout<<"the pointer is released."<<endl;}

void Box::volume()
{
    cout<<height*width*length<<endl;
}

int Box::height = 100;

int main()
{
    Box* p = new Box(10,20);
    delete p;
    cout<<p->height<<endl;
    cout<<Box::height<<endl;

    cout<<"width" <<p->width<<endl;
    cout<<"length "<<p->length<<endl;

    p->volume();
    return 0;
}
//输出：
/*100
100
width 16257288
length 16253120
-1812113408*/

例2

#include <iostream>
using namespace std;

int * func(){
    int * a = new int(10);
    return a;
}

int main(){
    int * p = func();
    cout << *p << endl;//10
    //delete关键字用来释放堆区数据
    delete p;
//    p = new int(5);
    cout << *p << endl;//10
    return 0;
}
//输出
//  10
//  16584968

解释：
访问 delete 之后的内存是一个未定义行为。 未定义行为可能产生任何结果，包括但不限于：产生期望的结果，产生未期望的结果，产生随机的结果，产生无法解释的结果，运行错误，随机的运行时错误，编译错误，等等 ---- 你只是放弃了对这片内存的所有权。获得所有权的人对这片内存做什么（或者说什么都不做）都不关你的事

static 变量的储存区域

https://blog.csdn.net/qq_32900237/article/details/107094377?utm_medium=distribute.pc_relevant.none-task-blog-2~default~baidujs_title~default-0.no_search_link&spm=1001.2101.3001.4242参考文章

例1

#include <iostream>
using namespace std;

class Box
{
public:

    Box(int,int);

    ~Box();

    void volume();

    static int height;

    int width;

    int length;
};

Box::Box(int wi, int le)
{
    width = wi;
    length = le;
}

Box::~Box(){cout<<"width: "<< width <<"the pointer is released."<<endl;}

void Box::volume()
{
    cout<<height*width*length<<endl;
}

int Box::height = 100;

int main()
{
    Box* p = new Box(10,20);
    cout<<"point  "<<p<<endl;  //point  0xe91470
    cout<<&(p->height)<<endl;  //0x405004
    cout<<&(p->width)<<endl;   //0xe91470
    cout<<&(p->length)<<endl;  //0xe91474
    cout<<sizeof(p)<<endl;    //4
    cout<<sizeof(*p)<<endl;   //8
    cout<<sizeof(Box)<<endl;  //8
	//delete p;              //width: 10the pointer is released.  用new创建的对象，必须自己用delete回收，不然系统不会帮助回收，出现内存泄漏

    Box a = Box(1,2);
    Box *pa = &a;
    cout<<"point  "<<pa<<endl;  //point  0x61ff00
    cout<<&(pa->height)<<endl;  //0x405004
    cout<<&(pa->width)<<endl;   //0x61fefc
    cout<<&(pa->length)<<endl;  //0x61ff00
    cout<<sizeof(pa)<<endl;     //4
    cout<<sizeof(*pa)<<endl;    //8
    cout<<sizeof(a)<<endl;      //8

    Box b = Box(3,4);
    Box *pb = &b;
    cout<<"point  "<<pb<<endl;  //point  0x61fef4
    cout<<&(pb->height)<<endl;  //0x61fef4
    cout<<&(pb->width)<<endl;   //0x61fef4
    cout<<&(pb->length)<<endl;  //0x61fef8
    cout<<sizeof(pb)<<endl;
    cout<<sizeof(*pb)<<endl;
    return 0;
}
/*
point  0xe91470       新对象的地址
0x405004              静态变量和普通变量地址不连续，是静态变量存在数据段
0xe91470              普通变量存在 开辟的堆上
0xe91474
4                    指针大小
8                    对象所占内存大小
8                    类大小
point  0x61fefc      新对象a的地址
0x405004             静态变量地址不变，静态变量属于整个类
0x61fefc             属于局部变量，普通变量存在 栈空间上
0x61ff00
4
8
8
point  0x61fef4     新对象b的地址， b与a之间相差8个字节
0x405004            静态变量地址不变，静态变量属于整个类
0x61fef4            属于局部变量，普通变量存在 栈空间上，地址连续
0x61fef8
4
8
width: 3the pointer is released.   自动调用析构函数
width: 1the pointer is released.   自动调用析构函数
*/

例2 帮助理解

#include <iostream>
using namespace std;

class Box
{
public:

    Box(int,int);

    ~Box();

    void volume();

    static int height;

    int width;

    int length;
};

Box::Box(int wi, int le)
{
    width = wi;
    length = le;
}

Box::~Box(){cout<<"width: "<< width <<"the pointer is released."<<endl;}

void Box::volume()
{
    cout<<height*width*length<<endl;
}

int Box::height = 100;

int main()
{
    Box* p = new Box(10,20);
    cout<<"point  "<<p<<endl;
    cout<<&(p->height)<<endl;
    cout<<&(p->width)<<endl;
    cout<<&(p->length)<<endl;
    cout<<sizeof(p)<<endl;
    cout<<sizeof(*p)<<endl;
    cout<<sizeof(Box)<<endl;
    // delete p;

    Box* p1 = new Box(30,40);
    cout<<"point  "<<p1<<endl;
    cout<<&(p1->height)<<endl;
    cout<<&(p1->width)<<endl;
    cout<<&(p1->length)<<endl;
    cout<<sizeof(p1)<<endl;
    cout<<sizeof(*p1)<<endl;
    cout<<sizeof(Box)<<endl;
    delete p;
    delete p1;

    Box a = Box(1,2);
    Box *pa = &a;
    cout<<"point  "<<pa<<endl;
    cout<<&(pa->height)<<endl;
    cout<<&(pa->width)<<endl;
    cout<<&(pa->length)<<endl;
    cout<<sizeof(pa)<<endl;
    cout<<sizeof(*pa)<<endl;
    cout<<sizeof(a)<<endl;

    Box b = Box(3,4);
    Box *pb = &b;
    cout<<"point  "<<pb<<endl;
    cout<<&(pb->height)<<endl;
    cout<<&(pb->width)<<endl;
    cout<<&(pb->length)<<endl;
    cout<<sizeof(pb)<<endl;
    cout<<sizeof(*pb)<<endl;
    return 0;
}
/*
point  0x791470
0x405004       
0x791470       
0x791474       
4
8
8
point  0x791108
0x405004       
0x791108       
0x79110c       
4
8
8
width: 10the pointer is released.
width: 30the pointer is released.
point  0x61fef8
0x405004
0x61fef8
0x61fefc
4
8
8
point  0x61fef0
0x405004
0x61fef0
0x61fef4
4
8
width: 3the pointer is released.
width: 1the pointer is released.
*/