【合集】【更新中】C++ pointer 之指针大乱指

Problem 1:

What's the result of the program:

class base {
public:
  ~base() {
    printf("base disconcrete\n");
  }
};
class child:public base {
public:
  ~child() {
    printf("child disconcrete\n");
  }
};
int main() {
  base* b = new child();
  delete b;
  return 0;
}

Take notice that base* points to derive, so :

Only

"base disconcrete" is printed.

But if

child* c = new child();
  delete c;

Problem 2:

char* getMem(void) {
  char p[] = "hello world";
  p[5] = 0x0;
  return p;
}
void test(void) {
  char *s = 0x0;
  s = getMem();
  printf(s);
}

int main() {
  test();
  return 0;
}

The result is Not Sure

Problem 3: Quite Dangerous

void f(char **p) {
  *p += 2;
}

int main() {
  char *a[] = {"123","abc","456"}, **r = a;
  f(r);
  printf("%s\r\n",*r);
  return 0;
}

The result is 3

But if

void f(char **p) {
  *p += 2;
}
void g(char **p) {
  p += 2;
}

int main() {
  char *a[] = {"123","abc","456"}, **r = a;
  //f(r);
  g(r);
  printf("%s\r\n",*r);
  return 0;
}

The result is still 123

Only

printf("%s\r\n",*(r+2)); output 456

Problem 4:

[cpp] view
plaincopy





class str{

public:

int x;

char s[0];

};



class foo{

public:

str * p;

};

int main() {

foo f = {0};

str* p1 = NULL;

if (f.p->s){

printf("%d\n", f.p->s);

}

return 0;

}

This sentence sets p to NULL:

[cpp] view
plaincopy





foo f = {0};

f.p is a pointer of str, so f.p is an offset. I.e., f.p->s = base address + 4

So the result is 4

Problem 5： Dangerous：

#include <stdio.h>
#include <assert.h>

int main()
{
    const int i = 10; 
    int* p = (int*)&i;
    *p += 10; 
    printf("%d\t%d\n", i, *p);
    printf("%x\t%x\n", &i, p); 
}

The value of i is 10, *p = 20. The address of i and p is same.





But for struct.

#include <iostream>
#include <assert.h>
using namespace std;

struct mbLJ{
    int mb; 
    int LJ; 
    mbLJ(){
        mb=0;
        LJ=1;
    }   
};

int main()
{
    const struct mbLJ hehe;
    int* p = (int*)(&hehe.mb);
    *p += 10; 
    printf("%d\t%d\n", hehe.mb, *p);
}
输出 10 10

对全局变量取地址不会出错，对全局变量的const修改直接程序崩溃：

const int j = 10;
int main()
{
    const int i = 10; 
    int* p = (int*)&i;
    int* q = (int*)&j;
    
    *q += 10;
    //*p += 10; 
    printf("%d\t%d\n", i, *p);
    printf("%x\t%x\n", &i, p);

    //printf("%d\t%d\n", j, *q);
    //printf("%x\t%x\n", &j, q);
    
    return 0;

}

#include <iostream>
#include <assert.h>
using namespace std;

struct mbLJ{
    static int mb ; 
    int LJ; 
    mbLJ(){
        mb=0;
        LJ=1;
    }   
};
int mbLJ::mb  = 0;//没有这句编译不会过，因为静态成员变量只是声明，没有实现定义。。。

int main()
{
    const struct mbLJ hehe;
    
    int* p = (int*)(&hehe.mb);
    *p += 10; 
    printf("%d\t%d\n", hehe.mb, *p);
    return 0;
}