C ++如何将内存分配给结构的指针，该结构是另一个第二结构的成员？

Question

This is the code:

# include<iostream>
#include<stdio.h>
using namespace std;
    struct hub
    {
        int info;
        int info2;
    };
    struct hub2
    {
        hub *p;
    };
    int main()
    {
        hub *pi;
        pi = new hub; // allocate memory for pi
        pi->info = 30;
        pi->info2 = 45;
        cout<<pi->info<<" "<<pi->info2; // shows 30 and 45
        hub2 obj; // declare a hub2 data type
        obj.p = new hub; // allocate memory for obj.p
        obj.p = pi; // now obj.p should be identical to what pi points to right?
        cout<<"\n"<<obj.p->info<<" "<<obj.p->info2; // This will show 30 and 45
        cout<<endl;
        delete pi;
        cout<<"\n"<<obj.p->info<<" "<<obj.p->info2;//This shows random numbers,why ???
    }

Why doesn't obj.p allocate memory after I delete the memory allocated for pi pointer? I really need a solution to this.I have to finish a very important project and I am stuck here with at this memory allocating part :(

Answer 1

obj.p = pi; // now obj.p should be identical to what pi points to right?

Yes, both pointers are now equal. As a consequence, delete pi is equivalent to delete obj.p . Once this is done, the statement

cout<<"\n"<<obj.p->info<<" "<<obj.p->info2;//This shows random numbers,why ???

tries to access the memory which has just been freed. This results in undefined behavior.

As a side note, be careful here:

obj.p = new hub; // allocate memory for obj.p
obj.p = pi;

The second statement overwrites the pointer to the memory allocated by new hub . After that, such memory is no longer accessible, nor can be freed. This is a memory leak.

Answer 2

obj.p = new hub; // Not necessary ,you can directly write obj.p = pi

delete pi; //After this instruction, pi is free (BUT NOT AUTOMATICALLY TO NULL ) pi points now to a "trash" , it's why you see random number.
So you have to reallocate it again .