how to correctly use vector of pointers to objects of class

Question

I wonder why the second part of the following code I wrote doesn't work. I am practicing vector of pointers to class objects. I tried two ways, one is to define a class object; the other is to define a pointer to the object. The second way failed.

#include <iostream>
#include <vector>

using namespace std;

class A{
    public:
        int id;
        A(int id):id(id){}
};

int main()
{
    vector<A*> A_vec, A_vec2;
    A a(5);
    A_vec.push_back(&a);
    cout << A_vec.size() << "; id " << A_vec[0]->id << endl;


    A *a1;
    a1->id = 5;
    A_vec2.push_back(a1);
    cout << A_vec2.size() << "; id " << A_vec2[0]->id << endl;
}

Here is your answer stackoverflow.com/questions/817263/… you need to use new — Khalil Khalaf
– Khalil Khalaf, Commented Apr 26, 2016 at 14:29
The correct use of pointers in a vector is to have a vector of smart pointers, the kind that handle deallocation of the target's memory. — Thomas Matthews
– Thomas Matthews, Commented Apr 26, 2016 at 14:51

NPE · Accepted Answer · 2016-04-26 14:29:20Z

2

The second snippet doesn't work because you've never allocated memory for the object, so a1 points nowhere.

A *a1 =  new A(5);

...

// Once you're done with `a1', release the memory.
delete a1;

answered Apr 26, 2016 at 14:29

NPE

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1 Comment

daydayup Over a year ago

Can I ask one more question? delete a1 will release the memory of the object in the second way, but it looks like in the first way memory will not be released. So does this mean we should use the second way? @NPE

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