具有指针变量的C ++分段故障类

Question

[Edited post, based on previous answers and comments. I have tried several strategies, none of which have worked.]

I have a class D, with a member variable of class type E. E has two subclasses, E1 and E2. When I try the following code, I get a segmentation fault (see detailed output below). Can someone tell me why, and provide changes to the code that will make it work? Thanks.

Any observations on acceptable or professional coding style would be welcome also.

---

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

class E {
    public:
        E() {cout << "In E constructor" << endl;}

        virtual ~E() {cout << "In E destructor" << endl;}
};

class E1 : public E {
    public:
        E1() {cout << "New E1" << endl;}

        ~E1() {cout << "In E1 destructor" << endl;}
};

class E2 : public E {
    public:
        E2() {cout << "New E2" << endl;}

        ~E2() {cout << "In E2 destructor" << endl;}
};

class D {
    public:
        D(const vector<int>& a, int b)
        {
            cout << "y1" << endl;
            if (b == 1)
                p = new E1();
            else
                p = new E2();
        }

        D() {cout << "y2" << endl; p = new E1();}

        ~D() {if (p != nullptr) {cout << "x" << endl; delete p;}}

    private:
        E *p;
};

int main(int argc, char **argv)
{
    D d;

    vector<int> a;
    a.push_back(1);

    int b = 2;

    d = D(a, b);
}

---

Here's the output.

y2
In E constructor
New E1
y1
In E constructor
New E2
x
In E2 destructor
In E destructor
x
Segmentation fault

Answer 1

Your original code is deleting the new'd E2 object twice due to the statement.

   d = D(a, b);

Since you didn't provide an assignment operator, the compiler will provide a memberwise assignment implementation. Immediately after the assignment, both the variable d and the temporary, unnamed variable created from D(a, b) will have their p member variables pointing to the same E2 object.

The D::~D() destructor will be invoked on both the d and the unnamed variable. Deleting the E2 object twice causes the segmentation fault.