崩溃问题-使用指针和析构函数的C ++代码设计

Question

I have a question regarding the following code, which crashes. I am creating a local variable in testfunction() and then pushing it (variable "y") into a list. This variable has a member pointer "b" of object type Ball. As I understand, this local variable "y" is on the stack, so its' destructor will be called after testfunction() is completed. Also, as I understand, a vector "copies" an object into its' list. From what I've learned, it is best practice to delete a pointer in the destructor if one exists in its' class. So, there is "delete b" in the destructor of Example.

The issue that I am having is that the object yb is being destroyed at the completion of testfunction(). In main(), I am able to see the value of "name" and the address of "b", but the object "b" has already been deleted. I would like to avoid this.

I think there is an issue with the design of the code/use of pointers vs references, etc. Please guide me in the right direction, I am an idiot!

#include <iostream>
#include <string>
#include <vector>
using namespace std;

class Ball
{
    public:
        int a;

        Ball()
        {
            a = 0;
        }

        ~Ball()
        {
            cout << "destroyed Ball()" << endl;
        }
};

class Example
{
    public:
        string name;
        Ball* b;

        Example()
        {
            name = "";
            b = NULL;
        }

        ~Example()
        {
            cout << "destroying Example()" << endl; 
            delete b;
        }
};

void testfunction(vector<Example>& list)
{
    cout << "entered testfunction1()" << endl;

    Example y;
    y.name = "myName";
    y.b = new Ball();
    y.b->a = 5;

    cout << "y.b->a = " << y.b->a << endl;
    list.push_back(y);

    cout << "exit testfunction1()" << endl;
}

void testfunction2()
{
    cout << "entered testfunction2()" << endl;
    Example* y = new Example();
    cout << "exit testfunction2()" << endl;
}

int main() {
    vector<Example> list;
    testfunction(list);
    //testfunction2();

    if(list[0].b == NULL)
        cout << "b is null" << endl;
    else
        cout << "b is not null" << endl;

    cout << list[0].name << endl;
    cout << list[0].b << endl;
    cout << "list[0].b->a = " << list[0].b->a << endl;
    return 0;
}

Answer 1

Since class Example has a pointer member and it tries to own a dynamically allocated resource, it needs non-default copy operations, in other words, it needs user-defined copy constructor and assignment operator.

Inside testfunction , when you copy y into vector , both local y and y copied to the vector point to very same Ball object. The local y is destroyed at the end of the function and Ball is deleted. However, that deleted Ball still pointed by the y in vector

void testfunction(vector<Example>& list)
{
    // ...

    Example y;
    y.name = "myName";
    y.b = new Ball();
    y.b->a = 5;

    list.push_back(y);

    // ...
} // <-- destructor for Example y is called and y.b is deleted

Answer 2

Define a copy constructor and an assignement operator for your class Example . These shall copy properly your object (creating a duplicated Ball object) when pushed back on the vector.

Example(const Example& a)  
{
    name = a.name;  // attention no dynamic allocation
    cout << "copy" <<endl; 
    if (a.b) {
        b = new Ball(*a.b);   // create a new duplicated Ball 
    }
    else b = NULL; 
}

The problem in your example is that the default copy constructor is called when you pushback the object. It copies memberwise and so the pointer to Ball is copied, not the object pointed to.

Yet another alternative could be to replace your Ball* with shared_ptr<Ball> (and accordingly, new Ball with make_shared<Ball>() and the delete b of the object with a b.reset() ). The principle is that this smart pointer keeps track of the number of time the object pointed to is used, so that it will not delete it twice, but only when its no longer used anywhere.