Is this some sort of pointer error?

Question

I'm working on a small game in my free time and while coding it I came across an issue.

I've isolated the issue and made it as small as possible here:

#include <iostream>
#include <memory>
#include <vector>
#include <Windows.h>
class Base
{
public:
    std::string p;
    virtual ~Base()
    {

    }
    virtual std::vector<std::shared_ptr<std::string>> getText() = 0;
};
class Derived1 : public Base
{
public:
    Derived1()
    {
        p = "from1";
    }
    std::vector<std::shared_ptr<std::string>> getText()
    {
        return std::vector<std::shared_ptr<std::string>>{std::shared_ptr<std::string>(&p)};
    }
};
class Derived2 : public Base
{
public:
    Derived2()
    {
        p = "from2";
    }
    std::vector<std::shared_ptr<std::string>> getText()
    {
        return std::vector<std::shared_ptr<std::string>>{std::shared_ptr<std::string>(&p)};
    }
};
int main()
{
    std::unique_ptr<Base> state;
    std::vector<std::shared_ptr<std::string>> displayText;
    state.reset(new Derived1());
    displayText = state->getText();
    while(1)
    {
        for(auto i = displayText.begin(); i != displayText.end(); ++i)
            std::cout << **i;
        if (GetKeyState('2') < 0)
        {
            state.reset(new Derived2());
            displayText.clear();
            displayText = state->getText();
        }
        else if (GetKeyState('1') < 0)
        {
            state.reset(new Derived1());
            displayText.clear();
            displayText = state->getText();
        }
    }
    return 0;
}

When pressing "1" and "2" back and forth it seems to work sometimes, and when I exit out I get something like -1073741510 or similar as the return value. In my game code it also seems to crash a lot.

(By exit out I mean using the X button on the command window, sorry for not clarifying)

After doing some research on unique_ptr and pointers in general along with polymorphic classes I still can't understand why this behaves the way it does.

So, what's going on here?

Define "exit out". The shown code provides no means to cleanly terminate the program. It runs in an infinite loop. — Sam Varshavchik, Dec 03 '16 at 00:29

score 0 · Answer 1 · edited May 23 '17 at 12:01

0

You are creating a shared_ptr from a stack variable. This will end up calling delete on p which is not safe. See Calling delete on variable allocated on the stack

EDIT: You would need to make p a pointer. You could do something like the following:

class Base
{
public:
    std::shared_ptr<std::string> p;
    virtual ~Base()
    {

    }
    virtual std::vector<std::shared_ptr<std::string>> getText() = 0;
};
class Derived1 : public Base
{
public:
    Derived1()
    {
        p = std::make_shared<std::string>("from1");
    }
    std::vector<std::shared_ptr<std::string>> getText()
    {
        return std::vector<std::shared_ptr<std::string>>{p};
    }
};
class Derived2 : public Base
{
public:
    Derived2()
    {
        p = std::make_shared<std::string>("from2");
    }
    std::vector<std::shared_ptr<std::string>> getText()
    {
        return std::vector<std::shared_ptr<std::string>>{p};
    }
};

edited May 23 '17 at 12:01

Community

1
1

answered Dec 03 '16 at 00:44

Chris Ray

31
2

You would need to make p a pointer. You could do something like the following: `class Base { public: std::shared_ptr p; virtual ~Base() {} virtual std::vector> getText() = 0; }; class Derived1 : public Base { public: Derived1() { p = std::make_shared("from1"); } std::vector> getText() { return std::vector>{p}; } };` – Chris Ray Dec 03 '16 at 01:03
Yes, there is a way. See `shared_ptr`'s documentation. – Sam Varshavchik Dec 03 '16 at 01:03
1

It's not a stack variable. It is, however, a member variable, owned by the complete object that contains it, and arranging for a shared_ptr to smash it to bits is going to make the true owner very very mad. – Ben Voigt Dec 03 '16 at 01:13
You are correct. I was in a hurry, and should have read the code fully before my first answer. – Chris Ray Dec 03 '16 at 01:37

Is this some sort of pointer error?

1 Answers1