Question

In my game I created a base class called Entity which I store in a set for processing. All my game objects derive from this class, and I have no problem adding the derived pointer types to the set in my initialization function.

The problem lies in adding new elements from within an Entity’s Step() function. Now, before I get too far into it I’ll show you some simplified code:

class GameState
{
    public:
        GameState();
        ~GameState();
        ...
        set<Entity*> entities;
        void Add(Entity* e);
        void Remove(Entity* e);
    protected:
        set<Entity*> added, removed;
};

class Entity
{
    public:
        Entity();
        Entity(GameState* parent);
        virtual ~Entity();
        virtual void Step(const sf::Input& input);
        ...
        virtual void Destroy();
    protected:
        GameState* game;
};

The functions Add and Remove in GameState simply add the argument e to the added and removed sets respectively. In the main loop (elsewhere in GameState), I move the elements from added to entities before processing and after processing I remove elements from removed from entities. This ensures that entities is not modified during iteration.

The Add/Remove functions are very simple:

void GameState::Add(Entity* e)
{
    added.insert(e);
}

void GameState::Remove(Entity* e)
{
    removed.insert(e);
}

Every derived Entity is passed a pointer to GameState in it’s constructor that it keeps as game. So theoretically from the Step function I should be able to Add and Remove entities with a simple call like game->Remove(this);, but instead I get a segfault. After a night of googling and coming up with nothing, I was able to work around (part of) the problem by implementing Entity::Destroy() like so:

void Entity::Destroy()
{
    game->Remove(this);
}

So my first question is: Why does this work when I’m in the base class but not in the derived class?

Even more puzzling to me is Add(). Why does Add(new Explosion(16,16,this)) work in GameState but game->Add(new Explosion(16,16,game)) doesn’t work inside my object?

I ran it through gdb and it tells me:

Program received signal SIGSEGV, Segmentation fault.
At c:/program files (x86)/codeblocks/mingw/bin/../lib/gcc/mingw32/4.4.1/include/c++/bits/stl_tree.h:482

The code that throws the error is:

_Link_type
  _M_begin()
  { return static_cast<_Link_type>(this->_M_impl._M_header._M_parent); } //this line

So to sum it up I have no idea why my pointers break the STL… and I get that grave feeling that I’m missing something very basic and its causing all these headaches. Can anyone give me advice?

Answer 1

Why does Add(new Explosion(16,16,this)) work in GameState but
game->Add(new Explosion(16,16,game)) doesn’t work inside my object?

If that is the case then the only possible explanation is that the Entity‘s game member doesn’t actually point to the GameState. Check that it is being set properly on construction and verify before you use it.

This has nothing to do with std::set. The problem is that you are using an std::set that is part of a class that you are accessing via a corrupt pointer.