Copying a vector of structs containing unique_ptrs - c++

Question

I'm working on a simple game using C++ and Allegro. I am running into an Access violation runtime error regarding a vector of structs that contain unique_ptrs to ALLEGRO_BITMAPs.
Here is my struct declaration.
struct Skin {
std::unique_ptr<ALLEGRO_BITMAP> img;
Skin();
Skin(ALLEGRO_BITMAP*);
Skin& operator=(const Skin& s);
Skin(const Skin& s);
};
And here are the definitions of the constructors in another file.
Skin::Skin() {
img.reset();
}
Skin::Skin(ALLEGRO_BITMAP* bitmap) {
img.reset(bitmap);
}
Skin::Skin(const Skin& s) {
img.reset(s.img.get());
}
Skin& Skin::operator=(const Skin& s) {
img.reset(s.img.get());
return *this;
}
Here is the code that gets called before my access violation.
generateBase(world, display.get());
Which calls this function.
void generateBase(World& world, ALLEGRO_DISPLAY* display) {
int x = TILESIZE - WIDTH;
int y = HEIGHT - TILESIZE;
int groundWidth = 3 * WIDTH - 2 * TILESIZE;
Point min{ x, y };
Point max{ x + groundWidth, y + (int)TILESIZE };
ALLEGRO_BITMAP* black = al_create_bitmap(groundWidth, TILESIZE);
ALLEGRO_BITMAP* white = al_create_bitmap(groundWidth, TILESIZE);
al_set_target_bitmap(black);
al_clear_to_color(al_map_rgb(0, 0, 0));
al_set_target_bitmap(white);
al_clear_to_color(al_map_rgb(255, 255, 255));
al_set_target_bitmap(al_get_backbuffer(display));
std::cout << "Errors incoming!" << endl;
createPlayer(world, x, y, 0, 0, 5, vector < AABB > { AABB(min, max) }, vector < Skin > { Skin(black), Skin(white) });
std::cout << "Did we make it?" << endl;
}
Which in turn calls this function.
unsigned int createPlayer(World& world, int x, int y, float dx, float dy, float speed, vector<AABB>& mesh, vector<Skin>& imgs) {
unsigned int entity = newEntityIndex(world);
world.masks[entity].set(COMPONENT_TYPE);
world.masks[entity].set(COMPONENT_POINT);
world.masks[entity].set(COMPONENT_UNITVECTOR);
world.masks[entity].set(COMPONENT_SPEED);
world.masks[entity].set(COMPONENT_COLLISIONMESH);
world.masks[entity].set(COMPONENT_SKINLIST);
world.types[entity] = TYPE_PLAYER;
world.points[entity] = Point(x, y);
world.unitVectors[entity] = UnitVector(dx, dy);
world.speeds[entity] = Speed(speed);
world.collisionMeshes[entity].mesh = mesh;
cout << "Starting vector copy" << endl;
for (auto skin : imgs) {
world.skinLists[entity].imgs.push_back(move(skin));
}
cout << "Ending vector copy" << endl;
return entity;
}
Here is my deleter for unique_ptr.
namespace std {
template<>
class default_delete < ALLEGRO_BITMAP > {
public:
void operator()(ALLEGRO_BITMAP* ptr) {
cout << ptr << endl;
al_destroy_bitmap(ptr);
}
};
}
Here is the output.
Errors incoming!
Starting vector copy
00AF9468
00AF9468
When I modified my createPlayer call in generateBase by removing Skin(white), the output changed to.
Errors incoming!
Starting vector copy
00799468
Ending vector copy
00799468
The change in output had me a bit puzzled, but my biggest question is what do I need to change about how I copy my vector of structs of unique_ptrs so that I don't try to delete the same pointer twice.
Thanks in advance!

Answer 1

The first thing to understand is you can only have one std::unique_ptr object containing a pointer to a particular object. Your Skin(const Skin& s) constructor violates this principle, resulting in two copies of a unique_ptr. If you have an object containing unique_ptr members, you'll need to do one of the following:
Not have a copy constructor or assignment operator.
In the copy constructor and/or assignment operator, allocate a new copy of the underlying resource. This would require calling al_clone_bitmap to duplicate the resource.
Second, when you are holding a resource in a unique_ptr, you want to initialize the unique_ptr at the same location where you create the resource. For example, instead of creating a local variable ALLEGRO_BITMAP* black, use the following:
std::unique_ptr<ALLEGRO_BITMAP> black(al_create_bitmap(groundWidth, TILESIZE));
Since this code is creating the unique_ptr directly from the result of al_create_bitmap, you'll want to remove the Skin constructor that takes an ALLEGRO_BITMAP* and replace it with this:
Skin::Skin(std::unique_ptr<ALLEGRO_BITMAP>&& bitmap)
: img(bitmap)
{
}
You can then create a Skin by moving your unique_ptr into it:
Skin(std::move(black))
Putting the above together, a working copy constructor might look like the following. It's not particularly efficient, but it's safe.
Skin::Skin(const Skin& s)
: img(al_clone_bitmap(s.img.get()))
{
}

Answer 2

The problem is here:
Skin::Skin(const Skin& s) {
img.reset(s.img.get());
}
Skin& Skin::operator=(const Skin& s) {
img.reset(s.img.get());
You are stealing the raw pointer from one unique_ptr and assigning it to another one. Now unique_ptr belongs to the RAII category. They expect to own the lifetime of an object as long as they are alive.
When you do this
img.reset(s.img.get());
You took out the pointer from one unique_ptr and handed over to the other unique_ptr. Now unique_ptr1 believes that it own the underlying object unaware that there is another unique_ptr2 that believes the same. So when they die they will happily free the memory allocated for _Ptr. So your code is bound to end up accessing/freeing memory which has already being freed by the first unique_ptr which dies.
You must either move the unique_ptr (thereby yielding ownership) or explicitly calling release on s.img