C++ vector of boost::weak_ptr troubles

chiranjivi · 2013-08-04T21:58:23

I have a game that has a number of ships in space. The ships exist in the gameworld as boost::shared_ptr<Ship>. These ships can attack one another. The Ship class has a boost::weak_ptr<Ship> called target, which holds a reference to whatever it's currently firing at, and it also has a vector of weak_ptr<Ship> called aggro_vector. The aggro vector records all ships that have attacked this Ship, so that when it kills its current target, it can consult its aggro vector to see which other ships it is currently in combat with, and pick a new target. This is the Ship class, sanitised: class Ship { public: Ship (int x, int y); boost::weak_ptr<Ship> target; std::vector< boost::weak_ptr<Ship> > aggro_vector; void determine_next_target(); void set_as_target (boost::shared_ptr<Ship> next_target); } What I'm having trouble with is getting the Ship to correctly look through its aggro_vector and choose a new target. Here is the function that picks the next target from the aggro vector. It iterates through all the weak_ptrs, and if .lock() successfully returns a shared_ptr, it hands this shared_ptr off to the member function that registers the next target. Otherwise the weak_ptr refers to a ship that has since been destroyed and is removed from the aggro vector: void Ship::determine_next_target() { std::vector< boost::weak_ptr<Ship> >::iterator it = aggro_vector.begin(); while(it != aggro_vector.end()) { if (boost::shared_ptr<Ship> this_ship = (*it).lock()) { set_as_target(this_ship); ++it; } else { it = aggro_vector.erase(it); } } } When this code is compiled and run, it crashes at the line set_as_target(this_ship). If this line is commented out, the remainder of the code works as it should (although obviously the determine_next_target() function now does nothing other than remove expired weak_ptrs). To be clear, the set_as_target(boost::shared_ptr<Ship>) function works fine and is called from a bunch of other places in the code with no problem. It doesn't appear to be that function, but rather the fact that I'm attempting to call that function in this way. Why am I having problems passing a shared_ptr to an outside function in this fashion? Is there something about iterating over a vector of weak_ptr that I'm missing, or am I just failing to grasp, in the broader sense, some aspect of how these pointers should be used? Thanks in advance for any help.

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Started by chiranjivi August 04, 2013 01:48 AM

9 comments, last by GameDev.net 10 years, 9 months ago

Anonymous

August 04, 2013 09:58 PM

AllEightUp, on 03 Aug 2013 - 10:23 PM, said:
My first guess is some odd behavior with scoping. Basically I don't remember for sure the scope rule but I bet if you change the code as follows, it may at least give you some better information on what the problem is:
void Ship::determine_next_target()
{
std::vector< boost::weak_ptr >::iterator it = aggro_vector.begin();
while(it != aggro_vector.end()) {
boost::shared_ptr this_ship( (*it).lock() );
if ( this_ship ) {
set_as_target(this_ship);
++it;
} else {
it = aggro_vector.erase(it);
}
}
}

That's actually the exact code I was using to begin with; I just simplified it for the purpose of posting the thread! It doesn't work in the same way that the original code doesn't work.

Ah, good, defining something within a condition is just a bad idea and scares me..

Now, given the other information I can think of a couple things to look at:

1. As others mentioned, if the call itself fails, check the caller is calling on a valid object.

2. If the declaration of the function is "void set_as_target( boost::shared_ptr ptr )" then the failure could be during the pass by value of the shared_ptr itself which leads to a couple possibilities:

a) while the pointer may look valid, have you ensured a single source of the shared_ptr? I.e. if you don't have "enable_shared_from_this" then you could have two raw pointers assigned into different shared_ptr's and they each keep separate ref counts. So, in the case of pass by value, the copy constructor could be accessing the ref count of an object which was accidentally destroyed via a separate instance of a shared_ptr. Hope this makes sense, shared_ptr's can be a bitch if you are not careful with how you create them and make copies.

b) even with enable_shared_from_this, you can still end up with different ref counts if you in one case you construct a smart_ptr from a raw pointer which is abstract and doesn't know about the actual derivative. This is quite a bit more difficult to have happen but I've seen it once or twice with a goof.

Beyond those items, I think folks will need a bit more context to help too much further.

C++ vector of boost::weak_ptr troubles

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C++ vector of boost::weak_ptr troubles

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