List iterator not dereferencable run time

Question

I am getting this error on runtime Expression: List iterator not dereferencable. below is the code I am using.

header file

//-----------------------------------------------------------------------------
//
//  Name:   Fletcher_TargetingSystem.h
//
//  Author: Alan Fletcher (20040797)
//
//  Desc:   class to select a target from the opponents currently in a bot's
//          perceptive memory.
//-----------------------------------------------------------------------------
#include "2d/Vector2D.h"
#include <list>
#include "../../AbstTargetingSystem.h"


class AbstRaven_Bot;

class Fletcher_TargetingSystem : public AbstTargetingSystem
{

public:
    Fletcher_TargetingSystem(AbstRaven_Bot* owner);

    //each time this method is called the opponents in the owner's sensory 
    //memory are examined and the closest  is assigned to m_pCurrentTarget.
    //if there are no opponents that have had their memory records updated
    //within the memory span of the owner then the current target is set
    //to null

    void       Update();
    void         closestBotStrategy();

    void      setCurrentTarget(AbstRaven_Bot* t);
    AbstRaven_Bot* getCurrentTarget();

    void      setCurrentOwner(AbstRaven_Bot* t);
    AbstRaven_Bot* getCurrentOwner();
};

class Fletcher_getClosestBotStrategy
{

public:
    Fletcher_getClosestBotStrategy()
    {}
    Fletcher_getClosestBotStrategy(AbstRaven_Bot* owner);

    //each time this method is called the opponents in the owner's sensory 
    //memory are examined and the closest  is assigned to m_pCurrentTarget.
    //if there are no opponents that have had their memory records updated
    //within the memory span of the owner then the current target is set
    //to null
    void      pickTarget(AbstRaven_Bot& owner);
};

#endif

c++ file

#include "Fletcher_TargetingSystem.h"
#include "../../AbstRaven_Bot.h"
#include "../../Raven_SensoryMemory.h"
#include "../../Debug/DebugConsole.h"


//-------------------------------- ctor ---------------------------------------
//-----------------------------------------------------------------------------
Fletcher_TargetingSystem::Fletcher_TargetingSystem(AbstRaven_Bot* owner):
AbstTargetingSystem(owner){

}

//-------------------------------- ctor ---------------------------------------
//-----------------------------------------------------------------------------
Fletcher_getClosestBotStrategy::Fletcher_getClosestBotStrategy(AbstRaven_Bot* owner){

}
std::list<AbstRaven_Bot*> SensedBots;
std::list<AbstRaven_Bot*>::const_iterator curBot;
//----------------------------- Update ----------------------------------------

//-----------------------------------------------------------------------------
//----------------------------- closestPlayer ----------------------------------------

//-----------------------------------------------------------------------------
void Fletcher_getClosestBotStrategy::pickTarget(AbstRaven_Bot& owner)
{

    double ClosestDistSoFar = MaxDouble;

    Fletcher_TargetingSystem *fTS = new Fletcher_TargetingSystem(&owner);


    for (curBot; curBot != SensedBots.end(); ++curBot)
    {
        //make sure the bot is alive and that it is not the owner
        if ((*curBot)->isAlive() && (*curBot != fTS->getCurrentOwner()) )
        {
            double dist = Vec2DDistanceSq((*curBot)->Pos(), fTS->getCurrentOwner()->Pos());
            if (dist < ClosestDistSoFar)
            {
                ClosestDistSoFar = dist;
                fTS->setCurrentTarget(*curBot);// = *curBot;
            }

        }
    }
    //return *fTS;
}

void Fletcher_TargetingSystem::Update()
{
    // currentStrategy = targetClosestBotStrategy;
    // target = currentStrategy.pickTarget();
    //std::list<AbstRaven_Bot*> SensedBots;
    SensedBots = getCurrentOwner()->GetSensoryMem()->GetListOfRecentlySensedOpponents();
    curBot = SensedBots.begin();
    //std::list<AbstRaven_Bot*>::const_iterator curBot = SensedBots.begin();
    setCurrentTarget(0);//       = 0;

    Fletcher_getClosestBotStrategy* fGCBS = new Fletcher_getClosestBotStrategy(this->getCurrentOwner());
    fGCBS->pickTarget(**curBot);
}

AbstRaven_Bot* Fletcher_TargetingSystem::getCurrentOwner(){
    return m_pOwner;
}
AbstRaven_Bot* Fletcher_TargetingSystem::getCurrentTarget(){
    return m_pCurrentTarget;
}

void Fletcher_TargetingSystem::setCurrentTarget(AbstRaven_Bot* t){
    m_pCurrentTarget = t;
}
void Fletcher_TargetingSystem::setCurrentOwner(AbstRaven_Bot* t){
    m_pOwner = m_pOwner;
}

any help in telling me how to fix this would be appreciated Thanking You

Also what does the error mean exactly and how/when does it occur

Answer 1

You're initialising curBot here:

void Fletcher_TargetingSystem::Update()
{
    // ...
    curBot = SensedBots.begin();
    // ...
}

But you don't appear to call Update() anywhere, so on this line:

for (curBot; curBot != SensedBots.end(); ++curBot)
{ // ... }

You're attempting to use an uninitialised iterator, which is not dereferencable, as the error message indicates.

The solution: initialise curBot before you use it.

Side note: why is curBot declared at global scope? It would make more sense to declare/initialise it where you actually use it ie at the for loop.

Answer 2

You nitialize the global:

std::list<AbstRaven_Bot*>::const_iterator curBot;

only inside Update() with:

SensedBots = getCurrentOwner()->GetSensoryMem()
              ->GetListOfRecentlySensedOpponents();
curBot = SensedBots.begin();

It is posible that you change SensedBots in other place (for example to other getCurrentOwner()->GetSensoryMem()->GetListOfRecentlySensedOpponents();) and dont set curBot , invalidating the previus value.

Typically you do:

for (curBot=SensedBots.begin();  curBot != SensedBots.end(); ++curBot)

Even if you call Update() making curBot=SensedBots.begin(); it is possible that you call pickTarget() 2 time, and SensedBots changed in size in betwen, invalidating the endlich first-call-value of curBot= SensedBots.end(); entering the second time into the for cycle with a "falch" value.