c++ - deque iterator not dereferencable error in thread pool -

i've written thread pool using sdl2's threading library, uses following thread function:

int threadfunc(void * pdata) {     threaddata* data = (threaddata*)pdata;     sdltaskmanager* pool = data->pool;     task* task = nullptr;     while (true)     {         sdl_lockmutex(pool->getmutex());         while (!pool->isrunning() && !pool->hastasks())         {             sdl_condwait(pool->getcondition(), pool->getmutex());         }         if (pool->shuttingdown())         {             return 0;         }         task = pool->getnexttask(); //sdl_condwait relocks mutex, *should* thread safe.         if (task != nullptr)         {             pool->notifythreadworking(true);             data->taskcount++;         }         else         {             pool->stop();             sdl_unlockmutex(pool->getmutex());             continue;         }         sdl_unlockmutex(pool->getmutex());         task->execute();         sdl_lockmutex(pool->getmutex());         pool->notifythreadworking(false);         pool->addcompletetask(task);         sdl_unlockmutex(pool->getmutex());     }     return 0; } 

i debug assertation error "deque iterator not dereferencable", when pool->getnexttask() called. i've checked contents of deque pool maintains via call stack , found it's not empty when function called.

the function code follows:

task * sdltaskmanager::getnexttask() {     task* t = nullptr;     if (!mcurrenttasks.empty())     {            t = mcurrenttasks.front();  //this part causing error         mcurrenttasks.pop_front();  //this std::deque<task*>     }     return t; } 

the deque has valid data in it, i'm @ loss causing problem.