ios - Bad Access using NSMutableArray with ARC -

at moment creating game cocos2d v3.1 using box2d, have use .mm extensions source files indicate using c++. game contains „endless terrain“ consists of terrain-key-points. created class saves key points named terrainelement. @ beginning of game create nsmutablearray called _elements array , fill random terrainelements.

within game loop check iterate on elements, , draw elements visible. app crashes after rather random amount of time.

header

@interface terrainbase : ccnode{      nsmutablearray * _elements; } 

part of game loop:

while (((terrainelement*)[_elements objectatindex:tokeypoint]).startpoint.x < _currentx+winsize.width*9/8) {      //crashes here exc_bad_access, _elements array null      terrainelement *element = (terrainelement*)[_elements objectatindex:tokeypoint];       [element drawelement];     tokeypoint++;  } 

i not able figure out why nsmutablearray null. iterate on elements in each loop, , not touch them basically. made property out of _elements array(nonatomic,strong), did not either. decided use normal array instead of nsmutablearray. did not change code @ all, except indexing of array:

@interface terrainbase : ccnode{     terrainelement *_elements[4000]; } 

gameloop:

while ( _elements[tokeypoint].startpoint.x< _currentx+winsize.width*9/8) {              [_elements[tokeypoint] drawelement];      tokeypoint++;  } 

this time works, , array never null. mystery me why second solution works first keeps crashing. prefer use nsmutablearray gives me more flexibility. can help?

you allowed send messages nil (null) not reason crash on own (you nil in case).

i'm not sure have given enough information determine actual cause. please post stacktrace , exceptions. posted code contain happening in loop?

your loop condition looks flawed logic me in couple of ways:

1. unless array sorted startpoint.x stop drawing terrain encounter first element >= _currentx+winsize.width*9/8
2. as stated above, can send messages nil , nil (or null or 0 or whatever want call it) value returned. means have infinite loop whenever _elements nil (because [_elements objectatindex:tokeypoint]).startpoint.x evaluate 0 < _currentx+winsize.width*9/8 presumably.

i need more information further.