Question

I have a 3D vector defined like this…

std::vector<std::vector<std::list<Object*> > > m_objectTiles;

I have this code…

void ObjectManager::AddObject( Object *object ) {
  m_objects.push_back( object );
  m_objectTypes.insert( std::make_pair(
    ObjectAttorney::GetType( object ), object )); 

  int x = ObjectAttorney::GetTileX( object );
  int y = ObjectAttorney::GetTileY( object );
  m_objectTiles[x][y].push_back( object ); // SEG FAULT HERE
}

that receives this error 0x0805ccdb in std::vector<std::list<Object*, std::allocator<Object*> >, std::allocator<std::list<Object*, std::allocator<Object*> > > >::operator[] ( this=0x8157758, object=0x8173f30) at /usr/include/c++/4.4/bits/stl_vector.h:611 { return *(this->_M_impl._M_start + __n); }

I changed it to this to test it…

void ObjectManager::AddObject( Object *object ) {
  m_objects.push_back( object );
  m_objectTypes.insert( std::make_pair(
    ObjectAttorney::GetType( object ), object )); 

  int x = ObjectAttorney::GetTileX( object );
  int y = ObjectAttorney::GetTileY( object );
  std::list<Object*> *l = &m_objectTiles[x][y];
  if ( l ) { // SEG FAULT HERE
    l->push_back( object );
  } else {
    std::cout << "List null.\n";
  }
}

which just gives an error message saying where the seg fault occured ObjectManager::AddObject (this=0x81577a0, object=0x8165760) at ObjectManager.cpp:381 if ( l ) {

Why would a seg fault occur when testing for a null pointer? Obviously operator [] is returning something corrupted or invalid. Not sure what the problem is here. Any help is appreciated. Thanks.

Answer 1

std::vector‘s [] for performance reasons doesn’t do range checks. Obviously that second variant doesn’t help if x or y are out of range.

Add a check like that:

m_objectTiles.size() < x && m_objectTiles[x].size() < y

It is hard to judge from the quoted code, but it might be that you want std::vector to grow automatically. It will not. For that you would need something like that:

m_objectTiles.resize(x);
m_objectTiles[x].resize(y);

before accessing the m_objectTiles[x][y].