Question

I have std::vector of cells. Each cell has other std::vector to store some pointers to entities. Now I want to move pointer from one cell to another based on calculation new cell index.
But I am getting vector iterators incompatible. I know that is caused by push_back invalid the iterator, but I do not know why, because push_back doesn’t manipulate with current existing entityIter. How should I modified the following example to make it work?

    for(uint32 cellIndex = 0; cellIndex < m_cells.size(); ++cellIndex)
    {
        std::vector<entity_type> & entitiesInCurrentCell = m_cells[cellIndex].entities;
        std::vector<entity_type>::iterator entityIter = entitiesInCurrentCell.begin();
        while(entityIter != entitiesInCurrentCell.end())
        {
            entity_type entity = *entityIter;
            uint32 entityNewIndex = calculateIndex(entity->getPosition());

            if(entityNewIndex == cellIndex) 
            {
                ++entityIter;
                continue;
            }

            m_cells[entityNewIndex].entities.push_back(entity);
            entitiesInCurrentCell.erase(entityIter++);
        }
    }

entity_type is a pointer type which point to entity allocated elsewhere, I do not want to delete it, just move the pointer between cells.

(I know this approach is not the best way – relocating pointer to higher-index cell causes recalculation it – but this is aim of this question)

thank you

Answer 1

The line that erases from entitiesInCurrentCell has a bug. Correct it by writing

entityIter = entitiesInCurrentCell.erase(entityIter);

When erasing from a container, the next iterator is returned by the erase function, so you do not need to increment the iterator.