strict-aliasing violation in stl

struct TreeNodeBase
{
    int color;
    TreeNodeBase *parent;
    TreeNodeBase *left;
    TreeNodeBase *right;
};

struct TreeNode : public TreeNodeBase
{
    double value;
};

struct Tree
{
    TreeNodeBase header;
    unsigned long node_count;
};

extern void side_effect(Tree *);

double write_eps(int n)
{
    Tree cells;
    cells.node_count = 0;
    cells.header.color = 0;
    cells.header.parent = 0;
    cells.header.left = &cells.header;
    cells.header.right = &cells.header;

    for (int i = 0; i < n; ++i)
    {
        side_effect(&cells);
    }

    return static_cast<TreeNode*>(cells.header.left)->value;
}