std::unique_ptr 如何表示空指针？

这样写会编译错误：

struct TreeNode {
  lint w, lzy;
  size_t l, r;
  std::unique_ptr<TreeNode> lson, rson;

  TreeNode() {
    w = lzy = 0;
    l = r = -1;
    lson = rson = nullptr;
  }

  TreeNode(size_t _l, size_t _r) {
    l = _l, r = _r;
  }
};