Class ResizeContext

native/thirdpart/flatbuffers/reflection.cpp:177–305

Resize a FlatBuffer in-place by iterating through all offsets in the buffer and adjusting them by "delta" if they straddle the start offset. Once that is done, bytes can now be inserted/deleted safely. "delta" may be negative (shrinking). Unless "delta" is a multiple of the largest alignment, you'll create a small amount of garbage space in the buffer (usually 0..7 bytes). If your FlatBuffer's roo

// If your FlatBuffer's root table is not the schema's root table, you should
// pass in your root_table type as well.
class ResizeContext {
 public:
  ResizeContext(const reflection::Schema &schema, uoffset_t start, int delta,
                std::vector<uint8_t> *flatbuf,
                const reflection::Object *root_table = nullptr)
      : schema_(schema),
        startptr_(vector_data(*flatbuf) + start),
        delta_(delta),
        buf_(*flatbuf),
        dag_check_(flatbuf->size() / sizeof(uoffset_t), false) {
    auto mask = static_cast<int>(sizeof(largest_scalar_t) - 1);
    delta_ = (delta_ + mask) & ~mask;
    if (!delta_) return;  // We can't shrink by less than largest_scalar_t.
    // Now change all the offsets by delta_.
    auto root = GetAnyRoot(vector_data(buf_));
    Straddle<uoffset_t, 1>(vector_data(buf_), root, vector_data(buf_));
    ResizeTable(root_table ? *root_table : *schema.root_table(), root);
    // We can now add or remove bytes at start.
    if (delta_ > 0)
      buf_.insert(buf_.begin() + start, delta_, 0);
    else
      buf_.erase(buf_.begin() + start + delta_, buf_.begin() + start);
  }

  // Check if the range between first (lower address) and second straddles
  // the insertion point. If it does, change the offset at offsetloc (of
  // type T, with direction D).
  template<typename T, int D>
  void Straddle(const void *first, const void *second, void *offsetloc) {
    if (first <= startptr_ && second >= startptr_) {
      WriteScalar<T>(offsetloc, ReadScalar<T>(offsetloc) + delta_ * D);
      DagCheck(offsetloc) = true;
    }
  }

  // This returns a boolean that records if the corresponding offset location
  // has been modified already. If so, we can't even read the corresponding
  // offset, since it is pointing to a location that is illegal until the
  // resize actually happens.
  // This must be checked for every offset, since we can't know which offsets
  // will straddle and which won't.
  uint8_t &DagCheck(const void *offsetloc) {
    auto dag_idx = reinterpret_cast<const uoffset_t *>(offsetloc) -
                   reinterpret_cast<const uoffset_t *>(vector_data(buf_));
    return dag_check_[dag_idx];
  }

  void ResizeTable(const reflection::Object &objectdef, Table *table) {
    if (DagCheck(table)) return;  // Table already visited.
    auto vtable = table->GetVTable();
    // Early out: since all fields inside the table must point forwards in
    // memory, if the insertion point is before the table we can stop here.
    auto tableloc = reinterpret_cast<uint8_t *>(table);
    if (startptr_ <= tableloc) {
      // Check if insertion point is between the table and a vtable that
      // precedes it. This can't happen in current construction code, but check
      // just in case we ever change the way flatbuffers are built.
      Straddle<soffset_t, -1>(vtable, table, table);