// reserved capacity of an Element_vector may be lost in copying.
for (size_t i = 0; i < this->chunks_.size(); ++i)
this->chunks_[i].reserve(chunk_size);
// reserved capacity of an Element_vector may be lost in copying.
for (size_t i = 0; i < this->chunks_.size(); ++i)
this->chunks_[i].reserve(chunk_size);