Function push_back

src/include/buffer.h:1051–1057

      _num = 0;
    }
    void push_back(const ptr& bp) {
      if (bp.length() == 0)
	return;
      _buffers.push_back(*ptr_node::create(bp).release());
      _len += bp.length();
      _num += 1;
    }
    void push_back(ptr&& bp) {
      if (bp.length() == 0)
	return;