1  // Obstack-related utilities.
       2  // Copyright (C) 2020-2023 Free Software Foundation, Inc.
       3  //
       4  // This file is part of GCC.
       5  //
       6  // GCC is free software; you can redistribute it and/or modify it under
       7  // the terms of the GNU General Public License as published by the Free
       8  // Software Foundation; either version 3, or (at your option) any later
       9  // version.
      10  //
      11  // GCC is distributed in the hope that it will be useful, but WITHOUT ANY
      12  // WARRANTY; without even the implied warranty of MERCHANTABILITY or
      13  // FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
      14  // for more details.
      15  //
      16  // You should have received a copy of the GNU General Public License
      17  // along with GCC; see the file COPYING3.  If not see
      18  // <http://www.gnu.org/licenses/>.
      19  
      20  #ifndef GCC_OBSTACK_UTILS_H
      21  #define GCC_OBSTACK_UTILS_H
      22  
      23  // This RAII class automatically frees memory allocated on an obstack,
      24  // unless told not to via keep ().  It automatically converts to an
      25  // obstack, so it can (optionally) be used in place of the obstack
      26  // to make the scoping clearer.  For example:
      27  //
      28  //     obstack_watermark watermark (ob);
      29  //     auto *ptr1 = XOBNEW (watermark, struct1);
      30  //     if (...)
      31  //       // Frees ptr1.
      32  //       return false;
      33  //
      34  //     auto *ptr2 = XOBNEW (watermark, struct2);
      35  //     if (...)
      36  //       // Frees ptr1 and ptr2.
      37  //       return false;
      38  //
      39  //     // Retains ptr1 and ptr2.
      40  //     watermark.keep ();
      41  //
      42  //     auto *ptr3 = XOBNEW (watermark, struct3);
      43  //     if (...)
      44  //       // Frees ptr3.
      45  //       return false;
      46  //
      47  //     // Retains ptr3 (in addition to ptr1 and ptr2 above).
      48  //     watermark.keep ();
      49  //     return true;
      50  //
      51  // The move constructor makes it possible to transfer ownership to a caller:
      52  //
      53  //     obstack_watermark
      54  //     foo ()
      55  //     {
      56  //       obstack_watermark watermark (ob);
      57  //       ...
      58  //       return watermark;
      59  //     }
      60  //
      61  //     void
      62  //     bar ()
      63  //     {
      64  //       // Inherit ownership of everything that foo allocated.
      65  //       obstack_watermark watermark = foo ();
      66  //       ...
      67  //     }
      68  class obstack_watermark
      69  {
      70  public:
      71    obstack_watermark (obstack *ob) : m_obstack (ob) { keep (); }
      72    constexpr obstack_watermark (obstack_watermark &&) = default;
      73    ~obstack_watermark () { obstack_free (m_obstack, m_start); }
      74  
      75    operator obstack *() const { return m_obstack; }
      76    void keep () { m_start = XOBNEWVAR (m_obstack, char, 0); }
      77  
      78  private:
      79    DISABLE_COPY_AND_ASSIGN (obstack_watermark);
      80  
      81  protected:
      82    obstack *m_obstack;
      83    char *m_start;
      84  };
      85  
      86  #endif