gdbsupport/offset-type.h

   1 /* Offset types for GDB.
   2
   3    Copyright (C) 2017-2020 Free Software Foundation, Inc.
   4
   5    This file is part of GDB.
   6
   7    This program is free software; you can redistribute it and/or modify
   8    it under the terms of the GNU General Public License as published by
   9    the Free Software Foundation; either version 3 of the License, or
  10    (at your option) any later version.
  11
  12    This program is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  15    GNU General Public License for more details.
  16
  17    You should have received a copy of the GNU General Public License
  18    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
  19
  20 /* Define an "offset" type.  Offset types are distinct integer types
  21    that are used to represent an offset into anything that is
  22    addressable.  For example, an offset into a DWARF debug section.
  23    The idea is catch mixing unrelated offset types at compile time, in
  24    code that needs to manipulate multiple different kinds of offsets
  25    that are easily confused.  They're safer to use than native
  26    integers, because they have no implicit conversion to anything.
  27    And also, since they're implemented as "enum class" strong
  28    typedefs, they're still integers ABI-wise, making them a bit more
  29    efficient than wrapper structs on some ABIs.
  30
  31    Some properties of offset types, loosely modeled on pointers:
  32
  33    - You can compare offsets of the same type for equality and order.
  34      You can't compare an offset with an unrelated type.
  35
  36    - You can add/substract an integer to/from an offset, which gives
  37      you back a shifted offset.
  38
  39    - You can subtract two offsets of the same type, which gives you
  40      back the delta as an integer (of the enum class's underlying
  41      type), not as an offset type.
  42
  43    - You can't add two offsets of the same type, as that would not
  44      make sense.
  45
  46    However, unlike pointers, you can't deference offset types.  */
  47
  48 #ifndef COMMON_OFFSET_TYPE_H
  49 #define COMMON_OFFSET_TYPE_H
  50
  51 /* Declare TYPE as being an offset type.  This declares the type and
  52    enables the operators defined below.  */
  53 #define DEFINE_OFFSET_TYPE(TYPE, UNDERLYING)    \
  54   enum class TYPE : UNDERLYING {};              \
  55   void is_offset_type (TYPE)
  56
  57 /* The macro macro is all you need to know use offset types.  The rest
  58    below is all implementation detail.  */
  59
  60 /* For each enum class type that you want to support arithmetic
  61    operators, declare an "is_offset_type" overload that has exactly
  62    one parameter, of type that enum class.  E.g.,:
  63
  64      void is_offset_type (sect_offset);
  65
  66    The function does not need to be defined, only declared.
  67    DEFINE_OFFSET_TYPE declares this.
  68
  69    A function declaration is preferred over a traits type, because the
  70    former allows calling the DEFINE_OFFSET_TYPE macro inside a
  71    namespace to define the corresponding offset type in that
  72    namespace.  The compiler finds the corresponding is_offset_type
  73    function via ADL.
  74 */
  75
  76 /* Adding or subtracting an integer to an offset type shifts the
  77    offset.  This is like "PTR = PTR + INT" and "PTR += INT".  */
  78
  79 #define DEFINE_OFFSET_ARITHM_OP(OP)                                     \
  80   template<typename E,                                                  \
  81            typename = decltype (is_offset_type (std::declval<E> ()))>   \
  82   constexpr E                                                           \
  83   operator OP (E lhs, typename std::underlying_type<E>::type rhs)       \
  84   {                                                                     \
  85     using underlying = typename std::underlying_type<E>::type;          \
  86     return (E) (static_cast<underlying> (lhs) OP rhs);                  \
  87   }                                                                     \
  88                                                                         \
  89   template<typename E,                                                  \
  90            typename = decltype (is_offset_type (std::declval<E> ()))>   \
  91   constexpr E                                                           \
  92   operator OP (typename std::underlying_type<E>::type lhs, E rhs)       \
  93   {                                                                     \
  94     using underlying = typename std::underlying_type<E>::type;          \
  95     return (E) (lhs OP static_cast<underlying> (rhs));                  \
  96   }                                                                     \
  97                                                                         \
  98   template<typename E,                                                  \
  99            typename = decltype (is_offset_type (std::declval<E> ()))>   \
 100   E &                                                                   \
 101   operator OP ## = (E &lhs, typename std::underlying_type<E>::type rhs) \
 102   {                                                                     \
 103     using underlying = typename std::underlying_type<E>::type;          \
 104     lhs = (E) (static_cast<underlying> (lhs) OP rhs);                   \
 105     return lhs;                                                         \
 106   }
 107
 108 DEFINE_OFFSET_ARITHM_OP(+)
 109 DEFINE_OFFSET_ARITHM_OP(-)
 110
 111 /* Adding two offset types doesn't make sense, just like "PTR + PTR"
 112    doesn't make sense.  This is defined as a deleted function so that
 113    a compile error easily brings you to this comment.  */
 114
 115 template<typename E,
 116          typename = decltype (is_offset_type (std::declval<E> ()))>
 117 constexpr typename std::underlying_type<E>::type
 118 operator+ (E lhs, E rhs) = delete;
 119
 120 /* Subtracting two offset types, however, gives you back the
 121    difference between the offsets, as an underlying type.  Similar to
 122    how "PTR2 - PTR1" returns a ptrdiff_t.  */
 123
 124 template<typename E,
 125          typename = decltype (is_offset_type (std::declval<E> ()))>
 126 constexpr typename std::underlying_type<E>::type
 127 operator- (E lhs, E rhs)
 128 {
 129   using underlying = typename std::underlying_type<E>::type;
 130   return static_cast<underlying> (lhs) - static_cast<underlying> (rhs);
 131 }
 132
 133 #endif /* COMMON_OFFSET_TYPE_H */