mfbt/MacroArgs.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 /*
   8  * Implements various macros meant to ease the use of variadic macros.
   9  */
  10
  11 #ifndef mozilla_MacroArgs_h
  12 #define mozilla_MacroArgs_h
  13
  14 // Concatenates pre-processor tokens in a way that can be used with __LINE__.
  15 #define MOZ_CONCAT2(x, y) x##y
  16 #define MOZ_CONCAT(x, y) MOZ_CONCAT2(x, y)
  17
  18 /*
  19  * MOZ_ARG_COUNT(...) counts the number of variadic arguments.
  20  * You must pass in between 0 and 50 (inclusive) variadic arguments.
  21  * For example:
  22  *
  23  *   MOZ_ARG_COUNT() expands to 0
  24  *   MOZ_ARG_COUNT(a) expands to 1
  25  *   MOZ_ARG_COUNT(a, b) expands to 2
  26  *
  27  * Implementation notes:
  28  * The `##__VA_ARGS__` form is a GCC extension that removes the comma if
  29  * __VA_ARGS__ is empty. It is supported by Clang too. MSVC ignores ##,
  30  * and its default behavior is already to strip the comma when __VA_ARGS__
  31  * is empty.
  32  *
  33  * So MOZ_MACROARGS_ARG_COUNT_HELPER() expands to
  34  *   (_, 50, 49, ...)
  35  * MOZ_MACROARGS_ARG_COUNT_HELPER(a) expands to
  36  *   (_, a, 50, 49, ...)
  37  * etc.
  38  */
  39 #define MOZ_ARG_COUNT(...) \
  40   MOZ_MACROARGS_ARG_COUNT_HELPER2(MOZ_MACROARGS_ARG_COUNT_HELPER(__VA_ARGS__))
  41
  42 #define MOZ_MACROARGS_ARG_COUNT_HELPER(...)                                    \
  43   (_, ##__VA_ARGS__, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37,   \
  44    36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, \
  45    17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
  46
  47 #define MOZ_MACROARGS_ARG_COUNT_HELPER2(aArgs) \
  48   MOZ_MACROARGS_ARG_COUNT_HELPER3 aArgs
  49
  50 #define MOZ_MACROARGS_ARG_COUNT_HELPER3(                                       \
  51     a0, a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, a11, a12, a13, a14, a15, a16, \
  52     a17, a18, a19, a20, a21, a22, a23, a24, a25, a26, a27, a28, a29, a30, a31, \
  53     a32, a33, a34, a35, a36, a37, a38, a39, a40, a41, a42, a43, a44, a45, a46, \
  54     a47, a48, a49, a50, a51, ...)                                              \
  55   a51
  56
  57 /*
  58  * MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) counts the number of variadic
  59  * arguments and prefixes it with |aPrefix|. For example:
  60  *
  61  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(, foo, 42) expands to 2
  62  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(A, foo, 42, bar) expands to A3
  63  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(A) expands to A0
  64  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT() expands to 0, but MSVC warns there
  65  *   aren't enough arguments given.
  66  *
  67  * You must pass in between 0 and 50 (inclusive) variadic arguments, past
  68  * |aPrefix|.
  69  */
  70 #define MOZ_PASTE_PREFIX_AND_ARG_COUNT_GLUE(a, b) a b
  71 #define MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) \
  72   MOZ_PASTE_PREFIX_AND_ARG_COUNT_GLUE(MOZ_CONCAT,    \
  73                                       (aPrefix, MOZ_ARG_COUNT(__VA_ARGS__)))
  74
  75 /*
  76  * MOZ_ARGS_AFTER_N expands to its arguments excluding the first |N|
  77  * arguments. For example:
  78  *
  79  *   MOZ_ARGS_AFTER_2(a, b, c, d) expands to: c, d
  80  */
  81 #define MOZ_ARGS_AFTER_1(a1, ...) __VA_ARGS__
  82 #define MOZ_ARGS_AFTER_2(a1, a2, ...) __VA_ARGS__
  83
  84 /*
  85  * MOZ_ARG_N expands to its |N|th argument.
  86  */
  87 #define MOZ_ARG_1(a1, ...) a1
  88 #define MOZ_ARG_2(a1, a2, ...) a2
  89 #define MOZ_ARG_3(a1, a2, a3, ...) a3
  90 #define MOZ_ARG_4(a1, a2, a3, a4, ...) a4
  91 #define MOZ_ARG_5(a1, a2, a3, a4, a5, ...) a5
  92 #define MOZ_ARG_6(a1, a2, a3, a4, a5, a6, ...) a6
  93 #define MOZ_ARG_7(a1, a2, a3, a4, a5, a6, a7, ...) a7
  94 #define MOZ_ARG_8(a1, a2, a3, a4, a5, a6, a7, a8, ...) a8
  95 #define MOZ_ARG_9(a1, a2, a3, a4, a5, a6, a7, a8, a9, ...) a9
  96
  97 #endif /* mozilla_MacroArgs_h */