onlineupdate/source/update/inc/mozilla/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 /*
  15  * MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) counts the number of variadic
  16  * arguments and prefixes it with |aPrefix|. For example:
  17  *
  18  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(, foo, 42) expands to 2
  19  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(A, foo, 42, bar) expands to A3
  20  *
  21  * You must pass in between 1 and 50 (inclusive) variadic arguments, past
  22  * |aPrefix|. It is not legal to do
  23  *
  24  *   MOZ_PASTE_PREFIX_AND_ARG_COUNT(prefix)
  25  *
  26  * (that is, pass in 0 variadic arguments). To ensure that a compile-time
  27  * error occurs when these constraints are violated, use the
  28  * MOZ_STATIC_ASSERT_VALID_ARG_COUNT macro with the same variaidc arguments
  29  * wherever this macro is used.
  30  *
  31  * Passing (__VA_ARGS__, <rest of arguments>) rather than simply calling
  32  * MOZ_MACROARGS_ARG_COUNT_HELPER2(__VA_ARGS__, <rest of arguments>) very
  33  * carefully tiptoes around a MSVC bug where it improperly expands __VA_ARGS__
  34  * as a single token in argument lists. For details, see:
  35  *
  36  *   http://connect.microsoft.com/VisualStudio/feedback/details/380090/variadic-macro-replacement
  37  *   http://cplusplus.co.il/2010/07/17/variadic-macro-to-count-number-of-arguments/#comment-644
  38  */
  39 #define MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) \
  40   MOZ_MACROARGS_ARG_COUNT_HELPER((__VA_ARGS__, \
  41     aPrefix##50, aPrefix##49, aPrefix##48, aPrefix##47, aPrefix##46, \
  42     aPrefix##45, aPrefix##44, aPrefix##43, aPrefix##42, aPrefix##41, \
  43     aPrefix##40, aPrefix##39, aPrefix##38, aPrefix##37, aPrefix##36, \
  44     aPrefix##35, aPrefix##34, aPrefix##33, aPrefix##32, aPrefix##31, \
  45     aPrefix##30, aPrefix##29, aPrefix##28, aPrefix##27, aPrefix##26, \
  46     aPrefix##25, aPrefix##24, aPrefix##23, aPrefix##22, aPrefix##21, \
  47     aPrefix##20, aPrefix##19, aPrefix##18, aPrefix##17, aPrefix##16, \
  48     aPrefix##15, aPrefix##14, aPrefix##13, aPrefix##12, aPrefix##11, \
  49     aPrefix##10, aPrefix##9,  aPrefix##8,  aPrefix##7,  aPrefix##6,  \
  50     aPrefix##5,  aPrefix##4,  aPrefix##3,  aPrefix##2,  aPrefix##1, aPrefix##0))
  51
  52 #define MOZ_MACROARGS_ARG_COUNT_HELPER(aArgs) \
  53   MOZ_MACROARGS_ARG_COUNT_HELPER2 aArgs
  54
  55 #define MOZ_MACROARGS_ARG_COUNT_HELPER2( \
  56    a1,  a2,  a3,  a4,  a5,  a6,  a7,  a8,  a9, a10, \
  57   a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, \
  58   a21, a22, a23, a24, a25, a26, a27, a28, a29, a30, \
  59   a31, a32, a33, a34, a35, a36, a37, a38, a39, a40, \
  60   a41, a42, a43, a44, a45, a46, a47, a48, a49, a50, \
  61   a51, ...) a51
  62
  63 /*
  64  * MOZ_STATIC_ASSERT_VALID_ARG_COUNT ensures that a compile-time error occurs
  65  * when the argument count constraints of MOZ_PASTE_PREFIX_AND_ARG_COUNT are
  66  * violated. Use this macro wherever MOZ_PASTE_PREFIX_AND_ARG_COUNT is used
  67  * and pass it the same variadic arguments.
  68  *
  69  * This macro employs a few dirty tricks to function. To detect the zero
  70  * argument case, |(__VA_ARGS__)| is stringified, sizeof-ed, and compared to
  71  * what it should be in the absence of arguments.
  72  *
  73  * Detecting too many arguments is a little trickier. With a valid argument
  74  * count and a prefix of 1, MOZ_PASTE_PREFIX_AND_ARG_COUNT expands to e.g. 14.
  75  * With a prefix of 0.0, it expands to e.g. 0.04. If there are too many
  76  * arguments, it expands to the first argument over the limit. If this
  77  * exceeding argument is a number, the assertion will fail as there is no
  78  * number than can simultaneously be both > 10 and == 0. If the exceeding
  79  * argument is not a number, a compile-time error should still occur due to
  80  * the operations performed on it.
  81  */
  82 #define MOZ_MACROARGS_STRINGIFY_HELPER(x) #x
  83 #define MOZ_STATIC_ASSERT_VALID_ARG_COUNT(...) \
  84   static_assert( \
  85     sizeof(MOZ_MACROARGS_STRINGIFY_HELPER((__VA_ARGS__))) != sizeof("()") && \
  86       (MOZ_PASTE_PREFIX_AND_ARG_COUNT(1, __VA_ARGS__)) > 10 && \
  87       (int)(MOZ_PASTE_PREFIX_AND_ARG_COUNT(0.0, __VA_ARGS__)) == 0, \
  88     "MOZ_STATIC_ASSERT_VALID_ARG_COUNT requires 1 to 50 arguments") /* ; */
  89
  90 /*
  91  * MOZ_ARGS_AFTER_N expands to its arguments excluding the first |N|
  92  * arguments. For example:
  93  *
  94  *   MOZ_ARGS_AFTER_2(a, b, c, d) expands to: c, d
  95  */
  96 #define MOZ_ARGS_AFTER_1(a1, ...) __VA_ARGS__
  97 #define MOZ_ARGS_AFTER_2(a1, a2, ...) __VA_ARGS__
  98
  99 /*
 100  * MOZ_ARG_N expands to its |N|th argument.
 101  */
 102 #define MOZ_ARG_1(a1, ...) a1
 103 #define MOZ_ARG_2(a1, a2, ...) a2
 104
 105 #endif /* mozilla_MacroArgs_h */