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 */ 106