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106 lines
4.6 KiB
C
106 lines
4.6 KiB
C
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
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/* vim: set ts=8 sts=2 et sw=2 tw=80: */
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/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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/*
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* Implements various macros meant to ease the use of variadic macros.
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*/
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#ifndef mozilla_MacroArgs_h
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#define mozilla_MacroArgs_h
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/*
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* MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) counts the number of variadic
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* arguments and prefixes it with |aPrefix|. For example:
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*
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* MOZ_PASTE_PREFIX_AND_ARG_COUNT(, foo, 42) expands to 2
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* MOZ_PASTE_PREFIX_AND_ARG_COUNT(A, foo, 42, bar) expands to A3
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*
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* You must pass in between 1 and 50 (inclusive) variadic arguments, past
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* |aPrefix|. It is not legal to do
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*
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* MOZ_PASTE_PREFIX_AND_ARG_COUNT(prefix)
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*
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* (that is, pass in 0 variadic arguments). To ensure that a compile-time
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* error occurs when these constraints are violated, use the
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* MOZ_STATIC_ASSERT_VALID_ARG_COUNT macro with the same variaidc arguments
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* wherever this macro is used.
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*
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* Passing (__VA_ARGS__, <rest of arguments>) rather than simply calling
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* MOZ_MACROARGS_ARG_COUNT_HELPER2(__VA_ARGS__, <rest of arguments>) very
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* carefully tiptoes around a MSVC bug where it improperly expands __VA_ARGS__
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* as a single token in argument lists. For details, see:
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*
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* http://connect.microsoft.com/VisualStudio/feedback/details/380090/variadic-macro-replacement
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* http://cplusplus.co.il/2010/07/17/variadic-macro-to-count-number-of-arguments/#comment-644
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*/
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#define MOZ_PASTE_PREFIX_AND_ARG_COUNT(aPrefix, ...) \
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MOZ_MACROARGS_ARG_COUNT_HELPER((__VA_ARGS__, \
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aPrefix##50, aPrefix##49, aPrefix##48, aPrefix##47, aPrefix##46, \
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aPrefix##45, aPrefix##44, aPrefix##43, aPrefix##42, aPrefix##41, \
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aPrefix##40, aPrefix##39, aPrefix##38, aPrefix##37, aPrefix##36, \
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aPrefix##35, aPrefix##34, aPrefix##33, aPrefix##32, aPrefix##31, \
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aPrefix##30, aPrefix##29, aPrefix##28, aPrefix##27, aPrefix##26, \
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aPrefix##25, aPrefix##24, aPrefix##23, aPrefix##22, aPrefix##21, \
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aPrefix##20, aPrefix##19, aPrefix##18, aPrefix##17, aPrefix##16, \
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aPrefix##15, aPrefix##14, aPrefix##13, aPrefix##12, aPrefix##11, \
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aPrefix##10, aPrefix##9, aPrefix##8, aPrefix##7, aPrefix##6, \
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aPrefix##5, aPrefix##4, aPrefix##3, aPrefix##2, aPrefix##1, aPrefix##0))
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#define MOZ_MACROARGS_ARG_COUNT_HELPER(aArgs) \
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MOZ_MACROARGS_ARG_COUNT_HELPER2 aArgs
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#define MOZ_MACROARGS_ARG_COUNT_HELPER2( \
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a1, a2, a3, a4, a5, a6, a7, a8, a9, a10, \
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a11, a12, a13, a14, a15, a16, a17, a18, a19, a20, \
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a21, a22, a23, a24, a25, a26, a27, a28, a29, a30, \
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a31, a32, a33, a34, a35, a36, a37, a38, a39, a40, \
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a41, a42, a43, a44, a45, a46, a47, a48, a49, a50, \
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a51, ...) a51
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/*
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* MOZ_STATIC_ASSERT_VALID_ARG_COUNT ensures that a compile-time error occurs
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* when the argument count constraints of MOZ_PASTE_PREFIX_AND_ARG_COUNT are
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* violated. Use this macro wherever MOZ_PASTE_PREFIX_AND_ARG_COUNT is used
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* and pass it the same variadic arguments.
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*
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* This macro employs a few dirty tricks to function. To detect the zero
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* argument case, |(__VA_ARGS__)| is stringified, sizeof-ed, and compared to
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* what it should be in the absence of arguments.
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*
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* Detecting too many arguments is a little trickier. With a valid argument
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* count and a prefix of 1, MOZ_PASTE_PREFIX_AND_ARG_COUNT expands to e.g. 14.
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* With a prefix of 0.0, it expands to e.g. 0.04. If there are too many
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* arguments, it expands to the first argument over the limit. If this
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* exceeding argument is a number, the assertion will fail as there is no
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* number than can simultaneously be both > 10 and == 0. If the exceeding
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* argument is not a number, a compile-time error should still occur due to
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* the operations performed on it.
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*/
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#define MOZ_MACROARGS_STRINGIFY_HELPER(x) #x
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#define MOZ_STATIC_ASSERT_VALID_ARG_COUNT(...) \
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static_assert( \
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sizeof(MOZ_MACROARGS_STRINGIFY_HELPER((__VA_ARGS__))) != sizeof("()") && \
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(MOZ_PASTE_PREFIX_AND_ARG_COUNT(1, __VA_ARGS__)) > 10 && \
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(int)(MOZ_PASTE_PREFIX_AND_ARG_COUNT(0.0, __VA_ARGS__)) == 0, \
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"MOZ_STATIC_ASSERT_VALID_ARG_COUNT requires 1 to 50 arguments") /* ; */
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/*
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* MOZ_ARGS_AFTER_N expands to its arguments excluding the first |N|
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* arguments. For example:
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*
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* MOZ_ARGS_AFTER_2(a, b, c, d) expands to: c, d
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*/
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#define MOZ_ARGS_AFTER_1(a1, ...) __VA_ARGS__
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#define MOZ_ARGS_AFTER_2(a1, a2, ...) __VA_ARGS__
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/*
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* MOZ_ARG_N expands to its |N|th argument.
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*/
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#define MOZ_ARG_1(a1, ...) a1
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#define MOZ_ARG_2(a1, a2, ...) a2
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#endif /* mozilla_MacroArgs_h */
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