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