common: add va_args.h
This PR is upstreaming changes in the Seraphis lib here: https://github.com/UkoeHB/monero/pull/39. This header adds a macro `VA_ARGS_COMMAPREFIX` which, when passed `__VA_ARGS__`, expands to `, __VA_ARGS__` unless the length of `__VA_ARGS__` is 0, in which case it expands to nothing. This macro is useful for passing/declaring optional function arguments.
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// Copyright (c) 2024, The Monero Project
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//
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// All rights reserved.
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//
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// Redistribution and use in source and binary forms, with or without modification, are
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// permitted provided that the following conditions are met:
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//
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// 1. Redistributions of source code must retain the above copyright notice, this list of
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// conditions and the following disclaimer.
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//
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// 2. Redistributions in binary form must reproduce the above copyright notice, this list
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// of conditions and the following disclaimer in the documentation and/or other
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// materials provided with the distribution.
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//
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// 3. Neither the name of the copyright holder nor the names of its contributors may be
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// used to endorse or promote products derived from this software without specific
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// prior written permission.
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//
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// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
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// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
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// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
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// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
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// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
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// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
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// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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#pragma once
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// Check for __VA_OPT__ support
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// Apdated from cpplearner's StackOverflow answer: https://stackoverflow.com/a/48045656
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#define PP_THIRD_ARG(a,b,c,...) c
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#define VA_OPT_SUPPORTED_I(...) PP_THIRD_ARG(__VA_OPT__(,),true,false,)
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#define VA_OPT_SUPPORTED VA_OPT_SUPPORTED_I(?)
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// VA_ARGS_COMMAPREFIX(): VA_ARGS_COMMAPREFIX(__VA_ARGS__) expands to __VA_ARGS__ with a comma in
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// front if more than one argument, else nothing.
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// If __VA_OPT__ supported, use that. Else, use GCC's ,## hack
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#if VA_OPT_SUPPORTED
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# define VA_ARGS_COMMAPREFIX(...) __VA_OPT__(,) __VA_ARGS__
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#else
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# define VA_ARGS_COMMAPREFIX(...) , ## __VA_ARGS__
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#endif
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