variant.hpp

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/*
    Copyright (C) 2021 - 2025
    Part of the Battle for Wesnoth Project https://www.wesnoth.org/
 
    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY.
 
    See the COPYING file for more details.
*/
 
#pragma once
 
/**
 * @file
 *
 * MacOS doesn't support std::visit when targing MacOS < 10.14 (currently we target 10.11).
 * This provides a wrapper around the STL variant API on all platforms except MacOS, which
 * instead utilizes boost::variant.
 */
 
#ifdef __APPLE__
#define USING_BOOST_VARIANT
#endif
 
#ifndef USING_BOOST_VARIANT
#include <variant>
#else
#include <boost/variant.hpp>
#endif
 
namespace utils
{
#ifndef USING_BOOST_VARIANT
 
using std::get;
using std::get_if;
using std::holds_alternative;
using std::monostate;
using std::variant;
using std::visit;
 
#else
 
using boost::get;
using boost::variant;
 
using monostate = boost::blank;
 
template<typename... Args>
inline auto visit(Args&&... args)
{
    return boost::apply_visitor(std::forward<Args>(args)...);
}
 
template<typename Ret, typename... Types>
inline bool holds_alternative(const boost::variant<Types...>& variant)
{
    return boost::get<Ret>(&variant) != nullptr;
}
 
template<typename Ret, typename... Types>
inline Ret* get_if(boost::variant<Types...>* variant)
{
    return boost::get<Ret>(variant);
}
 
template<typename Ret, typename... Types>
inline const Ret* get_if(const boost::variant<Types...>* variant)
{
    return boost::get<Ret>(variant);
}
 
#endif
 
template<typename... Types>
std::size_t variant_index(const variant<Types...>& var)
{
#ifndef USING_BOOST_VARIANT
    return var.index();
#else
    return var.which();
#endif
}
 
} // namespace utils