visitor.h

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/**********************************************************************
 * LeechCraft - modular cross-platform feature rich internet client.
 * Copyright (C) 2006-2014  Georg Rudoy
 *
 * Distributed under the Boost Software License, Version 1.0.
 * (See accompanying file LICENSE or copy at https://www.boost.org/LICENSE_1_0.txt)
 **********************************************************************/
 
#pragma once
 
#include <variant>
#include "void.h"
#include "util.h"
 
namespace LC
{
namespace Util
{
    namespace detail
    {
        template<typename... Bases>
        struct VisitorBase : std::decay_t<Bases>...
        {
            VisitorBase (Bases&&... bases)
            : std::decay_t<Bases> { std::forward<Bases> (bases) }...
            {
            }
 
            using std::decay_t<Bases>::operator()...;
        };
 
        template<class... Ts> struct Overloaded : Ts...
        {
            using Ts::operator()...;
        };
        template<class... Ts> Overloaded (Ts...) -> Overloaded<Ts...>;
    }
 
    template<typename... Vars, typename... Args>
    decltype (auto) Visit (const std::variant<Vars...>& v, Args&&... args)
    {
        return std::visit (detail::Overloaded { std::forward<Args> (args)... }, v);
    }
 
    template<typename... Vars, typename... Args>
    decltype (auto) Visit (std::variant<Vars...>& v, Args&&... args)
    {
        return std::visit (detail::Overloaded { std::forward<Args> (args)... }, v);
    }
 
    namespace detail
    {
        struct VisitorFinallyTag {};
    }
 
    template<typename FinallyFunc, typename... Args>
    class Visitor
    {
        detail::VisitorBase<Args...> Base_;
 
        FinallyFunc Finally_;
    public:
        Visitor (Args&&... args)
        : Base_ { std::forward<Args> (args)... }
        {
        }
 
        Visitor (detail::VisitorFinallyTag, Args&&... args, FinallyFunc&& func)
        : Base_ { std::forward<Args> (args)... }
        , Finally_ { std::forward<FinallyFunc> (func) }
        {
        }
 
        template<typename T>
        decltype (auto) operator() (const T& var) const
        {
            if constexpr (std::is_same_v<FinallyFunc, Void>)
                return Visit (var, Base_);
            else
            {
                const auto guard = MakeScopeGuard (Finally_);
                return Visit (var, Base_);
            }
        }
 
        template<typename F>
        Visitor<F, detail::VisitorBase<Args...>> Finally (F&& func)
        {
            return { detail::VisitorFinallyTag {}, std::move (Base_), std::forward<F> (func) };
        }
    };
 
    template<typename... Args>
    Visitor (Args&&...) -> Visitor<Void, Args...>;
 
    template<typename T, typename... Args>
    auto InvokeOn (T&& t, Args&&... args)
    {
        return detail::VisitorBase<Args...> { std::forward<Args> (args)... } (std::forward<T> (t));
    }
}
}