noncopyable_function.hh

noncopyable_function.hh

/*
 * This file is open source software, licensed to you under the terms
 * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
 * distributed with this work for additional information regarding copyright
 * ownership.  You may not use this file except in compliance with the License.
 *
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
/*
 * Copyright (C) 2017 ScyllaDB Ltd.
 */

#pragma once

#include <utility>
#include <type_traits>
#include <functional>

namespace seastar {

template <typename Signature>
class noncopyable_function;

template <typename Ret, typename... Args>
class noncopyable_function<Ret (Args...)> {
    static constexpr size_t nr_direct = 32;
    [[gnu::may_alias]]
    union storage {
        char direct[nr_direct];
        void* indirect;
    };
    using call_type = Ret (*)(const noncopyable_function* func, Args...);
    using move_type = void (*)(noncopyable_function* from, noncopyable_function* to);
    using destroy_type = void (*)(noncopyable_function* func);
    struct vtable {
        const call_type call;
        const move_type move;
        const destroy_type destroy;
    };
private:
    storage _storage;
    const vtable* _vtable;
private:
    static Ret empty_call(const noncopyable_function* func, Args... args) {
        throw std::bad_function_call();
    }
    static Ret empty_move(noncopyable_function* from, noncopyable_function* to) {}
    static Ret empty_destroy(noncopyable_function* func) {}
    static constexpr vtable _s_empty_vtable = {empty_call, empty_move, empty_destroy};
    // Accessor templates select either direct or indirect storage for Func
    template <typename Func>
    struct direct_accessor {
        static Func* access(noncopyable_function* func) { return reinterpret_cast<Func*>(func->_storage.direct); }
        static const Func* access(const noncopyable_function* func) { return reinterpret_cast<const Func*>(func->_storage.direct); }
    };
    template <typename Func>
    struct indirect_accessor {
        static Func* access(noncopyable_function* func) { return reinterpret_cast<Func*>(func->_storage.indirect); }
        static const Func* access(const noncopyable_function* func) { return reinterpret_cast<const Func*>(func->_storage.indirect); }
    };
    template <typename Func, template <class> class Accessor>
    struct make_vtable_for {
        static Ret call(const noncopyable_function* func, Args... args) {
            return (*Accessor<Func>::access(func))(std::forward<Args>(args)...);
        }
        static void move(noncopyable_function* from, noncopyable_function* to) {
            new (Accessor<Func>::access(to)) Func(std::move(*Accessor<Func>::access(from)));
        }
        static void destroy(noncopyable_function* func) {
            Accessor<Func>::access(func)->~Func();
        }
        static void initialize(Func&& from, noncopyable_function* to) {
            new (Accessor<Func>::access(to)) Func(std::move(from));
        }
        static constexpr vtable s_vtable = { &make_vtable_for::call, &make_vtable_for::move, &make_vtable_for::destroy };
    };
    template <typename Func, bool Direct = true>
    struct select_vtable_for : make_vtable_for<Func, direct_accessor> {};
    template <typename Func>
    struct select_vtable_for<Func, false> : make_vtable_for<Func, indirect_accessor> {};
    template <typename Func>
    static constexpr bool is_direct() {
        return sizeof(Func) <= nr_direct && alignof(Func) <= alignof(storage);
    }
    template <typename Func>
    struct vtable_for : select_vtable_for<Func, is_direct<Func>()> {};
public:
    noncopyable_function() noexcept : _vtable(&_s_empty_vtable) {}
    template <typename Func>
    noncopyable_function(Func&& func) noexcept : _vtable(&vtable_for<Func>::s_vtable) {
        vtable_for<Func>::initialize(std::move(func), this);
    }
    template <typename Object, typename... AllButFirstArg>
    noncopyable_function(Ret (Object::*member)(AllButFirstArg...)) noexcept : noncopyable_function(std::mem_fn(member)) {}
    template <typename Object, typename... AllButFirstArg>
    noncopyable_function(Ret (Object::*member)(AllButFirstArg...) const) noexcept : noncopyable_function(std::mem_fn(member)) {}

    Ret operator()(Args... args) const {
        return _vtable->call(this, std::forward<Args>(args)...);
    }
};


template <typename Ret, typename... Args>
template <typename Func, template <class> class Accessor>
const typename noncopyable_function<Ret (Args...)>::vtable noncopyable_function<Ret (Args...)>::template make_vtable_for<Func, Accessor>::s_vtable;

}