libcruft-util/singleton.hpp

/*
 * Licensed under the Apache License, Version 2.0 (the "License");
 * 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 2018 Danny Robson <danny@nerdcruft.net>
 */

#ifndef CRUFT_UTIL_SINGLETON_HPP
#define CRUFT_UTIL_SINGLETON_HPP

#include "debug.hpp"

#include <memory>


namespace util {
    /// statically stores a single value of the named type.
    ///
    /// this does not prevent the instantiation of the named type, but
    /// instead defines a method to refer to a single instance across an
    /// application.
    template <typename SelfT>
    class singleton {
    private:
        singleton () = default;

        static SelfT *instance;

    public:
        /// instantiates the one blessed value of this type.
        ///
        /// `instantiate` must be called once and once only before any calls
        /// to `get`. it is not threadsafe. there are some (unsafe)
        /// assertions against multiple instantiations but they may not be
        /// relied upon.
        ///
        /// returns a unique_ptr to the value which the caller must store
        /// until it will never be needed again.
        template <typename ...Args>
        static std::unique_ptr<SelfT>
        instantiate [[nodiscard]] (Args &&...args)
        {
            CHECK (!instance);

            auto cookie =std::make_unique<SelfT> (std::forward<Args> (args)...);
            instance = cookie.get ();
            return cookie;
        }


        /// returns a reference to sole instantiated value
        ///
        /// `instantiate` must have already been called before `get` is called.
        static SelfT&
        get (void)
        {
            CHECK (instance);
            return *instance;
        }
    };
};


template <typename SelfT>
SelfT*
util::singleton<SelfT>::instance;

#endif