100 lines
2.7 KiB
C++
100 lines
2.7 KiB
C++
/*
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* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/.
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*
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* Copyright 2018 Danny Robson <danny@nerdcruft.net>
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*/
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#pragma once
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#include "debug/assert.hpp"
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#include <memory>
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namespace cruft {
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/// statically stores a single value of the named type.
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///
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/// this does not prevent the instantiation of the named type, but
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/// instead defines a method to refer to a single instance across an
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/// application.
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template <typename SelfT>
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class singleton {
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private:
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singleton () = default;
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static SelfT *instance;
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public:
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static bool
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is_valid (void) noexcept
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{
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return instance;
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}
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/// instantiates the one blessed value of this type.
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///
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/// `instantiate` must be called once and once only before any calls
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/// to `get`. it is not threadsafe. there are some (unsafe)
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/// assertions against multiple instantiations but they may not be
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/// relied upon.
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///
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/// returns a unique_ptr to the value which the caller must store
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/// until it will never be needed again.
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template <typename ...Args>
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static std::unique_ptr<SelfT>
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instantiate [[nodiscard]] (Args &&...args)
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{
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CHECK (!instance);
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auto cookie =std::make_unique<SelfT> (std::forward<Args> (args)...);
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instance = cookie.get ();
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return cookie;
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}
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/// Swap the stored object pointer for the provided one and return
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/// the old value.
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///
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/// Useful for bypassing `instantiate` by initialising the pointer
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/// to a pre-existing object.
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///
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/// The caller to swap must free any resources associated with the
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/// prior value and arrange for the new object to be safely destructed
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/// in the future.
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static SelfT*
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swap (SelfT *next)
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{
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return std::exchange (instance, next);
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}
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/// Returns a reference to the instantiated value.
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///
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/// `instantiate` must have already been called before `get` is called
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/// otherwise the results are undefined.
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static SelfT&
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get (void)
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{
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CHECK (instance);
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return *instance;
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}
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/// returns a pointer to the instantiated value if it exists, else we
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/// return nullptr.
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static SelfT*
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maybe (void)
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{
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return instance;
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}
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};
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};
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template <typename SelfT>
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SelfT*
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cruft::singleton<SelfT>::instance;
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