Optional.h

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/*
 * Copyright (c) Onix Solutions Limited. All rights reserved.
 *
 * This software owned by Onix Solutions Limited and is protected by copyright law
 * and international copyright treaties.
 *
 * Access to and use of the software is governed by the terms of the applicable ONIXS Software
 * Services Agreement (the Agreement) and Customer end user license agreements granting
 * a non-assignable, non-transferable and non-exclusive license to use the software
 * for it's own data processing purposes under the terms defined in the Agreement.
 *
 * Except as otherwise granted within the terms of the Agreement, copying or reproduction of any part
 * of this source code or associated reference material to any other location for further reproduction
 * or redistribution, and any amendments to this copyright notice, are expressly prohibited.
 *
 * Any reproduction or redistribution for sale or hiring of the Software not in accordance with
 * the terms of the Agreement is a violation of copyright law.
 */
 
#pragma once
 
#include "Compiler.h"
#include "Export.h"
 
#include <new>
#include <ostream>
 
namespace OnixS { namespace ICE { namespace iMpact { namespace MarketData {
 
template <typename T>
class Optional
{
public:
    Optional()
      : init_(false)
    {
    }

    Optional(const T& val)
      : init_(true)
    {
        construct(val);
    }

    Optional(const Optional<T>& opt)
      : init_(opt.init_)
    {
        if (init_)
        {
            construct(opt.get());
        }
    }

    Optional& operator=(const Optional<T>& opt)
    {
        // Self-assignment has to be caught before `destruct()`, which would
        // otherwise destroy the very value `construct()` is about to read.
        if (this == &opt)
        {
            return *this;
        }
 
        destruct();
 
        if (opt.init_)
        {
            // `init_` is raised only once the value is there. `destruct()` has
            // already cleared it, so a throwing copy constructor leaves this
            // object empty rather than leaving the destructor to run `~T()`
            // over storage no object was ever built in.
            construct(opt.get());
 
            init_ = true;
        }
 
        return *this;
    }

    Optional& operator=(const T& val)
    {
        // `val` may be the contained value itself -- `opt = *opt`, or
        // `opt = opt.value()`. Destroying first would leave `construct()`
        // copying from an object that no longer exists.
        if (init_ && &val == &get())
        {
            return *this;
        }
 
        destruct();
 
        // Raised after the value is there, for the reason given above.
        construct(val);
 
        init_ = true;
 
        return *this;
    }

    ~Optional()
    {
        destruct();
    }

    operator bool() const
    {
        return init_;
    }

    T& operator*()
    {
        return get(); // undefined if not initialized
    }

    const T& operator*() const
    {
        return get(); // undefined if not initialized
    }

    T& value()
    {
        return **this;
    }

    const T& value() const
    {
        return **this;
    }

    void reset()
    {
        destruct();
 
        init_ = false;
    }
 
protected:
    void* storage()
    {
        return data_;
    }
 
    const void* storage() const
    {
        return data_;
    }
 
    // Placement `new` starts the lifetime of a `T` inside `data_`, but a pointer
    // produced by casting the storage is not by itself a pointer to that object.
    // Laundering it is what makes it one.
    T& get()
    {
        return *ONIXS_ICEMDH_LAUNDER(static_cast<T*>(storage()));
    }
 
    const T& get() const
    {
        return *ONIXS_ICEMDH_LAUNDER(static_cast<const T*>(storage()));
    }
 
    void destruct()
    {
        if (init_)
        {
            get().~T();
        }
        init_ = false;
    }
 
    void construct(const T& data)
    {
        new(storage()) T(data);
    }
 
    // `alignas` is what keeps the contained value properly aligned: a bare
    // `char` array has an alignment of 1, which makes every access to the value
    // constructed inside it a misaligned one, and so undefined behaviour.
    alignas(T) char data_[sizeof(T)];
    bool init_;
};

template <typename T>
std::ostream& operator<<(std::ostream& os, const Optional<T>& value)
{
    if (!value)
    {
        return os << "null";
    }
 
    return os << *value;
}
 
}}}} // namespace OnixS::ICE::iMpact::MarketData