optional_value.h

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/*
 * Copyright © 2014 Canonical Ltd.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Lesser General Public License version 2 or 3 as
 * published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 *
 * Authored by: Alberto Aguirre <alberto.aguirre@canonical.com>
 */
 
#ifndef MIR_OPTIONAL_VALUE_H_
#define MIR_OPTIONAL_VALUE_H_
 
#include "mir/fatal.h"
#include <utility>
 
namespace mir
{
template<typename T>
class optional_value
{
public:
    optional_value() = default;
    optional_value(T const& value) : value_(value), is_set_{true} {}
 
    optional_value& operator=(T const& value)
    {
        value_ = value;
        is_set_ = true;
        return *this;
    }
 
    bool is_set() const { return is_set_; }
 
    T const& value() const
    {
        die_if_unset();
        return value_;
    }
 
    T& value()
    {
        die_if_unset();
        return value_;
    }
 
    T&& consume()
    {
        die_if_unset();
        is_set_ = false;
        return std::move(value_);
    }
 
    operator bool() const
    {
        return is_set();
    }
 
private:
    void die_if_unset() const
    {
        if (!is_set())
        {
            (*fatal_error)("Accessing value of unset optional");
        }
    }
 
    T value_;
    bool is_set_{false};
};
 
 
template<typename T>
inline bool operator == (optional_value<T> const& lhs, optional_value<T> const& rhs)
{
    return lhs.is_set() == rhs.is_set() &&
           (!lhs.is_set() || lhs.value() == rhs.value());
}
 
template<typename T>
inline bool operator != (optional_value<T> const& lhs, optional_value<T> const& rhs)
{
    return !(lhs == rhs);
}
 
template<typename T>
inline bool operator == (optional_value<T> const& lhs, T const& rhs)
{
    return lhs.is_set() && (lhs.value() == rhs);
}
 
template<typename T>
inline bool operator != (optional_value<T> const& lhs, T const& rhs)
{
    return !(lhs == rhs);
}
 
template<typename T>
inline bool operator == (T const& lhs, optional_value<T> const& rhs)
{
    return rhs == lhs;
}
 
template<typename T>
inline bool operator != (T const& lhs, optional_value<T> const& rhs)
{
    return rhs != lhs;
}
}
 
#endif /* MIR_OPTIONAL_VALUE_H_ */