Optional.h

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/*
 * Copyright (C) 1996-2022 The Squid Software Foundation and contributors
 *
 * Squid software is distributed under GPLv2+ license and includes
 * contributions from numerous individuals and organizations.
 * Please see the COPYING and CONTRIBUTORS files for details.
 */
 
#ifndef SQUID__SRC_BASE_OPTIONAL_H
#define SQUID__SRC_BASE_OPTIONAL_H
 
#include <exception>
#include <type_traits>
#include <utility>
 
class BadOptionalAccess: public std::exception
{
public:
    BadOptionalAccess() {}
    /* std::exception API */
    virtual const char* what() const noexcept override { return "bad-optional-access"; }
    virtual ~BadOptionalAccess() noexcept = default;
};
 
template <typename Value>
class Optional
{
public:
    // std::optional supports non-trivial types as well, but we
    // do not want to fiddle with unions to disable default Value constructor
    // until that work becomes necessary
    static_assert(std::is_trivial<Value>::value, "Value is trivial");
 
    constexpr Optional() noexcept {}
    constexpr explicit Optional(const Value &v): value_(v), hasValue_(true) {}
 
    constexpr explicit operator bool() const noexcept { return hasValue_; }
    constexpr bool has_value() const noexcept { return hasValue_; }
 
    const Value &value() const &
    {
        if (!hasValue_)
            throw BadOptionalAccess();
        return value_;
    }
 
    template <class Other>
    constexpr Value value_or(Other &&defaultValue) const &
    {
        return hasValue_ ? value_ : static_cast<Value>(std::forward<Other>(defaultValue));
    }
 
    template <class Other = Value>
    Optional &operator =(Other &&otherValue)
    {
        value_ = std::forward<Other>(otherValue);
        hasValue_ = true;
        return *this;
    }
 
private:
    Value value_; // stored value; inaccessible/uninitialized unless hasValue_
    bool hasValue_ = false;
};
 
#endif /* SQUID__SRC_BASE_OPTIONAL_H */