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        It is recommended to use optional<T>
        in situations where there is exactly one, clear (to all parties) reason for
        having no value of type T,
        and where the lack of value is as natural as having any regular value of
        T. One example of such situation
        is asking the user in some GUI form to optionally specify some limit on an
        int value, but the user is allowed
        to say 'I want the number not to be constrained by the maximum'. For another
        example, consider a config parameter specifying how many threads the application
        should launch. Leaving this parameter unspecified means that the application
        should decide itself. For yet another example, consider a function returning
        the index of the smallest element in a vector.
        We need to be prepared for the situation, where the vector
        is empty. Therefore a natural signature for such function would be:
      
template <typename T> optional<size_t> find_smallest_elem(const std::vector<T>& vec);
        Here, having received an empty vec
        and having no size_t to return
        is not a failure but a normal,
        albeit irregular, situation.
      
Another typical situation is to indicate that we do not have a value yet, but we expect to have it later. This notion can be used in implementing solutions like lazy initialization or a two-phase initialization.
        optional can be used to take
        a non-DefaultConstructible type T and create a sibling type with a default
        constructor. This is a way to add a null-state to any
        type that doesn't have it already.
      
        Sometimes type T already
        provides a built-in null-state, but it may still be useful to wrap it into
        optional. Consider std::string.
        When you read a piece of text from a GUI form or a DB table, it is hardly
        ever that the empty string indicates anything else but a missing text. And
        some data bases do not even distinguish between a null string entry and a
        non-null string of length 0. Still, it may be practical to use optional<string>
        to indicate in the returned type that we want to treat the empty string in
        a special dedicated program path:
      
if(boost::optional<std::string> name = ask_user_name()) { assert(*name != ""); logon_as(*name); } else { skip_logon(); }
        In the example above, the assertion indicates that if we choose to use this
        technique, we must translate the empty string state to an optional object
        with no contained value (inside function ask_user_name).
      
        It is not recommended to use optional
        to indicate that we were not able to compute a value because of a failure.
        It is difficult to define what a failure is, but it usually has one common
        characteristic: an associated information on the cause of the failure. This
        can be the type and member data of an exception object, or an error code.
        It is a bad design to signal a failure and not inform about the cause. If
        you do not want to use exceptions, and do not like the fact that by returning
        error codes you cannot return the computed value, you can use Expected
        library. It is sort of Boost.Variant
        that contains either a computed value or a reason why the computation failed.
      
        Sometimes the distinction into what is a failure and what is a valid but
        irregular result is blurry and depends on a particular usage and personal
        preference. Consider a function that converts a string
        to an int. Is it a failure that
        you cannot convert? It might in some cases, but in other you may call it
        exactly for the purpose of figuring out if a given string
        is convertible, and you are not even interested in the resulting value. Sometimes
        when a conversion fails you may not consider it a failure, but you need to
        know why it cannot be converted; for instance at which character it is determined
        that the conversion is impossible. In this case returning optional<T>
        will not suffice. Finally, there is a use case where an input string that
        does not represent an int is
        not a failure condition, but during the conversion we use resources whose
        acquisition may fail. In that case the natural representation is to both
        return optional<int> and
        signal failure:
      
optional<int> convert1(const string& str); // throws expected<ErrorT, optional<int>> convert2(const string& str); // return either optional or error