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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