libs/optional/doc/11_development.qbk
[/
Boost.Optional
Copyright (c) 2003-2007 Fernando Luis Cacciola Carballal
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE_1_0.txt or copy at
http://www.boost.org/LICENSE_1_0.txt)
]
[section Design Overview]
[section The models]
In C++, we can ['declare] an object (a variable) of type `T`, and we can give this
variable an ['initial value] (through an ['initializer]. (cf. 8.5)).
When a declaration includes a non-empty initializer (an initial value is given),
it is said that the object has been initialized.
If the declaration uses an empty initializer (no initial value is given), and
neither default nor value initialization applies, it is said that the object is
[*uninitialized]. Its actual value exist but has an ['indeterminate initial value]
(cf. 8.5/11).
`optional<T>` intends to formalize the notion of initialization (or lack of it)
allowing a program to test whether an object has been initialized and stating
that access to the value of an uninitialized object is undefined behavior. That
is, when a variable is declared as `optional<T>` and no initial value is given,
the variable is ['formally] uninitialized. A formally uninitialized optional object
has conceptually no value at all and this situation can be tested at runtime. It
is formally ['undefined behavior] to try to access the value of an uninitialized
optional. An uninitialized optional can be assigned a value, in which case its initialization state changes to initialized. Furthermore, given the formal
treatment of initialization states in optional objects, it is even possible to
reset an optional to ['uninitialized].
In C++ there is no formal notion of uninitialized objects, which means that
objects always have an initial value even if indeterminate.
As discussed on the previous section, this has a drawback because you need
additional information to tell if an object has been effectively initialized.
One of the typical ways in which this has been historically dealt with is via
a special value: `EOF`, `npos`, -1, etc... This is equivalent to adding the
special value to the set of possible values of a given type. This super set of
`T` plus some ['nil_t]—where `nil_t` is some stateless POD—can be modeled in modern
languages as a [*discriminated union] of T and nil_t. Discriminated unions are
often called ['variants]. A variant has a ['current type], which in our case is either
`T` or `nil_t`.
Using the __BOOST_VARIANT__ library, this model can be implemented in terms of `boost::variant<T,nil_t>`.
There is precedent for a discriminated union as a model for an optional value:
the __HASKELL__ [*Maybe] built-in type constructor. Thus, a discriminated union
`T+nil_t` serves as a conceptual foundation.
A `variant<T,nil_t>` follows naturally from the traditional idiom of extending
the range of possible values adding an additional sentinel value with the
special meaning of ['Nothing]. However, this additional ['Nothing] value is largely
irrelevant for our purpose since our goal is to formalize the notion of
uninitialized objects and, while a special extended value can be used to convey
that meaning, it is not strictly necessary in order to do so.
The observation made in the last paragraph about the irrelevant nature of the
additional `nil_t` with respect to [_purpose] of `optional<T>` suggests an
alternative model: a ['container] that either has a value of `T` or nothing.
As of this writing I don't know of any precedent for a variable-size
fixed-capacity (of 1) stack-based container model for optional values, yet I
believe this is the consequence of the lack of practical implementations of
such a container rather than an inherent shortcoming of the container model.
In any event, both the discriminated-union or the single-element container
models serve as a conceptual ground for a class representing optional—i.e.
possibly uninitialized—objects.
For instance, these models show the ['exact] semantics required for a wrapper
of optional values:
Discriminated-union:
* [*deep-copy] semantics: copies of the variant implies copies of the value.
* [*deep-relational] semantics: comparisons between variants matches both
current types and values
* If the variant's current type is `T`, it is modeling an ['initialized] optional.
* If the variant's current type is not `T`, it is modeling an ['uninitialized]
optional.
* Testing if the variant's current type is `T` models testing if the optional
is initialized
* Trying to extract a `T` from a variant when its current type is not `T`, models
the undefined behavior of trying to access the value of an uninitialized optional
Single-element container:
* [*deep-copy] semantics: copies of the container implies copies of the value.
* [*deep-relational] semantics: comparisons between containers compare container
size and if match, contained value
* If the container is not empty (contains an object of type `T`), it is modeling
an ['initialized] optional.
* If the container is empty, it is modeling an ['uninitialized] optional.
* Testing if the container is empty models testing if the optional is
initialized
* Trying to extract a `T` from an empty container models the undefined behavior
of trying to access the value of an uninitialized optional
[endsect]
[section The semantics]
Objects of type `optional<T>` are intended to be used in places where objects of
type `T` would but which might be uninitialized. Hence, `optional<T>`'s purpose is
to formalize the additional possibly uninitialized state.
From the perspective of this role, `optional<T>` can have the same operational
semantics of `T` plus the additional semantics corresponding to this special
state.
As such, `optional<T>` could be thought of as a ['supertype] of `T`. Of course, we
can't do that in C++, so we need to compose the desired semantics using a
different mechanism.
Doing it the other way around, that is, making `optional<T>` a ['subtype] of `T`
is not only conceptually wrong but also impractical: it is not allowed to
derive from a non-class type, such as a built-in type.
We can draw from the purpose of `optional<T>` the required basic semantics:
* [*Default Construction:] To introduce a formally uninitialized wrapped
object.
* [*Direct Value Construction via copy:] To introduce a formally initialized
wrapped object whose value is obtained as a copy of some object.
* [*Deep Copy Construction:] To obtain a new yet equivalent wrapped object.
* [*Direct Value Assignment (upon initialized):] To assign a value to the
wrapped object.
* [*Direct Value Assignment (upon uninitialized):] To initialize the wrapped
object with a value obtained as a copy of some object.
* [*Assignment (upon initialized):] To assign to the wrapped object the value
of another wrapped object.
* [*Assignment (upon uninitialized):] To initialize the wrapped object with
value of another wrapped object.
* [*Deep Relational Operations (when supported by the type T):] To compare
wrapped object values taking into account the presence of uninitialized states.
* [*Value access:] To unwrap the wrapped object.
* [*Initialization state query:] To determine if the object is formally
initialized or not.
* [*Swap:] To exchange wrapped objects. (with whatever exception safety
guarantees are provided by `T`'s swap).
* [*De-initialization:] To release the wrapped object (if any) and leave the
wrapper in the uninitialized state.
Additional operations are useful, such as converting constructors and
converting assignments, in-place construction and assignment, and safe
value access via a pointer to the wrapped object or null.
[endsect]
[section The Interface]
Since the purpose of optional is to allow us to use objects with a formal
uninitialized additional state, the interface could try to follow the
interface of the underlying `T` type as much as possible. In order to choose
the proper degree of adoption of the native `T` interface, the following must
be noted: Even if all the operations supported by an instance of type `T` are
defined for the entire range of values for such a type, an `optional<T>`
extends such a set of values with a new value for which most
(otherwise valid) operations are not defined in terms of `T`.
Furthermore, since `optional<T>` itself is merely a `T` wrapper (modeling a `T`
supertype), any attempt to define such operations upon uninitialized optionals
will be totally artificial w.r.t. `T`.
This library chooses an interface which follows from `T`'s interface only for
those operations which are well defined (w.r.t the type `T`) even if any of the
operands are uninitialized. These operations include: construction,
copy-construction, assignment, swap and relational operations.
For the value access operations, which are undefined (w.r.t the type `T`) when
the operand is uninitialized, a different interface is chosen (which will be
explained next).
Also, the presence of the possibly uninitialized state requires additional
operations not provided by `T` itself which are supported by a special interface.
[heading Lexically-hinted Value Access in the presence of possibly
uninitialized optional objects: The operators * and ->]
A relevant feature of a pointer is that it can have a [*null pointer value].
This is a ['special] value which is used to indicate that the pointer is not
referring to any object at all. In other words, null pointer values convey
the notion of nonexistent objects.
This meaning of the null pointer value allowed pointers to became a ['de
facto] standard for handling optional objects because all you have to do
to refer to a value which you don't really have is to use a null pointer
value of the appropriate type. Pointers have been used for decades—from
the days of C APIs to modern C++ libraries—to ['refer] to optional (that is,
possibly nonexistent) objects; particularly as optional arguments to a
function, but also quite often as optional data members.
The possible presence of a null pointer value makes the operations that
access the pointee's value possibly undefined, therefore, expressions which
use dereference and access operators, such as: `( *p = 2 )` and `( p->foo() )`,
implicitly convey the notion of optionality, and this information is tied to
the ['syntax] of the expressions. That is, the presence of operators `*` and `->`
tell by themselves —without any additional context— that the expression will
be undefined unless the implied pointee actually exist.
Such a ['de facto] idiom for referring to optional objects can be formalized
in the form of a concept: the __OPTIONAL_POINTEE__ concept.
This concept captures the syntactic usage of operators `*`, `->` and
contextual conversion to `bool` to convey the notion of optionality.
However, pointers are good to [_refer] to optional objects, but not particularly
good to handle the optional objects in all other respects, such as initializing
or moving/copying them. The problem resides in the shallow-copy of pointer
semantics: if you need to effectively move or copy the object, pointers alone
are not enough. The problem is that copies of pointers do not imply copies of
pointees. For example, as was discussed in the motivation, pointers alone
cannot be used to return optional objects from a function because the object
must move outside from the function and into the caller's context.
A solution to the shallow-copy problem that is often used is to resort to
dynamic allocation and use a smart pointer to automatically handle the details
of this. For example, if a function is to optionally return an object `X`, it can
use `shared_ptr<X>` as the return value. However, this requires dynamic allocation
of `X`. If `X` is a built-in or small POD, this technique is very poor in terms of
required resources. Optional objects are essentially values so it is very
convenient to be able to use automatic storage and deep-copy semantics to
manipulate optional values just as we do with ordinary values. Pointers do
not have this semantics, so are inappropriate for the initialization and
transport of optional values, yet are quite convenient for handling the access
to the possible undefined value because of the idiomatic aid present in the
__OPTIONAL_POINTEE__ concept incarnated by pointers.
[heading Optional<T> as a model of OptionalPointee]
For value access operations `optional<>` uses operators `*` and `->` to
lexically warn about the possibly uninitialized state appealing to the
familiar pointer semantics w.r.t. to null pointers.
[caution
However, it is particularly important to note that `optional<>` objects
are not pointers. [_`optional<>` is not, and does not model, a pointer].
]
For instance, `optional<>` does not have shallow-copy so does not alias:
two different optionals never refer to the ['same] value unless `T` itself is
a reference (but may have ['equivalent] values).
The difference between an `optional<T>` and a pointer must be kept in mind,
particularly because the semantics of relational operators are different:
since `optional<T>` is a value-wrapper, relational operators are deep: they
compare optional values; but relational operators for pointers are shallow:
they do not compare pointee values.
As a result, you might be able to replace `optional<T>` by `T*` on some
situations but not always. Specifically, on generic code written for both,
you cannot use relational operators directly, and must use the template
functions __FUNCTION_EQUAL_POINTEES__ and __FUNCTION_LESS_POINTEES__ instead.
[endsect]
[endsect]
