In C++, we can <spanclass="emphasis"><em>declare</em></span> an object (a variable) of type
<codeclass="computeroutput"><spanclass="identifier">T</span></code>, and we can give this variable
an <spanclass="emphasis"><em>initial value</em></span> (through an <spanclass="emphasis"><em>initializer</em></span>.
(c.f. 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
<spanclass="bold"><strong>uninitialized</strong></span>. Its actual value exist but
has an <spanclass="emphasis"><em>indeterminate initial value</em></span> (c.f. 8.5.9). <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code> 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 <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
and no initial value is given, the variable is <spanclass="emphasis"><em>formally</em></span>
uninitialized. A formally uninitialized optional object has conceptually
no value at all and this situation can be tested at runtime. It is formally
<spanclass="emphasis"><em>undefined behavior</em></span> 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 <spanclass="emphasis"><em>uninitialized</em></span>.
</p>
<p>
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: <codeclass="computeroutput"><spanclass="identifier">EOF</span></code>, <codeclass="computeroutput"><spanclass="identifier">npos</span></code>, -1, etc... This is equivalent to
adding the special value to the set of possible values of a given type. This
super set of <codeclass="computeroutput"><spanclass="identifier">T</span></code> plus some
is some stateless POD-can be modeled in modern languages as a <spanclass="bold"><strong>discriminated
union</strong></span> of T and nil_t. Discriminated unions are often called <spanclass="emphasis"><em>variants</em></span>.
A variant has a <spanclass="emphasis"><em>current type</em></span>, which in our case is either
<codeclass="computeroutput"><spanclass="identifier">T</span></code> or <codeclass="computeroutput"><spanclass="identifier">nil_t</span></code>.
Using the <ahref="../../../../variant/index.html"target="_top">Boost.Variant</a>
library, this model can be implemented in terms of <codeclass="computeroutput"><spanclass="identifier">boost</span><spanclass="special">::</span><spanclass="identifier">variant</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">,</span><spanclass="identifier">nil_t</span><spanclass="special">></span></code>.
There is precedent for a discriminated union as a model for an optional value:
the <ahref="http://www.haskell.org/"target="_top">Haskell</a><spanclass="bold"><strong>Maybe</strong></span>
built-in type constructor. Thus, a discriminated union <codeclass="computeroutput"><spanclass="identifier">T</span><spanclass="special">+</span><spanclass="identifier">nil_t</span></code>
serves as a conceptual foundation.
</p>
<p>
A <codeclass="computeroutput"><spanclass="identifier">variant</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">,</span><spanclass="identifier">nil_t</span><spanclass="special">></span></code> follows naturally from the traditional
idiom of extending the range of possible values adding an additional sentinel
value with the special meaning of <spanclass="emphasis"><em>Nothing</em></span>. However,
this additional <spanclass="emphasis"><em>Nothing</em></span> 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.
</p>
<p>
The observation made in the last paragraph about the irrelevant nature of
the additional <codeclass="computeroutput"><spanclass="identifier">nil_t</span></code> with
respect to <spanclass="underline">purpose</span> of <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code> suggests
an alternative model: a <spanclass="emphasis"><em>container</em></span> that either has a
value of <codeclass="computeroutput"><spanclass="identifier">T</span></code> or nothing.
If the container is not empty (contains an object of type <codeclass="computeroutput"><spanclass="identifier">T</span></code>), it is modeling an <spanclass="emphasis"><em>initialized</em></span>
Objects of type <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
are intended to be used in places where objects of type <codeclass="computeroutput"><spanclass="identifier">T</span></code>
would but which might be uninitialized. Hence, <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>'s
purpose is to formalize the additional possibly uninitialized state. From
the perspective of this role, <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
can have the same operational semantics of <codeclass="computeroutput"><spanclass="identifier">T</span></code>
plus the additional semantics corresponding to this special state. As such,
<codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code> could
be thought of as a <spanclass="emphasis"><em>supertype</em></span> of <codeclass="computeroutput"><spanclass="identifier">T</span></code>.
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
<codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code> a
<spanclass="emphasis"><em>subtype</em></span> of <codeclass="computeroutput"><spanclass="identifier">T</span></code>
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.
</p>
<p>
We can draw from the purpose of <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
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 <codeclass="computeroutput"><spanclass="identifier">T</span></code> type
as much as possible. In order to choose the proper degree of adoption of
the native <codeclass="computeroutput"><spanclass="identifier">T</span></code> interface, the
following must be noted: Even if all the operations supported by an instance
of type <codeclass="computeroutput"><spanclass="identifier">T</span></code> are defined for
the entire range of values for such a type, an <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
extends such a set of values with a new value for which most (otherwise valid)
operations are not defined in terms of <codeclass="computeroutput"><spanclass="identifier">T</span></code>.
</p>
<p>
Furthermore, since <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
itself is merely a <codeclass="computeroutput"><spanclass="identifier">T</span></code> wrapper
(modeling a <codeclass="computeroutput"><spanclass="identifier">T</span></code> supertype),
any attempt to define such operations upon uninitialized optionals will be
This library chooses an interface which follows from <codeclass="computeroutput"><spanclass="identifier">T</span></code>'s
interface only for those operations which are well defined (w.r.t the type
<codeclass="computeroutput"><spanclass="identifier">T</span></code>) even if any of the operands
are uninitialized. These operations include: construction, copy-construction,
assignment, swap and relational operations.
</p>
<p>
For the value access operations, which are undefined (w.r.t the type <codeclass="computeroutput"><spanclass="identifier">T</span></code>) when the operand is uninitialized, a
different interface is chosen (which will be explained next).
</p>
<p>
Also, the presence of the possibly uninitialized state requires additional
operations not provided by <codeclass="computeroutput"><spanclass="identifier">T</span></code>
itself which are supported by a special interface.
and <codeclass="computeroutput"><spanclass="special">(</span><spanclass="identifier">p</span><spanclass="special">-></span><spanclass="identifier">foo</span><spanclass="special">()</span><spanclass="special">)</span></code>, implicitly
convey the notion of optionality, and this information is tied to the <spanclass="emphasis"><em>syntax</em></span>
of the expressions. That is, the presence of operators <codeclass="computeroutput"><spanclass="special">*</span></code>
and <codeclass="computeroutput"><spanclass="special">-></span></code> tell by themselves
—without any additional context— that the expression will be undefined
unless the implied pointee actually exist.
</p>
<p>
Such a <spanclass="emphasis"><em>de facto</em></span> idiom for referring to optional objects
can be formalized in the form of a concept: the <ahref="../../../../utility/OptionalPointee.html"target="_top">OptionalPointee</a>
concept. This concept captures the syntactic usage of operators <codeclass="computeroutput"><spanclass="special">*</span></code>, <codeclass="computeroutput"><spanclass="special">-></span></code>
and conversion to <codeclass="computeroutput"><spanclass="keyword">bool</span></code> to convey
the notion of optionality.
</p>
<p>
However, pointers are good to <spanclass="underline">refer</span>
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.
</p>
<p>
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 <codeclass="computeroutput"><spanclass="identifier">X</span></code>, it can use <codeclass="computeroutput"><spanclass="identifier">shared_ptr</span><spanclass="special"><</span><spanclass="identifier">X</span><spanclass="special">></span></code>
as the return value. However, this requires dynamic allocation of <codeclass="computeroutput"><spanclass="identifier">X</span></code>. If <codeclass="computeroutput"><spanclass="identifier">X</span></code>
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 <ahref="../../../../utility/OptionalPointee.html"target="_top">OptionalPointee</a> concept
For value access operations <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><></span></code> uses operators <codeclass="computeroutput"><spanclass="special">*</span></code>
and <codeclass="computeroutput"><spanclass="special">-></span></code> to lexically warn
about the possibly uninitialized state appealing to the familiar pointer
However, it is particularly important to note that <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><></span></code> objects are not pointers. <spanclass="underline"><codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><></span></code> is not, and does not model, a pointer</span>.
For instance, <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><></span></code>
does not have shallow-copy so does not alias: two different optionals never
refer to the <spanclass="emphasis"><em>same</em></span> value unless <codeclass="computeroutput"><spanclass="identifier">T</span></code>
itself is a reference (but may have <spanclass="emphasis"><em>equivalent</em></span> values).
The difference between an <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
and a pointer must be kept in mind, particularly because the semantics of
relational operators are different: since <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
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 <codeclass="computeroutput"><spanclass="identifier">optional</span><spanclass="special"><</span><spanclass="identifier">T</span><spanclass="special">></span></code>
by <codeclass="computeroutput"><spanclass="identifier">T</span><spanclass="special">*</span></code>
on some situations but not always. Specifically, on generic code written
for both, you cannot use relational operators directly, and must use the
and <ahref="../../../../utility/OptionalPointee.html#less"target="_top"><codeclass="computeroutput"><spanclass="identifier">less_pointees</span><spanclass="special">()</span></code></a>
