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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.development"></a><a href="development.html" title="Development">Development</a>
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</h2></div></div></div>
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<div class="toc"><dl>
|
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<dt><span class="section"><a href="development.html#boost_optional.development.the_models">The models</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
|
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</dl></div>
|
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<div class="section" lang="en">
|
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.development.the_models"></a><a href="development.html#boost_optional.development.the_models" title="The models">The models</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
|
||||
an <span class="emphasis"><em>initial value</em></span> (through an <span class="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
|
||||
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
|
||||
has an <span class="emphasis"><em>indeterminate initial value</em></span> (c.f. 8.5.9). <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="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 <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and no initial value is given, the variable is <span class="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
|
||||
<span class="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 <span class="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: <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="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 <code class="computeroutput"><span class="identifier">T</span></code> plus some
|
||||
<span class="emphasis"><em>nil_t</em></span>—were <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD-can be modeled in modern languages as a <span class="bold"><strong>discriminated
|
||||
union</strong></span> of T and nil_t. Discriminated unions are often called <span class="emphasis"><em>variants</em></span>.
|
||||
A variant has a <span class="emphasis"><em>current type</em></span>, which in our case is either
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
|
||||
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>.
|
||||
There is precedent for a discriminated union as a model for an optional value:
|
||||
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
|
||||
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="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 <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="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 <code class="computeroutput"><span class="identifier">nil_t</span></code> with
|
||||
respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> suggests
|
||||
an alternative model: a <span class="emphasis"><em>container</em></span> that either has a
|
||||
value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedence 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.
|
||||
</p>
|
||||
<p>
|
||||
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 <span class="emphasis"><em>exact</em></span>
|
||||
semantics required for a wrapper of optional values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul type="disc">
|
||||
<li>
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li>
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li>
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li>
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li>
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code> from
|
||||
a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul type="disc">
|
||||
<li>
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li>
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li>
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li>
|
||||
Testing if the container is empty models testing if the optional is initialized
|
||||
</li>
|
||||
<li>
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code> from
|
||||
an empty container models the undefined behavior of trying to access the
|
||||
value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section" lang="en">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_semantics"></a><a href="development.html#boost_optional.development.the_semantics" title="The semantics">The semantics</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
are intended to be used in places where objects of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
would but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
purpose is to formalize the additional possibly uninitialized state. From
|
||||
the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> could
|
||||
be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="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
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
|
||||
<span class="emphasis"><em>subtype</em></span> of <code class="computeroutput"><span class="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 <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul type="disc">
|
||||
<li>
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce a formally
|
||||
uninitialized wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span> To
|
||||
introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a new
|
||||
yet equivalent wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of some
|
||||
object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
|
||||
to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To initialize
|
||||
the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by the
|
||||
type T):</strong></span> To compare wrapped object values taking into account
|
||||
the presence of uninitialized states.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped object.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
|
||||
whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
swap).
|
||||
</li>
|
||||
<li>
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
|
||||
object (if any) and leave the wrapper in the uninitialized state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
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.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section" lang="en">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_interface"></a><a href="development.html#boost_optional.development.the_interface" title="The Interface">The Interface</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
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 <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface, the
|
||||
following must be noted: Even if all the operations supported by an instance
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code> are defined for
|
||||
the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="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 <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
itself is merely a <code class="computeroutput"><span class="identifier">T</span></code> wrapper
|
||||
(modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will be
|
||||
totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="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 <code class="computeroutput"><span class="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 <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a><h5>
|
||||
<a name="id2615242"></a>
|
||||
<a href="development.html#boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly untitialized optional objects: The
|
||||
operators * and -></a>
|
||||
</h5>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> 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 inexistent objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
facto</em></span> 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 <span class="emphasis"><em>refer</em></span>
|
||||
to optional (that is, possibly inexistent) objects; particularly as optional
|
||||
arguments to a function, but also quite often as optional data members.
|
||||
</p>
|
||||
<p>
|
||||
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: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span> <span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code>
|
||||
and <code class="computeroutput"><span class="special">(</span> <span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly
|
||||
convey the notion of optionality, and this information is tied to the <span class="emphasis"><em>syntax</em></span>
|
||||
of the expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="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 <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and conversion to <code class="computeroutput"><span class="keyword">bool</span></code> to convey
|
||||
the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="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 <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="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 <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a> concept
|
||||
incarnated by pointers.
|
||||
</p>
|
||||
<a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a><h5>
|
||||
<a name="id2615580"></a>
|
||||
<a href="development.html#boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h5>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top">
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
</td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
does not have shallow-copy so does not alias: two different optionals never
|
||||
refer to the <span class="emphasis"><em>same</em></span> value unless <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself is a reference (but may have <span class="emphasis"><em>equivalent</em></span> values).
|
||||
The difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and a pointer must be kept in mind, particularly because the semantics of
|
||||
relational operators are different: since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="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 <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="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
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
||||
<td align="left"></td>
|
||||
<td align="right"><small>Copyright <20> 2003 -2007 Fernando Luis Cacciola Carballal</small></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
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|
||||
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||||
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|
||||
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|
||||
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Reference in New Issue
Block a user