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- </div>
- <div class="section">
- <div class="titlepage"><div><div><h2 class="title" style="clear: both">
- <a name="boost_optional.development"></a><a class="link" href="development.html" title="Development">Development</a>
- </h2></div></div></div>
- <div class="toc"><dl class="toc">
- <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>
- </dl></div>
- <div class="section">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_optional.development.the_models"></a><a class="link" 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>.
- (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
- <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
- has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <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>—where <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 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.
- </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 class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
- implies copies of the value.
- </li>
- <li class="listitem">
- <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
- between variants matches both current types and values
- </li>
- <li class="listitem">
- 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 class="listitem">
- 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 class="listitem">
- 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 class="listitem">
- 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 class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
- implies copies of the value.
- </li>
- <li class="listitem">
- <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
- between containers compare container size and if match, contained value
- </li>
- <li class="listitem">
- 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 class="listitem">
- If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
- optional.
- </li>
- <li class="listitem">
- Testing if the container is empty models testing if the optional is initialized
- </li>
- <li class="listitem">
- 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">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_optional.development.the_semantics"></a><a class="link" 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 class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <span class="bold"><strong>Default Construction:</strong></span> To introduce a
- formally uninitialized wrapped object.
- </li>
- <li class="listitem">
- <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 class="listitem">
- <span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a
- new yet equivalent wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
- To assign a value to the wrapped object.
- </li>
- <li class="listitem">
- <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 class="listitem">
- <span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
- to the wrapped object the value of another wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
- initialize the wrapped object with value of another wrapped object.
- </li>
- <li class="listitem">
- <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 class="listitem">
- <span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
- object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Initialization state query:</strong></span> To determine
- if the object is formally initialized or not.
- </li>
- <li class="listitem">
- <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 class="listitem">
- <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">
- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_optional.development.the_interface"></a><a class="link" 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>
- <h5>
- <a name="boost_optional.development.the_interface.h0"></a>
- <span class="phrase"><a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_untitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" 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 nonexistent 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 nonexistent) 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 contextual 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>
- <h5>
- <a name="boost_optional.development.the_interface.h1"></a>
- <span class="phrase"><a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" 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="../../../../../doc/src/images/warning.png"></td>
- <th align="left">Warning</th>
- </tr>
- <tr><td align="left" valign="top"><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></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"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
- Distributed under the Boost Software License, Version 1.0. (See accompanying
- file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
- </p>
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