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| <div class="section" title="Development"> |
| <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> |
| <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" title="The models"> |
| <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>. |
| (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 class="itemizedlist" 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" 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" title="The semantics"> |
| <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" 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" title="The Interface"> |
| <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> |
| <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="id555569"></a> |
| <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 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="id555831"></a> |
| <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" title="Warning"><table border="0" summary="Warning"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/html/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> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003 -2007 Fernando Luis Cacciola Carballal<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>) |
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