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- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
- </h3></div></div></div>
- <div class="section">
- <div class="titlepage"><div><div><h4 class="title">
- <a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
- models</a>
- </h4></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>
- <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 © 2014-2018 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>
- </div></td>
- </tr></table>
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