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  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_optional.development"></a><a class="link" href="development.html" title="Development">Development</a>
  28 </h2></div></div></div>
  29 <div class="toc"><dl class="toc">
  30 <dt><span class="section"><a href="development.html#boost_optional.development.the_models">The models</a></span></dt>
  31 <dt><span class="section"><a href="development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
  32 <dt><span class="section"><a href="development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
  33 </dl></div>
  34 <div class="section">
  35 <div class="titlepage"><div><div><h3 class="title">
  36 <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>
  37 </h3></div></div></div>
  38 <p>
  39         In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
  40         <code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
  41         an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
  42         (cf. 8.5)). When a declaration includes a non-empty initializer (an initial
  43         value is given), it is said that the object has been initialized. If the
  44         declaration uses an empty initializer (no initial value is given), and neither
  45         default nor value initialization applies, it is said that the object is
  46         <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
  47         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">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> intends
  48         to formalize the notion of initialization (or lack of it) allowing a program
  49         to test whether an object has been initialized and stating that access to
  50         the value of an uninitialized object is undefined behavior. That is, when
  51         a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  52         and no initial value is given, the variable is <span class="emphasis"><em>formally</em></span>
  53         uninitialized. A formally uninitialized optional object has conceptually
  54         no value at all and this situation can be tested at runtime. It is formally
  55         <span class="emphasis"><em>undefined behavior</em></span> to try to access the value of an
  56         uninitialized optional. An uninitialized optional can be assigned a value,
  57         in which case its initialization state changes to initialized. Furthermore,
  58         given the formal treatment of initialization states in optional objects,
  59         it is even possible to reset an optional to <span class="emphasis"><em>uninitialized</em></span>.
  60       </p>
  61 <p>
  62         In C++ there is no formal notion of uninitialized objects, which means that
  63         objects always have an initial value even if indeterminate. As discussed
  64         on the previous section, this has a drawback because you need additional
  65         information to tell if an object has been effectively initialized. One of
  66         the typical ways in which this has been historically dealt with is via a
  67         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
  68         adding the special value to the set of possible values of a given type. This
  69         super set of <code class="computeroutput"><span class="identifier">T</span></code> plus some
  70         <span class="emphasis"><em>nil_t</em></span>&#8212;where <code class="computeroutput"><span class="identifier">nil_t</span></code>
  71         is some stateless POD&#8212;can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
  72         unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a <span class="emphasis"><em>current
  73         type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code>
  74         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
  75         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">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>.
  76         There is precedent for a discriminated union as a model for an optional value:
  77         the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
  78         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>
  79         serves as a conceptual foundation.
  80       </p>
  81 <p>
  82         A <code class="computeroutput"><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code> follows naturally from the traditional
  83         idiom of extending the range of possible values adding an additional sentinel
  84         value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
  85         this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
  86         for our purpose since our goal is to formalize the notion of uninitialized
  87         objects and, while a special extended value can be used to convey that meaning,
  88         it is not strictly necessary in order to do so.
  89       </p>
  90 <p>
  91         The observation made in the last paragraph about the irrelevant nature of
  92         the additional <code class="computeroutput"><span class="identifier">nil_t</span></code> with
  93         respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> suggests
  94         an alternative model: a <span class="emphasis"><em>container</em></span> that either has a
  95         value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
  96       </p>
  97 <p>
  98         As of this writing I don't know of any precedent for a variable-size fixed-capacity
  99         (of 1) stack-based container model for optional values, yet I believe this
 100         is the consequence of the lack of practical implementations of such a container
 101         rather than an inherent shortcoming of the container model.
 102       </p>
 103 <p>
 104         In any event, both the discriminated-union or the single-element container
 105         models serve as a conceptual ground for a class representing optional&#8212;i.e.
 106         possibly uninitialized&#8212;objects. For instance, these models show the <span class="emphasis"><em>exact</em></span>
 107         semantics required for a wrapper of optional values:
 108       </p>
 109 <p>
 110         Discriminated-union:
 111       </p>
 112 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 113 <li class="listitem">
 114             <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
 115             implies copies of the value.
 116           </li>
 117 <li class="listitem">
 118             <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
 119             between variants matches both current types and values
 120           </li>
 121 <li class="listitem">
 122             If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
 123             it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
 124           </li>
 125 <li class="listitem">
 126             If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
 127             it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
 128           </li>
 129 <li class="listitem">
 130             Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
 131             models testing if the optional is initialized
 132           </li>
 133 <li class="listitem">
 134             Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
 135             from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
 136             models the undefined behavior of trying to access the value of an uninitialized
 137             optional
 138           </li>
 139 </ul></div>
 140 <p>
 141         Single-element container:
 142       </p>
 143 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 144 <li class="listitem">
 145             <span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
 146             implies copies of the value.
 147           </li>
 148 <li class="listitem">
 149             <span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
 150             between containers compare container size and if match, contained value
 151           </li>
 152 <li class="listitem">
 153             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>
 154             optional.
 155           </li>
 156 <li class="listitem">
 157             If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
 158             optional.
 159           </li>
 160 <li class="listitem">
 161             Testing if the container is empty models testing if the optional is initialized
 162           </li>
 163 <li class="listitem">
 164             Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
 165             from an empty container models the undefined behavior of trying to access
 166             the value of an uninitialized optional
 167           </li>
 168 </ul></div>
 169 </div>
 170 <div class="section">
 171 <div class="titlepage"><div><div><h3 class="title">
 172 <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>
 173 </h3></div></div></div>
 174 <p>
 175         Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 176         are intended to be used in places where objects of type <code class="computeroutput"><span class="identifier">T</span></code>
 177         would but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>'s
 178         purpose is to formalize the additional possibly uninitialized state. From
 179         the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 180         can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
 181         plus the additional semantics corresponding to this special state. As such,
 182         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> could
 183         be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>.
 184         Of course, we can't do that in C++, so we need to compose the desired semantics
 185         using a different mechanism. Doing it the other way around, that is, making
 186         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> a
 187         <span class="emphasis"><em>subtype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>
 188         is not only conceptually wrong but also impractical: it is not allowed to
 189         derive from a non-class type, such as a built-in type.
 190       </p>
 191 <p>
 192         We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 193         the required basic semantics:
 194       </p>
 195 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 196 <li class="listitem">
 197             <span class="bold"><strong>Default Construction:</strong></span> To introduce a
 198             formally uninitialized wrapped object.
 199           </li>
 200 <li class="listitem">
 201             <span class="bold"><strong>Direct Value Construction via copy:</strong></span>
 202             To introduce a formally initialized wrapped object whose value is obtained
 203             as a copy of some object.
 204           </li>
 205 <li class="listitem">
 206             <span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a
 207             new yet equivalent wrapped object.
 208           </li>
 209 <li class="listitem">
 210             <span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
 211             To assign a value to the wrapped object.
 212           </li>
 213 <li class="listitem">
 214             <span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
 215             To initialize the wrapped object with a value obtained as a copy of some
 216             object.
 217           </li>
 218 <li class="listitem">
 219             <span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
 220             to the wrapped object the value of another wrapped object.
 221           </li>
 222 <li class="listitem">
 223             <span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
 224             initialize the wrapped object with value of another wrapped object.
 225           </li>
 226 <li class="listitem">
 227             <span class="bold"><strong>Deep Relational Operations (when supported by the
 228             type T):</strong></span> To compare wrapped object values taking into account
 229             the presence of uninitialized states.
 230           </li>
 231 <li class="listitem">
 232             <span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
 233             object.
 234           </li>
 235 <li class="listitem">
 236             <span class="bold"><strong>Initialization state query:</strong></span> To determine
 237             if the object is formally initialized or not.
 238           </li>
 239 <li class="listitem">
 240             <span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
 241             whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
 242             swap).
 243           </li>
 244 <li class="listitem">
 245             <span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
 246             object (if any) and leave the wrapper in the uninitialized state.
 247           </li>
 248 </ul></div>
 249 <p>
 250         Additional operations are useful, such as converting constructors and converting
 251         assignments, in-place construction and assignment, and safe value access
 252         via a pointer to the wrapped object or null.
 253       </p>
 254 </div>
 255 <div class="section">
 256 <div class="titlepage"><div><div><h3 class="title">
 257 <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>
 258 </h3></div></div></div>
 259 <p>
 260         Since the purpose of optional is to allow us to use objects with a formal
 261         uninitialized additional state, the interface could try to follow the interface
 262         of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
 263         as much as possible. In order to choose the proper degree of adoption of
 264         the native <code class="computeroutput"><span class="identifier">T</span></code> interface, the
 265         following must be noted: Even if all the operations supported by an instance
 266         of type <code class="computeroutput"><span class="identifier">T</span></code> are defined for
 267         the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 268         extends such a set of values with a new value for which most (otherwise valid)
 269         operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
 270       </p>
 271 <p>
 272         Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 273         itself is merely a <code class="computeroutput"><span class="identifier">T</span></code> wrapper
 274         (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
 275         any attempt to define such operations upon uninitialized optionals will be
 276         totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
 277       </p>
 278 <p>
 279         This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
 280         interface only for those operations which are well defined (w.r.t the type
 281         <code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
 282         are uninitialized. These operations include: construction, copy-construction,
 283         assignment, swap and relational operations.
 284       </p>
 285 <p>
 286         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
 287         different interface is chosen (which will be explained next).
 288       </p>
 289 <p>
 290         Also, the presence of the possibly uninitialized state requires additional
 291         operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
 292         itself which are supported by a special interface.
 293       </p>
 294 <h5>
 295 <a name="boost_optional.development.the_interface.h0"></a>
 296         <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
 297         Value Access in the presence of possibly untitialized optional objects: The
 298         operators * and -&gt;</a>
 299       </h5>
 300 <p>
 301         A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
 302         pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value which
 303         is used to indicate that the pointer is not referring to any object at all.
 304         In other words, null pointer values convey the notion of nonexistent objects.
 305       </p>
 306 <p>
 307         This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
 308         facto</em></span> standard for handling optional objects because all you have
 309         to do to refer to a value which you don't really have is to use a null pointer
 310         value of the appropriate type. Pointers have been used for decades&#8212;from
 311         the days of C APIs to modern C++ libraries&#8212;to <span class="emphasis"><em>refer</em></span>
 312         to optional (that is, possibly nonexistent) objects; particularly as optional
 313         arguments to a function, but also quite often as optional data members.
 314       </p>
 315 <p>
 316         The possible presence of a null pointer value makes the operations that access
 317         the pointee's value possibly undefined, therefore, expressions which use
 318         dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
 319         <span class="special">*</span><span class="identifier">p</span> <span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code>
 320         and <code class="computeroutput"><span class="special">(</span> <span class="identifier">p</span><span class="special">-&gt;</span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly
 321         convey the notion of optionality, and this information is tied to the <span class="emphasis"><em>syntax</em></span>
 322         of the expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
 323         and <code class="computeroutput"><span class="special">-&gt;</span></code> tell by themselves
 324         &#8212;without any additional context&#8212; that the expression will be undefined
 325         unless the implied pointee actually exist.
 326       </p>
 327 <p>
 328         Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
 329         can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
 330         concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-&gt;</span></code>
 331         and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
 332         to convey the notion of optionality.
 333       </p>
 334 <p>
 335         However, pointers are good to <span class="underline">refer</span>
 336         to optional objects, but not particularly good to handle the optional objects
 337         in all other respects, such as initializing or moving/copying them. The problem
 338         resides in the shallow-copy of pointer semantics: if you need to effectively
 339         move or copy the object, pointers alone are not enough. The problem is that
 340         copies of pointers do not imply copies of pointees. For example, as was discussed
 341         in the motivation, pointers alone cannot be used to return optional objects
 342         from a function because the object must move outside from the function and
 343         into the caller's context.
 344       </p>
 345 <p>
 346         A solution to the shallow-copy problem that is often used is to resort to
 347         dynamic allocation and use a smart pointer to automatically handle the details
 348         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">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span></code>
 349         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>
 350         is a built-in or small POD, this technique is very poor in terms of required
 351         resources. Optional objects are essentially values so it is very convenient
 352         to be able to use automatic storage and deep-copy semantics to manipulate
 353         optional values just as we do with ordinary values. Pointers do not have
 354         this semantics, so are inappropriate for the initialization and transport
 355         of optional values, yet are quite convenient for handling the access to the
 356         possible undefined value because of the idiomatic aid present in the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a> concept
 357         incarnated by pointers.
 358       </p>
 359 <h5>
 360 <a name="boost_optional.development.the_interface.h1"></a>
 361         <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&lt;T&gt;
 362         as a model of OptionalPointee</a>
 363       </h5>
 364 <p>
 365         For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
 366         and <code class="computeroutput"><span class="special">-&gt;</span></code> to lexically warn
 367         about the possibly uninitialized state appealing to the familiar pointer
 368         semantics w.r.t. to null pointers.
 369       </p>
 370 <div class="warning"><table border="0" summary="Warning">
 371 <tr>
 372 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
 373 <th align="left">Warning</th>
 374 </tr>
 375 <tr><td align="left" valign="top"><p>
 376           However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code> is not, and does not model, a pointer</span>.
 377         </p></td></tr>
 378 </table></div>
 379 <p>
 380         For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>
 381         does not have shallow-copy so does not alias: two different optionals never
 382         refer to the <span class="emphasis"><em>same</em></span> value unless <code class="computeroutput"><span class="identifier">T</span></code>
 383         itself is a reference (but may have <span class="emphasis"><em>equivalent</em></span> values).
 384         The difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 385         and a pointer must be kept in mind, particularly because the semantics of
 386         relational operators are different: since <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 387         is a value-wrapper, relational operators are deep: they compare optional
 388         values; but relational operators for pointers are shallow: they do not compare
 389         pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 390         by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
 391         on some situations but not always. Specifically, on generic code written
 392         for both, you cannot use relational operators directly, and must use the
 393         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>
 394         and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
 395         instead.
 396       </p>
 397 </div>
 398 </div>
 399 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 400 <td align="left"></td>
 401 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<p>
 402         Distributed under the Boost Software License, Version 1.0. (See accompanying
 403         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>)
 404       </p>
 405 </div></td>
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