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source: downloads/boost_1_33_1/libs/optional/doc/optional.html @ 12

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15	<H2><IMG SRC="../../../boost.png" WIDTH="276" HEIGHT="86">Header <<A
16	HREF="../../../boost/optional/optional.hpp">boost/optional/optional.hpp</A>> </H2>
17
18	<H2>Contents</H2>
19	<DL CLASS="page-index">
20	<DT><A HREF="#mot">Motivation</A></DT>
21	<DT><A HREF="#dev">Development</A></DT>
22	<DT><A HREF="#synopsis">Synopsis</A></DT>
23	<DT><A HREF="#semantics">Semantics</A></DT>
24	<DT><A HREF="#examples">Examples</A></DT>
25	<DT><A HREF="#ref">Optional references</A></DT>
26	<DT><A HREF="#refassign">Rebinding semantics for assignment of optional references</A></DT>
27	<DT><A HREF="#inplace">In-Place Factories</A></DT>
28	<DT><A HREF="#bool">A note about optional<bool></A></DT>
29	<DT><A HREF="#exsafety">Exception Safety Guarantees</A></DT>
30	<DT><A HREF="#requirements">Type requirements</A></DT>
31	<DT><A HREF="#impl">Implementation Notes</A></DT>
32	<DT><A HREF="#porta">Dependencies and Portability</A></DT>
33	<DT><A HREF="#credits">Acknowledgment</A></DT>
34	</DL>
35
36	<HR>
37
38	<H2><A NAME="mot"></A>Motivation</H2>
39
40	<P>Consider these functions which should return a value but which might not have
41	a value to return:</P>
42	<pre>(A) double sqrt(double n );
43	(B) char get_async_input();
44	(C) point polygon::get_any_point_effectively_inside();</pre>
45	<P>There are different approaches to the issue of not having a value to return.</P>
46	<P>A typical approach is to consider the existence of a valid return value as
47	a postcondition, so that if the function cannot compute the value to return,
48	it has either undefined behavior (and can use assert in a debug build)
49	or uses a runtime check and throws an exception if the postcondition is violated.
50	This is a reasonable choice for example, for function (A), because the
51	lack of a proper return value is directly related to an invalid parameter (out
52	of domain argument), so it is appropriate to require the callee to supply only
53	parameters in a valid domain for execution to continue normally.</P>
54	<P>However, function (B), because of its asynchronous nature, does not fail just
55	because it can't find a value to return; so it is incorrect to consider
56	such a situation an error and assert or throw an exception. This function must
57	return, and somehow, must tell the callee that it is not returning a meaningful
58	value.</P>
59	<P>A similar situation occurs with function (C): it is conceptually an error to
60	ask a <i>null-area</i> polygon to return a point inside itself, but in many
61	applications, it is just impractical for performance reasons to treat this as
62	an error (because detecting that the polygon has no area might be too expensive
63	to be required to be tested previously), and either an arbitrary point (typically
64	at infinity) is returned, or some efficient way to tell the callee that there
65	is no such point is used.</P>
66	<P>There are various mechanisms to let functions communicate that the returned
67	value is not valid. One such mechanism, which is quite common since it has zero
68	or negligible overhead, is to use a special value which is reserved to communicate
69	this. Classical examples of such special values are EOF, string::npos, points
70	at infinity, etc...</P>
71	<P>When those values exist, i.e. the return type can hold all meaningful values
72	<i>plus</i> the <i>signal</i> value, this mechanism is quite appropriate and
73	well known. Unfortunately, there are cases when such values do not exist. In
74	these cases, the usual alternative is either to use a wider type, such as 'int'
75	in place of 'char'; or a compound type, such as std::pair<point,bool>.
76	</P>
77	<P>Returning a std::pair<T,bool>, thus attaching a boolean flag to the result
78	which indicates if the result is meaningful, has the advantage that can be turned
79	into a consistent idiom since the first element of the pair can be whatever
80	the function would conceptually return. For example, the last two functions
81	could have the following interface:</P>
82	<pre>std::pair<char,bool> get_async_input();
83	std::pair<point,bool> polygon::get_any_point_effectively_inside();</pre>
84	<p>These functions use a consistent interface for dealing with possibly inexistent
85	results:</p>
86	<pre>std::pair<point,bool> p = poly.get_any_point_effectively_inside();
87	if ( p.second )
88	flood_fill(p.first);
89	</pre>
90
91	<P>However, not only is this quite a burden syntactically, it is also error
92	prone since the user can easily use the function result (first element of the
93	pair) without ever checking if it has a valid value.</P>
94	<P>Clearly, we need a better idiom.</P>
95
96	<H2><A NAME="dev"></A>Development</H2>
97
98	<h3><u>The models:</u></h3>
99	<P>In C++, we can <i>declare</i> an object (a variable) of type T, and we can give this variable
100	an <i>initial value</i> (through an <i>initializer</i>. (c.f. 8.5)).<br>
101	When a declaration includes a non-empty initializer (an initial value is given), it is said that
102	the object has been <i><b>initialized</b></i>.<br>
103	If the declaration uses an empty initializer (no initial value is given),
104	and neither default nor value initialization applies, it is said that the object is
105	<i><b>uninitialized</b></i>. Its actual value exist but has an
106	<i>indeterminate initial value</i> (c.f. 8.5.9).<br>
107	<code>optional<T></code> intends to formalize the notion of initialization
108	(or lack of it)
109	allowing a program to test whether an object has been initialized and stating that access to
110	the value of an uninitialized object is undefined behavior. That is,
111	when a variable is declared as optional<T> and no initial value is given,
112	the variable is <i>formally</i> uninitialized. A formally uninitialized optional object has conceptually
113	no value at all and this situation can be tested at runtime. It is formally <i>
114	undefined behavior</i>
115	to try to access the value of an uninitialized optional. An uninitialized optional can be <i>assigned</i> a value, in which case its initialization state changes to initialized. Furthermore, given the formal
116	treatment of initialization states in optional objects, it is even possible to reset an optional to <i>uninitialized</i>.</P>
117	<P>In C++ there is no formal notion of uninitialized objects, which
118	means that objects always have an initial value even if indeterminate.<br>
119	As discussed on the previous section, this has a drawback because you need additional
120	information to tell if an object has been effectively initialized.<br>
121	One of the typical ways in which this has been historically
122	dealt with is via a special value: EOF,npos,-1, etc... This is equivalent to adding
123	the special value to the set of possible values of a given type. This super set of
124	T plus some <i>nil_t</i>—were nil_t is some stateless POD-can be modeled in modern
125	languages as a <b>discriminated union</b> of <code>T</code> and <code>nil_t</code>.
126	Discriminated unions are often called <i>variants</i>. A variant has a <i>current type</i>,
127	which in our case is either <code>T</code> or <code>nil_t</code>.<br>
128	Using the <a href="../../../doc/html/variant.html">Boost.Variant</a> library, this model can be implemented
129	in terms of <code>boost::variant<T,nil_t></code>.<br>
130	There is precedence for a discriminated union as a model for an optional value: the
131	<a href="http://www.haskell.org/"><u>Haskell</u></a> <b>Maybe</b> built-in type constructor.
132	Thus, a discriminated union <code>T+nil_t</code> serves as a conceptual foundation.</p>
133	<p>A <code>variant<T,nil_t></code> follows naturally from the traditional idiom of extending
134	the range of possible values adding an additional sentinel value with the special meaning of <i>Nothing. </i>
135	However, this additional <i>Nothing</i> value is largely irrelevant for our purpose
136	since our goal is to formalize the notion of uninitialized objects and, while a special extended value <i>can</i> be used to convey that meaning, it is not strictly
137	necessary in order to do so.</p>
138	<p>The observation made in the last paragraph about the irrelevant nature of the additional <code>nil_t</code> with respect to
139	<u>purpose</u> of optional<T> suggests
140	an alternative model: a <i>container</i> that either has a value of T or nothing.
141	</p>
142	<p>As of this writing I don't know of any precedence for a variable-size fixed-capacity (of 1)
143	stack-based container model for optional values, yet I believe this is the consequence of
144	the lack of practical implementations of such a container rather than an inherent shortcoming
145	of the container model.</p>
146	<p>In any event, both the discriminated-union or the single-element container models serve as a conceptual
147	ground for a class representing optional—i.e. possibly uninitialized—objects.<br>
148	For instance, these models show the <i>exact</i> semantics required for a wrapper of optional values:</p>
149	<p>Discriminated-union:</p>
150	<blockquote>
151	<li><b>deep-copy</b> semantics: copies of the variant implies copies of the value.</li>
152	<li><b>deep-relational</b> semantics: comparisons between variants matches both current types and values</li>
153	<li>If the variant's current type is T, it is modeling an <i>initialized</i> optional.</li>
154	<li>If the variant's current type is not T, it is modeling an <i>uninitialized</i> optional.</li>
155	<li>Testing if the variant's current type is T models testing if the optional is initialized</li>
156	<li>Trying to extract a T from a variant when its current type is not T, models the undefined
157	behavior
158	of trying to access the value of an uninitialized optional</li>
159	</blockquote>
160	<p>Single-element container:</p>
161	<blockquote>
162	<li><b>deep-copy</b> semantics: copies of the container implies copies of the value.</li>
163	<li><b>deep-relational</b> semantics: comparisons between containers compare container size and if match, contained value</li>
164	<li>If the container is not empty (contains an object of type T), it is modeling an <i>initialized</i> optional.</li>
165	<li>If the container is empty, it is modeling an <i>uninitialized</i> optional.</li>
166	<li>Testing if the container is empty models testing if the optional is initialized</li>
167	<li>Trying to extract a T from an empty container models the undefined behavior
168	of trying to access the value of an uninitialized optional</li>
169	</blockquote>
170
171	<h3><u>The semantics:</u></h3>
172	<p>Objects of type <code>optional<T></code> are intended to be used in places where objects of type T would
173	but which might be uninitialized. Hence, <code>optional<T></code>'s purpose is to formalize the
174	additional possibly uninitialized state.<br>
175	From the perspective of this role, <code>optional<T></code> can have the same operational semantics of T
176	plus the additional semantics corresponding to this special state.<br>
177	As such, <code>optional<T></code> could be thought of as a <i>supertype</i> of T. Of course,
178	we can't do that in C++, so we need to compose the desired semantics using a different mechanism.<br>
179	Doing it the other way around, that is, making <code>optional<T></code> a <i>subtype</i> of T is not only
180	conceptually wrong but also impractical: it is not allowed to derive from a non-class type, such as a
181	built-in type.</p>
182
183	<p>We can draw from the purpose of optional<T> the required basic semantics:</p>
184
185	<blockquote>
186	<p><b>Default Construction:</b> To introduce a formally uninitialized wrapped
187	object.</p>
188
189	<p><b>Direct Value Construction via copy:</b> To introduce a formally
190	initialized wrapped object whose value is obtained as a copy of some object.</p>
191
192	<p><b>Deep Copy Construction:</b> To obtain a new yet equivalent wrapped
193	object.</p>
194
195	<p><b>Direct Value Assignment (upon initialized):</b> To assign a value to the wrapped object.</p>
196
197	<p><b>Direct Value Assignment (upon uninitialized):</b> To initialize the wrapped object
198	with a value obtained
199	as a copy of some object.</p>
200
201	<p><b>Assignment (upon initialized):</b> To assign to the wrapped object the value
202	of another wrapped object.</p>
203
204	<p><b>Assignment (upon uninitialized):</b> To initialize the wrapped object
205	with value of another wrapped object.</p>
206
207	<p><b>Deep Relational Operations (when supported by the type T):</b> To compare
208	wrapped object values taking into account the presence of uninitialized
209	states.</p>
210
211	<p><b>Value access:</b> To unwrap the wrapped object.</p>
212
213	<p><b>Initialization state query:</b> To determine if the object is formally
214	initialized or not.</p>
215
216	<p><b>Swap:</b> To exchange wrapped objects. (with whatever exception safety
217	guarantees are provided by T's swap).</p>
218
219	<p><b>De-initialization:</b> To release the wrapped object (if any) and leave
220	the wrapper in the uninitialized state.</p>
221
222	</blockquote>
223
224	<p>Additional operations are useful, such as converting constructors and
225	converting assignments, in-place construction and assignment, and safe value
226	access via a pointer to the wrapped object or null.</p>
227	<h3><u>The Interface:</u></h3>
228	<p>Since the purpose of optional is to allow us to use objects with a formal
229	uninitialized additional state, the interface could try to follow the interface
230	of the underlying T type as much as possible. In order to choose the proper
231	degree of adoption of the native T interface, the following must be noted: <br>
232	Even if all the operations supported by an instance of type T are defined for
233	the entire range of values for such a type, an optional<T> extends such a set of
234	values with a new value for which most (otherwise valid) operations are not
235	defined in terms of T.<br>
236	Furthermore, since optional<T> itself is merely a T wrapper (modeling a T
237	supertype), any attempt to define such operations upon uninitialized optionals
238	will be totally artificial w.r.t. T.<br>
239	This library chooses an interface which follows from T's interface only for
240	those operations which are well defined (w.r.t the type T) even if any of the
241	operands are uninitialized. These operations include: construction,
242	copy-construction, assignment, swap and relational operations.<br>
243	For the value access operations, which are undefined (w.r.t the type T) when the
244	operand is uninitialized, a different interface is chosen (which will be
245	explained next).<br>
246	Also, the presence of the possibly uninitialized state requires additional
247	operations not provided by T itself which are supported by a special interface.</p>
248	<h3>Lexically-hinted Value Access in the presence of possibly untitialized
249	optional objects: The operators * and -></h3>
250	<p>A relevant feature of a pointer is that it can have a <b>null
251	pointer value</b>. This is a <i>special</i> value which is used to indicate that the
252	pointer is not referring to any object at all. In other words, null pointer
253	values convey the notion of inexistent objects.</P>
254	<P>This meaning of the null pointer value allowed pointers to became a <i>de facto</i> standard
255	for handling optional objects because all you have to do to refer to a value which you
256	don't really have is to use a null pointer value of the appropriate type.
257	Pointers have been used for decades—from the days of C APIs to modern C++ libraries—to
258	<i>refer</i> to optional (that is, possibly inexistent) objects; particularly
259	as optional arguments to a function, but also quite often as optional data members.</P>
260	<P>The possible presence of a null pointer value makes the operations that access the
261	pointee's value possibly undefined, therefore, expressions which use dereference
262	and access operators, such as: <code>( *p = 2 )</code> and <code>( p->foo())</code>,
263	implicitly convey the notion of optionality, and this information is tied to
264	the <i>syntax</i> of the expressions. That is, the presence of operators * and -> tell by
265	themselves—without any additional context—that the expression will be undefined unless
266	the implied pointee actually exist.</P>
267	<P>Such a <i>de facto</i> idiom for referring to optional objects can be formalized in the form of a
268	concept: the <a href="../../utility/OptionalPointee.html">OptionalPointee</a> concept.<br>
269	This concept captures the syntactic usage of operators *, -> and conversion to bool to convey
270	the notion of optionality.</P>
271	<P>However, pointers are good to <u>refer</u> to optional objects, but not particularly good
272	to handle the optional objects in all other respects, such as initializing or moving/copying
273	them. The problem resides in the shallow-copy of pointer semantics: if you need to
274	effectively move or copy the object, pointers alone are not enough. The problem
275	is that copies of pointers do not imply copies of pointees. For example, as
276	was discussed in the motivation, pointers alone cannot be used to return optional
277	objects from a function because the object must move outside from the function and
278	into the caller's context.<br>
279	A solution to the shallow-copy problem that is often used is to resort to dynamic
280	allocation and use a smart pointer to automatically handle the details of this.
281	For example, if a function is to optionally return an object X, it can use shared_ptr<X>
282	as the return value. However, this requires dynamic allocation of X. If X is
283	a built-in or small POD, this technique is very poor in terms of required resources.
284	Optional objects are essentially values so it is very convenient to be able to use automatic
285	storage and deep-copy semantics to manipulate optional values just as we do with ordinary
286	values. Pointers do not have this semantics, so are inappropriate for the initialization and
287	transport of optional values, yet are quite convenient for handling the access to the
288	possible undefined value because of the idiomatic aid present in the OptionalPointee
289	concept incarnated by pointers.
290	</p>
291	<h4>Optional<T> as a model of OptionalPointee</h4>
292	<P>For value access operations optional<> uses operators * and -> to lexically
293	warn about the possibly uninitialized state appealing to the familiar pointer
294	semantics w.r.t. to null pointers.<br>
295	<u><b>However, it is particularly important to note that optional<> objects are not pointers. optional<>
296	is not, and does not model, a pointer</b></u><b>.</b>
297	<P>For instance, optional<> does not have shallow-copy so does not alias: two different optionals
298	never refer to the <i>same</i> value unless T itself is a reference (but may have <i>equivalent</i> values).<br>
299	The difference between an optional<T> and a pointer must be kept in mind, particularly
300	because the semantics of relational operators are different: since optional<T>
301	is a value-wrapper, relational operators are deep: they compare optional values;
302	but relational operators for pointers are shallow: they do not compare pointee values.<br>
303	As a result, you might be able to replace optional<T> by T* on some situations but
304	not always. Specifically, on generic code written for both, you cannot use relational
305	operators directly, and must use the template functions
306	<a href="../../utility/OptionalPointee.html#equal">equal_pointees()</a> and
307	<a href="../../utility/OptionalPointee.html#less">less_pointees()</a> instead.
308	<HR>
309
310	<H2><A NAME="synopsis">Synopsis</A></H2>
311
312	<PRE>namespace boost {
313
314	template<class T>
315	class optional
316	{
317	public :
318
319	<i><u>(If T is of reference type, the parameters and results by reference are by value)</u></i>
320
321	optional () ;
322
323	optional ( none_t ) ;
324
325	optional ( T const& v ) ;
326
327	optional ( optional const& rhs ) ;
328
329	template<class U> explicit optional ( optional<U> const& rhs ) ;
330
331	template<class InPlaceFactory> explicit optional ( InPlaceFactory const& f ) ;
332
333	template<class TypedInPlaceFactory> explicit optional ( TypedInPlaceFactory const& f ) ;
334
335	optional& operator = ( none_t ) ;
336
337	optional& operator = ( T const& v ) ;
338
339	optional& operator = ( optional const& rhs ) ;
340
341	template<class U> optional& operator = ( optional<U> const&amp rhs ) ;
342
343	template<class InPlaceFactory> optional& operator = ( InPlaceFactory const&amp f ) ;
344
345	template<class TypedInPlaceFactory> optional& operator = ( TypedInPlaceFactory const&amp f ) ;
346
347	T const& get() const ;
348	T& get() ;
349
350	T const* operator ->() const ;
351	T* operator ->() ;
352
353	T const& operator *() const ;
354	T& operator *() ;
355
356	T const* get_ptr() const ;
357	T* get_ptr() ;
358
359	operator <i>unspecified-bool-type</i>() const ;
360
361	bool operator!() const ;
362
363	<i><u>deprecated methods</u></i>
364
365	void reset() ; (deprecated)
366	void reset ( T const& ) ; (deprecated)
367	bool is_initialized() const ; (deprecated)
368
369	} ;
370
371	template<class T> inline bool operator == ( optional<T> const& x, optional<T> const& y ) ;
372
373	template<class T> inline bool operator != ( optional<T> const& x, optional<T> const& y ) ;
374
375	template<class T> inline bool operator < ( optional<T> const& x, optional<T> const& y ) ;
376
377	template<class T> inline bool operator > ( optional<T> const& x, optional<T> const& y ) ;
378
379	template<class T> inline bool operator <= ( optional<T> const& x, optional<T> const& y ) ;
380
381	template<class T> inline bool operator >= ( optional<T> const& x, optional<T> const& y ) ;
382
383	template<class T> inline T const& get ( optional<T> const& opt ) ;
384
385	template<class T> inline T& get ( optional<T> & opt ) ;
386
387	template<class T> inline T const* get ( optional<T> const* opt ) ;
388
389	template<class T> inline T* get ( optional<T>* opt ) ;
390
391	template<class T> inline T const* get_pointer ( optional<T> const& opt ) ;
392
393	template<class T> inline T* get_pointer ( optional<T> & opt ) ;
394
395	template<class T> inline void swap( optional<T>& x, optional<T>& y ) ;
396
397	} // namespace boost
398	</PRE>
399
400	<HR>
401
402	<h2><A NAME="semantics">Detailed Semantics</a></h2>
403
404	<p><b><u>NOTES: </u></b></p>
405
406	<p><b>Because T might be of reference type, in the sequel, those entries whose
407	semantic depends on T being of reference type or not will be distinguished using
408	the following convention:<br>
409	If the entry reads: optional<T (not a ref)>, the description corresponds only to
410	the case where T is not of reference type.<br>
411	If the entry reads: optional<T&>, the description corresponds only to the case
412	where T is of reference type. <br>
413	If the entry reads: optional<T>, the description is the same for both cases.</b></p>
414
415	<p><i>The following section contains various assert() which are used only to
416	show the postconditions as sample code. It is not implied that the type T must
417	support each particular expression but that if the expression is supported, the
418	implied condition holds.</i></p>
419
420	<hr>
421
422	<pre>optional<T>::optional();</pre>
423	<blockquote>
424	<p><b>Effect:</b> Default-Constructs an <b>optional</b>.</p>
425	<p><b>Postconditions:</b> <b>*this</b> is <u>uninitialized</u>.</p>
426	<p><b>Throws:</b> Nothing.</p>
427	<p><b>Notes:</b> T's default constructor <u><i>is not</i></u> called.</p>
428	<p><b>Example:</b></p>
429	<blockquote>
430	<pre>optional<T> def ;
431	assert ( !def ) ;</pre>
432	</blockquote>
433	</blockquote>
434
435	<HR>
436
437	<pre>optional<T>::optional( none_t );</pre>
438	<blockquote>
439	<p><b>Effect:</b> Constructs an <b>optional </b>uninitialized.</p>
440	<p><b>Postconditions:</b> <b>*this</b> is <u>uninitialized</u>.</p>
441	<p><b>Throws:</b> Nothing.</p>
442	<p><b>Notes:</b></p>
443	<blockquote>
444	<p>T's default constructor <u><i>is not</i></u> called.<br>
445	The
446	expression <code>boost::none</code> denotes an instance of <code>boost::none_t</code> that can be
447	used as the parameter.</p>
448	</blockquote>
449	<p><b>Example:</b></p>
450	<blockquote>
451	<pre>#include <boost/none.hpp></pre>
452	<pre>optional<T> n(none) ;
453	assert ( !n ) ;</pre>
454	</blockquote>
455	</blockquote>
456
457	<HR>
458
459	<pre>optional<T <i>(not a ref)</i>>::optional( T const& v )</pre>
460	<blockquote>
461	<p><b>Effect:</b> Directly-Constructs an <b>optional</b>.</p>
462	<!-- TemplateName: general/sy_footer_inc.isml -->
463	<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is a <i>copy</i> of 'v'.</p>
464	<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
465	<p><b>Notes: </b> T::T( T const& ) is called.</p>
466	<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
467	in that case, this constructor has no effect.
468	</p>
469	<p><b>Example:</b></p>
470	<blockquote>
471	<pre>T v;
472	optional<T> opt(v);
473	assert ( *opt == v ) ;</pre>
474	</blockquote>
475	</blockquote>
476
477	<HR>
478
479	<pre>optional<T&>::optional( T& ref )</pre>
480	<blockquote>
481	<p><b>Effect:</b> Directly-Constructs an <b>optional</b>.</p>
482	<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is an
483	instance of an internal type wrapping the reference 'ref'.</p>
484	<p><b>Throws:</b> Nothing.</p>
485	<p><b>Example:</b></p>
486	<blockquote>
487	<pre>T v;
488	T& vref = v ;
489	optional<T&> opt(vref);
490	assert ( *opt == v ) ;
491	++ v ; // mutate referee
492	assert (*opt == v); </pre>
493	</blockquote>
494	</blockquote>
495
496	<HR>
497
498	<pre>optional<T <i>(not a ref)</i>>::optional( optional const& rhs );</pre>
499	<blockquote>
500	<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
501	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
502	and its value is a <i>copy</i> of the value of <b>rhs</b>; else <b>*this</b>
503	is uninitialized.</p>
504	<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
505	<p><b>Notes:</b> If <b>rhs</b> is initialized, T::T(T const& ) is called.</p>
506	<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
507	in that case, this constructor has no effect.
508	</p>
509	<p><b>Example:</b></p>
510	<blockquote>
511	<pre>optional<T> uninit ;
512	assert (!uninit);
513
514	optional<T> uinit2 ( uninit ) ;
515	assert ( uninit2 == uninit );
516
517	optional<T> init( T(2) );
518	assert ( *init == T(2) ) ;
519
520	optional<T> init2 ( init ) ;
521	assert ( init2 == init ) ;
522	</pre>
523
524	</blockquote>
525	</blockquote>
526
527	<HR>
528
529	<pre>optional<T&>::optional( optional const& rhs );</pre>
530	<blockquote>
531	<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
532	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
533	and its value is another reference to the same object referenced by <b>rhs</b>; else <b>this</b>
534	is uninitialized.</p>
535	<p><b>Throws:</b> Nothing.</p>
536	<p><b>Notes:</b> If <b>rhs</b> is initialized, both <b>this</b> and <b>rhs</b> will
537	reefer to the same object<b> </b>(they alias).</p>
538	<p><b>Example:</b></p>
539	<blockquote>
540	<pre>optional<T&> uninit ;
541	assert (!uninit);
542
543	optional<T&> uinit2 ( uninit ) ;
544	assert ( uninit2 == uninit );
545
546	T v = 2 ; T& ref = v ;
547	optional<T> init(ref);
548	assert ( *init == v ) ;
549
550	optional<T> init2 ( init ) ;
551	assert ( *init2 == v ) ;
552
553	v = 3 ;
554
555	assert ( *init == 3 ) ;
556	assert ( *init2 == 3 ) ;
557
558
559	</pre>
560
561	</blockquote>
562	</blockquote>
563
564	<HR>
565
566	<pre>template<U> explicit optional<T <i>(not a ref)</i>>::optional( optional<U> const& rhs );</pre>
567	<blockquote>
568	<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
569	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
570	and its value is a <i>copy</i> of the value of <b>rhs</b> <i>converted</i>
571	to type T; else <b>*this</b> is uninitialized.
572	</p>
573	<p><b>Throws:</b> Whatever T::T( U const& ) throws.</p>
574	<p><b>Notes:</b> T::T( U const& ) is called if <b>rhs</b> is initialized, which requires
575	a valid conversion from U to T.
576	</p>
577	<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( U const& );
578	in that case, this constructor has no effect.
579	</p>
580	<p><b>Example:</b></p>
581	<blockquote>
582
583	<pre>optional<double> x(123.4);
584	assert ( *x == 123.4 ) ;
585
586	optional<int> y(x) ;
587	assert( *y == 123 ) ;
588	</pre>
589	</blockquote>
590	</blockquote>
591
592	<HR>
593
594	<pre>template<<i>InPlaceFactory</i>> explicit optional<T <i>(not a ref)</i>>::optional( <i>InPlaceFactory</i> const& f );</pre>
595
596	<pre>template<<i>TypedInPlaceFactory</i>> explicit optional<T <i>(not a ref)</i>>::optional( <i>TypedInPlaceFactory</i> const& f );</pre>
597	<blockquote>
598	<p><b>Effect:</b> Constructs an <b>optional</b> with a value of T obtained from
599	the factory.</p>
600	<p><b>Postconditions:</b>  <b>*this</b> is <u>initialized</u> and its value is
601	<i>directly given</i> from the factory 'f' (i.e., the value<u> is not copied</u>).</p>
602	<p><b>Throws:</b> Whatever the T constructor called by the factory throws.</p>
603	<p><b>Notes:</b> See <A HREF="#inplace">In-Place Factories</A></p>
604	<p><b>Exception Safety:</b> Exceptions can only be thrown during the call to the
605	T constructor used by the factory;
606	in that case, this constructor has no effect.
607	</p>
608	<p><b>Example:</b></p>
609	<blockquote>
610
611	<pre>class C { C ( char, double, std::string ) ; } ;
612
613	C v('A',123.4,"hello");
614
615	optional<C> x( in_place ('A', 123.4, "hello") ); // InPlaceFactory used
616	optional<C> y( in_place<C>('A', 123.4, "hello") ); // TypedInPlaceFactory used
617
618	assert ( *x == v ) ;
619	assert ( *y == v ) ;
620
621	</pre>
622	</blockquote>
623	</blockquote>
624
625	<HR>
626
627	<pre>optional& optional<T <i>(not a ref)</i>>::operator= ( T const& rhs ) ;</pre>
628	<blockquote>
629	<p><b>Effect:</b> Assigns the value 'rhs' to an <b>optional</b>.</p>
630	<p><b>Postconditions:</b> <b>*this</b> is initialized
631	and its value is a <i>copy</i> of <b>rhs.</b></p>
632	<p><b>Throws:</b> Whatever T::operator=( T const& ) or T::T(T const&) throws.</p>
633	<p><b>Notes:</b> If <b>*this</b> was initialized, T's assignment operator is
634	used, otherwise, its copy-constructor is used.</p>
635	<p><b>Exception Safety:</b> In the event of an exception, the initialization
636	state of <b>*this</b> is unchanged and its value unspecified as far as optional
637	is concerned (it is up to T's operator=()) [If <b>*this</b> is initially
638	uninitialized and T's <i>copy constructor</i> fails, <b>*this</b> is left
639	properly uninitialized]</p>
640	<p><b>Example:</b></p>
641	<blockquote>
642	<pre>T x;
643	optional<T> def ;
644	optional<T> opt(x) ;
645
646	T y;
647	def = y ;
648	assert ( *def == y ) ;
649	opt = y ;
650	assert ( *opt == y ) ;</pre>
651	</blockquote>
652	</blockquote>
653
654	<HR>
655
656	<pre>optional<T&>& optional<T&>::operator= ( T& const& rhs ) ;</pre>
657	<blockquote>
658	<p><b>Effect:</b> (Re)binds thee wrapped reference.</p>
659	<p><b>Postconditions:</b> <b>*this</b> is initialized
660	and it references the same object referenced by <b>rhs.</b></p>
661	<p><b>Notes:</b> If <b>*this</b> was initialized, is is <i>rebound</i> to the
662	new object. See <A HREF="#refassign">here</a> for details on this behavior.</p>
663	<p><b>Example:</b></p>
664	<blockquote>
665	<pre>int a = 1 ;
666	int b = 2 ;
667	T& ra = a ;
668	T& rb = b ;
669	optional<int&> def ;
670	optional<int&> opt(ra) ;
671
672	def = rb ; // binds 'def' to 'b' through 'rb'
673	assert ( *def == b ) ;
674	*def = a ; // changes the value of 'b' to a copy of the value of 'a'
675	assert ( b == a ) ;
676	int c = 3;
677	int& rc = c ;
678	opt = rc ; // REBINDS to 'c' through 'rc'
679	c = 4 ;
680	assert ( *opt == 4 ) ;
681	</pre>
682	</blockquote>
683	</blockquote>
684
685	<HR>
686
687	<pre>optional& optional<T <i>(not a ref)</i>>::operator= ( optional const& rhs ) ;</pre>
688	<blockquote>
689	<p><b>Effect:</b> Assigns another <b>optional</b> to an <b>optional</b>.</p>
690	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
691	and its value is a <i>copy</i> of the value of <b>rhs</b>; else <b>*this</b>
692	is uninitialized.
693	</p>
694	<p><b>Throws:</b> Whatever T::operator( T const&) or  T::T( T const& ) throws.</p>
695	<p><b>Notes:</b> If both<b> *this</b> and <b>rhs</b> are initially initialized,
696	T's <i>assignment</i> <i>operator</i> is used. If <b>*this</b> is initially initialized but <b>
697	rhs</b> is uninitialized, T's <i>destructor</i> is called. If <b>*this</b> is initially
698	uninitialized but rhs is initialized, T's <i>copy constructor</i> is called.
699	</p>
700	<p><b>Exception Safety:</b> In the event of an exception, the initialization
701	state of <b>*this</b> is unchanged and its value unspecified as far as optional
702	is concerned (it is up to T's operator=()) [If <b>*this</b> is initially
703	uninitialized and T's <i>copy constructor</i> fails, <b>*this</b> is left
704	properly uninitialized]</p>
705	<p><b>Example:</b></p>
706	<blockquote>
707	<pre>T v;
708	optional<T> opt(v);
709	optional<T> def ;
710
711	opt = def ;
712	assert ( !def ) ;
713	// previous value (copy of 'v') destroyed from within 'opt'.
714
715	</pre>
716	</blockquote>
717	</blockquote>
718
719	<HR>
720
721	<pre>optional<T&> & optional<T&>::operator= ( optional<T&> const& rhs ) ;</pre>
722	<blockquote>
723	<p><b>Effect:</b> (Re)binds thee wrapped reference.</p>
724	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>this</b> is initialized
725	and it references the same object referenced by <b>rhs</b>; otherwise, <b>this</b>
726	is uninitialized (and references no object).</p>
727	<p><b>Notes:</b> If <b>this</b> was initialized and so is <b>rhs</b>, <b>this</b>
728	is is <i>rebound</i> to the new object. See <A HREF="#refassign">here</a> for details on this
729	behavior.</p>
730	<p><b>Example:</b></p>
731	<blockquote>
732	<pre>int a = 1 ;
733	int b = 2 ;
734	T& ra = a ;
735	T& rb = b ;
736	optional<int&> def ;
737	optional<int&> ora(ra) ;
738	optional<int&> orb(rb) ;
739
740	def = orb ; // binds 'def' to 'b' through 'rb' wrapped within 'orb'
741	assert ( *def == b ) ;
742	*def = ora ; // changes the value of 'b' to a copy of the value of 'a'
743	assert ( b == a ) ;
744	int c = 3;
745	int& rc = c ;
746	optional<int&> orc(rc) ;
747	ora = orc ; // REBINDS ora to 'c' through 'rc'
748	c = 4 ;
749	assert ( *ora == 4 ) ;
750	</pre>
751	</blockquote>
752	</blockquote>
753
754	<HR>
755
756	<pre>template<U> optional& optional<T <i>(not a ref)</i>>::operator= ( optional<U> const& rhs ) ;</pre>
757	<blockquote>
758	<p><b>Effect:</b> Assigns another <i>convertible</i> <b>optional</b> to an <b>optional</b>.</p>
759	<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
760	and its value is a <i>copy</i> of the value of <b>rhs</b> <i>converted</i>
761	to type T; else <b>*this</b> is uninitialized.
762	</p>
763	<p><b>Throws:</b> Whatever T::operator=( U const& ) or T::T( U const& ) throws.</p>
764	<p><b>Notes:</b> If both<b> *this</b> and <b>rhs</b> are initially initialized,
765	T's <i>assignment</i> <i>operator</i> (from U) is used. If <b>*this</b> is initially initialized but <b>
766	rhs</b> is uninitialized, T's <i>destructor</i> is called. If <b>*this</b> is initially
767	uninitialized but rhs is initialized, T's <i>converting constructor</i> (from U) is called.
768	</p>
769	<p><b>Exception Safety:</b> In the event of an exception, the initialization
770	state of <b>*this</b> is unchanged and its value unspecified as far as optional
771	is concerned (it is up to T's operator=()) [If <b>*this</b> is initially
772	uninitialized and T's <i>converting constructor</i> fails, <b>*this</b> is left
773	properly uninitialized]</p>
774	<p><b>Example:</b></p>
775	<blockquote>
776	<pre>T v;
777	optional<T> opt0(v);
778	optional<U> opt1;
779
780	opt1 = opt0 ;
781	assert ( *opt1 == static_cast<U>(v) ) ;
782	</pre>
783	</blockquote>
784	</blockquote>
785
786	<HR>
787	<pre>void optional<T <i>(not a ref)</i>>::reset( T const&amp v ) ;</pre>
788	<blockquote>
789	<p><b>Deprecated:</b> same as operator= ( T const& v) ;</p>
790	</blockquote>
791
792	<HR>
793	<pre>void optional<T>::reset() ;</pre>
794	<blockquote>
795	<p><b>Deprecated: </b>Same as operator=( detail::none_t );</p>
796	</blockquote>
797
798	<HR>
799
800
801	<pre>T const& optional<T <i>(not a ref)</i>>::operator*() const ;
802	T& optional<T<i> (not a ref)</i>>::operator*();</pre>
803
804	<pre>T const& optional<T <i>(not a ref)</i>>::get() const ;
805	T& optional<T <i>(not a ref)</i>>::get() ;
806
807	inline T const& get ( optional<T<i> (not a ref)</i>> const& ) ;
808	inline T& get ( optional<T <i>(not a ref)</i>> &) ;
809	</pre>
810	<blockquote>
811	<p><b>Requirements: *this</b> is initialized</p>
812	<p><b>Returns:</b> A reference to the contained value</p>
813	<p><b>Throws:</b> Nothing.</p>
814	<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
815	<p><b>Example:</b></p>
816	<blockquote>
817	<pre>T v ;
818	optional<T> opt ( v );
819	T const& u = *opt;
820	assert ( u == v ) ;
821	T w ;
822	*opt = w ;
823	assert ( *opt == w ) ;
824	</pre>
825	</blockquote>
826	<pre></pre>
827	</blockquote>
828
829	<HR>
830
831
832	<pre>T const& optional<T&>::operator*() const ;
833	T & optional<T<i>&</i>>::operator*();</pre>
834
835	<pre>T const& optional<T&>::get() const ;
836	T& optional<T&>::get() ;
837
838	inline T const& get ( optional<T<i>&</i>> const& ) ;
839	inline T& get ( optional<T&> &) ;
840	</pre>
841	<blockquote>
842	<p><b>Requirements: *this</b> is initialized</p>
843	<p><b>Returns:</b> <u>The</u> reference contained.</p>
844	<p><b>Throws:</b> Nothing.</p>
845	<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
846	<p><b>Example:</b></p>
847	<blockquote>
848	<pre>T v ;
849	T& vref = v ;
850	optional<T&> opt ( vref );
851	T const& vref2 = *opt;
852	assert ( vref2 == v ) ;
853	++ v ;
854	assert ( *opt == v ) ;</pre>
855	</blockquote>
856	</blockquote>
857
858	<HR>
859
860	<pre>T const* optional<T <i>(not a ref)</i>>::get_ptr() const ;
861	T* optional<T <i>(not a ref)</i>>::get_ptr() ;
862
863	inline T const* get_pointer ( optional<T <i>(not a ref)</i>> const& ) ;
864	inline T* get_pointer ( optional<T <i>(not a ref)</i>> &) ;
865	</pre>
866	<blockquote>
867	<p><b>Returns:</b> If <b>*this</b> is initialized, a pointer to the contained
868	value; else 0 (<i>null</i>).
869	</p>
870	<p><b>Throws:</b> Nothing.</p>
871	<p><b>Notes:</b> The contained value is permanently stored within *this, so
872	you should not hold nor delete this pointer
873	</p>
874	<p><b>Example:</b></p>
875	<blockquote>
876	<pre>T v;
877	optional<T> opt(v);
878	optional<T> const copt(v);
879	T* p = opt.get_ptr() ;
880	T const* cp = copt.get_ptr();
881	assert ( p == get_pointer(opt) );
882	assert ( cp == get_pointer(copt) ) ;
883	</pre>
884	</blockquote>
885	</blockquote>
886
887
888	<HR>
889
890
891	<pre>T const* optional<T <i>(not a ref)</i>>::operator ->() const ;
892	T* optional<T <i>(not a ref)</i>>::operator ->() ;
893	</pre>
894	<blockquote>
895	<p><b>Requirements: *this</b> is initialized.</p>
896	<p><b>Returns:</b> A pointer to the contained value.</p>
897	<p><b>Throws:</b> Nothing.</p>
898	<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
899	<p><b>Example:</b></p>
900	<blockquote>
901	<pre>struct X { int mdata ; } ;
902	X x ;
903	optional<X> opt (x);
904	opt->mdata = 2 ;
905	</pre>
906	</blockquote>
907	</blockquote>
908
909
910	<HR>
911
912
913	<pre>optional<T>::operator <i>unspecified-bool-type</i>() const ;</pre>
914	<blockquote>
915	<p><b>Returns:</b> An unspecified value which if used on a boolean context is equivalent to (get() != 0)</p>
916	<p><b>Throws:</b> Nothing.</p>
917	<blockquote>
918	<pre>optional<T> def ;
919	assert ( def == 0 );
920	optional<T> opt ( v ) ;
921	assert ( opt );
922	assert ( opt != 0 );
923	</pre>
924	</blockquote>
925	</blockquote>
926
927	<HR>
928
929
930	<pre> bool optional<T>::operator!() ;</pre>
931	<blockquote>
932	<p><b>Returns:</b> If <b>*this</b> is uninitialized, <code>true</code>; else <code>false.</code></p>
933	<p><b>Throws:</b> Nothing.</p>
934	<p><b>Notes:</b> This operator is provided for those compilers which can't use
935	the <i>unspecified-bool-type</i> operator in certain boolean contexts.
936	</p>
937	<p><b>Example:</b></p>
938	<blockquote>
939	<pre>optional<T> opt ;
940	assert ( !opt );
941	*opt = some_T ;
942
943	// Notice the "double-bang" idiom here.
944	assert ( !!opt ) ;
945	</pre>
946	</blockquote>
947	</blockquote>
948
949
950	<HR>
951
952
953	<pre>bool optional<T>::is_initialized() const ;</pre>
954	<blockquote>
955	<p><b>Returns:</b> <i>true</i> is the <b>optional</b> is initialized, <i>false</i>
956	otherwise.</p>
957	<p><b>Throws:</b> Nothing.</p>
958	<blockquote>
959	<pre>optional<T> def ;
960	assert ( !def.is_initialized() );
961	optional<T> opt ( v ) ;
962	assert ( opt.is_initialized() );</pre>
963	</blockquote>
964	</blockquote>
965
966	<HR>
967
968
969	<pre>bool operator == ( optional<T> const&amp x, optional<T&gt const&amp y );</pre>
970	<blockquote>
971	<p><b>Returns:</b> If both <b>x</b> and <b>y</b> are initialied, <code>(x == y)</code>.
972	If only x or y is initialized, <code>false</code>. If both are uninitialized, <code>true</code>.
973	</p>
974	<p><b>Throws:</b> Nothing.</p>
975	<p><b>Notes:</b> Pointers have shallow relational operators while <b>optional</b> has
976	deep relational operators. Do not use operator == directly in generic code
977	which expect to be given either an optional<T> or a pointer;
978	use <a href="../../utility/OptionalPointee.html#equal">equal_pointees()</a> instead
979	</p>
980	<p><b>Example:</b></p>
981	<blockquote>
982	<pre>T x(12);
983	T y(12);
984	T z(21);
985	optional<T> def0 ;
986	optional<T> def1 ;
987	optional<T> optX(x);
988	optional<T> optY(y);
989	optional<T> optZ(z);
990
991	// Identity always hold
992	assert ( def0 == def0 );
993	assert ( optX == optX );
994
995	// Both uninitialized compare equal
996	assert ( def0 == def1 );
997
998	// Only one initialized compare unequal.
999	assert ( def0 != optX );
1000
1001	// Both initialized compare as (lhs == rhs)
1002	assert ( optX == optY ) ;
1003	assert ( optX != optZ ) ;
1004	</pre>
1005	</blockquote>
1006	</blockquote>
1007
1008	<HR>
1009
1010
1011	<pre>bool operator < ( optional<T> const&amp x, optional<T&gt const&amp y );</pre>
1012	<blockquote>
1013	<p><b>Returns:</b> If <b>y</b> is not initialized, <code>false</code>.
1014	If <b>y</b> is initialized and <b>x</b> is not initialized, <code>true</code>.
1015	If both <b>x</b> and <b>y</b> are initialized, <code>(x < y)</code>.
1016	</p>
1017	<p><b>Throws:</b> Nothing.</p>
1018	<p><b>Notes:</b> Pointers have shallow relational operators while <b>optional</b> has
1019	deep relational operators. Do not use operator < directly in generic code
1020	which expect to be given either an optional<T> or a pointer;
1021	use <a href="../../utility/OptionalPointee.html#less">less_pointees()</a> instead
1022	</p>
1023	<p><b>Example:</b></p>
1024	<blockquote>
1025	<pre>T x(12);
1026	T y(34);
1027	optional<T> def ;
1028	optional<T> optX(x);
1029	optional<T> optY(y);
1030
1031	// Identity always hold
1032	assert ( !(def < def) );
1033	assert ( optX == optX );
1034
1035	// Both uninitialized compare equal
1036	assert ( def0 == def1 );
1037
1038	// Only one initialized compare unequal.
1039	assert ( def0 != optX );
1040
1041	// Both initialized compare as (lhs == rhs)
1042	assert ( optX == optY ) ;
1043	assert ( optX != optZ ) ;
1044	</pre>
1045	</blockquote>
1046	</blockquote>
1047
1048	<HR>
1049	<pre>bool operator != ( optional<T> const&amp x, optional<T&gt const&amp y );
1050	</pre>
1051	<blockquote>
1052	<p><b>Returns:</b> !( x == y );</p>
1053	<p><b>Throws:</b> Nothing.</p>
1054	</blockquote>
1055
1056	<HR>
1057	<pre>bool operator > ( optional<T> const&amp x, optional<T&gt const&amp y );
1058	</pre>
1059	<blockquote>
1060	<p><b>Returns:</b> ( y < x );</p>
1061	<p><b>Throws:</b> Nothing.</p>
1062	</blockquote>
1063
1064	<HR>
1065	<pre>bool operator <= ( optional<T> const&amp x, optional<T&gt const&amp y );
1066	</pre>
1067	<blockquote>
1068	<p><b>Returns:</b> !( y<x );</p>
1069	<p><b>Throws:</b> Nothing.</p>
1070	</blockquote>
1071
1072	<HR>
1073	<pre>bool operator >= ( optional<T> const&amp x, optional<T&gt const&amp y );
1074	</pre>
1075	<blockquote>
1076	<p><b>Returns:</b> !( x<y );</p>
1077	<p><b>Throws:</b> Nothing.</p>
1078	</blockquote>
1079
1080	<HR>
1081
1082	<pre>void swap ( optional<T>&amp x, optional<T&gt&amp y );</pre>
1083
1084	<blockquote>
1085	<p><b>Effect:</b> If both <b>x</b> and <b>y</b> are initialized, calls <code>swap(x,y)</code>
1086	using std::swap.<br>
1087	If only one is initialized, say x, calls: <code>y.reset(*x); x.reset();</code><br>
1088	If none is initialized, does nothing.
1089	</p>
1090	<p><b>Postconditions:</b> The states of x and y interchanged.</p>
1091	<p><b>Throws:</b> If both are initialized, whatever swap(T&,T&) throws.
1092	If only one is initialized, whatever T::T ( T const& ) throws.
1093	</p>
1094	<p><b>Notes:</b> If both are initialized, swap(T&,T&) is used <i>unqualified</i>
1095	but with std::swap introduced in scope.<br>
1096	If only one is initialized, T::~T() and T::T( T const& ) is called.
1097	</p>
1098	<p><b>Exception Safety:</b> If both are initialized, this operation has the exception
1099	safety guarantees of swap(T&,T&).<br>
1100	If only one is initialized, it has the same <b>basic</b> guarantee as optional<T>::reset( T const& ).
1101	</p>
1102	<p><b>Example:</b></p>
1103	<blockquote>
1104	<pre>T x(12);
1105	T y(21);
1106	optional<T> def0 ;
1107	optional<T> def1 ;
1108	optional<T> optX(x);
1109	optional<T> optY(y);
1110
1111	boost::swap(def0,def1); // no-op
1112
1113	boost::swap(def0,optX);
1114	assert ( *def0 == x );
1115	assert ( !optX );
1116
1117	boost::swap(def0,optX); // Get back to original values
1118
1119	boost::swap(optX,optY);
1120	assert ( *optX == y );
1121	assert ( *optY == x );
1122
1123	</pre>
1124	</blockquote>
1125	</blockquote>
1126	<HR>
1127
1128	<H2><A NAME="examples">Examples</A></H2>
1129
1130	<h3>Optional return values</h3>
1131	<PRE>optional<char> get_async_input()
1132	{
1133	if ( !queue.empty() )
1134	return optional<char>(queue.top());
1135	else return optional<char>(); // uninitialized
1136	}
1137
1138	void receive_async_message()
1139	{
1140	optional<char> rcv ;
1141	// The safe boolean conversion from 'rcv' is used here.
1142	while ( (rcv = get_async_input()) && !timeout() )
1143	output(*rcv);
1144	}
1145	</pre>
1146
1147	<h3>Optional local variables</h3>
1148	<pre>optional<string> name ;
1149	if ( database.open() )
1150	{
1151	name.reset ( database.lookup(employer_name) ) ;
1152	}
1153	else
1154	{
1155	if ( can_ask_user )
1156	name.reset ( user.ask(employer_name) ) ;
1157	}
1158
1159	if ( name )
1160	print(*name);
1161	else print("employer's name not found!");
1162	</pre>
1163
1164	<h3>Optional data members</h3>
1165	<pre>class figure
1166	{
1167	public:
1168
1169	figure()
1170	{
1171	// data member 'm_clipping_rect' is uninitialized at this point.
1172	}
1173
1174	void clip_in_rect ( rect const& rect )
1175	{
1176	....
1177	m_clipping_rect.reset ( rect ) ; // initialized here.
1178	}
1179
1180	void draw ( canvas& cvs )
1181	{
1182	if ( m_clipping_rect )
1183	do_clipping(*m_clipping_rect);
1184
1185	cvs.drawXXX(..);
1186	}
1187
1188	// this can return NULL.
1189	rect const* get_clipping_rect() { return get_pointer(m_clipping_rect); }
1190
1191	private :
1192
1193	optional<rect> m_clipping_rect ;
1194
1195	};
1196	</pre>
1197	<h3>Bypassing expensive unnecessary default construction</h3>
1198	<pre>class ExpensiveCtor { ... } ;
1199	class Fred
1200	{
1201	Fred() : mLargeVector(10000) {}
1202
1203	std::vector< optional<ExpensiveCtor> > mLargeVector ;
1204	} ;
1205	</pre>
1206
1207	<HR>
1208
1209	<H2><A NAME="ref">Optional references</A></H2>
1210	<p>This library allows the template parameter T to be of reference type: T&, and
1211	to some extent, T const&.</p>
1212
1213	<p>However, since references are not real objects some restrictions apply and
1214	some operations are not available in this case:</p>
1215
1216	<ul>
1217	<li>Converting constructors</li>
1218	<li>Converting assignment</li>
1219	<li>InPlace construction</li>
1220	<li>InPlace assignment</li>
1221	<li>Value-access via pointer</li>
1222	</ul>
1223	<p>Also, even though optional<T&> treats it wrapped pseudo-object much as a real
1224	value, a true real reference is stored so aliasing will ocurr: </p>
1225
1226	<ul>
1227	<li>Copies of optional<T&> will copy the references but all these references
1228	will nonetheless reefer to the same object.</li>
1229	<li>Value-access will actually provide access to the referenced object rather
1230	than the reference itself.</li>
1231	</ul>
1232
1233	<HR>
1234	<h2><A NAME="refassign">Rebinding semantics for assignment of optional
1235	references</a></h2>
1236	<p>If you assign to an <i>uninitialized</i> optional<T&> the effect is to bind (for the first time) to the object.
1237	Clearly, there is no other choice.</p>
1238	<pre>int x = 1 ;
1239	int& rx = x ;
1240	optional<int&> ora ;
1241	optional<int&> orb(x) ;
1242	ora = orb ; // now 'ora' is bound to 'x' through 'rx'
1243	*ora = 2 ; // Changes value of 'x' through 'ora'
1244	assert(x==2);
1245	</pre>
1246	<p>If you assign to a bare C++ reference, the assignment is forwarded to the
1247	referenced object; it's value changes but the reference is never rebound.</p>
1248	<pre>int a = 1 ;
1249	int& ra = a ;
1250	int b = 2 ;
1251	int& rb = b ;
1252	ra = rb ; // Changes the value of 'a' to 'b'
1253	assert(a==b);
1254	b = 3 ;
1255	assert(ra!=b); // 'ra' is not rebound to 'b'
1256	</pre>
1257	<p>Now, if you assign to an <i>initialized</i> optional<T&>, the effect is to <b>
1258	rebind</b> to the new object instead of assigning the referee. This is unlike
1259	bare C++ references.</p>
1260	<pre>int a = 1 ;
1261	int b = 2 ;
1262	int& ra = a ;
1263	int& rb = b ;
1264	optional<int&> ora(ra) ;
1265	optional<int&> orb(rb) ;
1266	ora = orb ; // 'ora' is <b>rebound</b> to 'b'
1267	*ora = 3 ; // Changes value of 'b' (not 'a')
1268	assert(a==1);
1269	assert(b==3);
1270	</pre>
1271	<h3>Rationale:</h3>
1272	<p>Rebinding semantics for the assignment of <i>initialized</i> optional
1273	references has been chosen to provide<b><i> </i>consistency among initialization
1274	states<i> </i></b>even at the expense of lack of consistency with the semantics of bare
1275	C++ references.<br>
1276	It is true that optional<U> strives to behave as much as possible as U does
1277	whenever it is initialized; but in the case when U is T&, doing so would result
1278	in inconsistent behavior w.r.t to the lvalue initialization state.</p>
1279	<p>Imagine optional<T&> forwarding assignment to the referenced object (thus
1280	changing the referenced object value but not rebinding), and consider the
1281	following code :</p>
1282	<pre>  optional<int&> a = get();
1283	int x = 1 ;
1284	int& rx = x ;
1285	optional<int&> b(rx);
1286	a = b ;
1287	</pre>
1288	<p>What does the assignment do?<br>
1289	If 'a' is <i>uninitialized</i>, the answer is clear: it binds to 'x' (we now have
1290	another reference to 'x').<br>
1291	But what if 'a' is already <i>initialized? </i>it would change the value of the
1292	referenced object (whatever that is); which is inconsistent with the other
1293	possible case.</p>
1294	<p>If optional<T&> would assign just like T& does, you would never be able to
1295	use Optional's assignment without explicitly handling the previous
1296	initialization state unless your code is capable of functioning whether after
1297	the assignment, 'a'
1298	aliases the same object as 'b' or not.</p>
1299	<p>That is, you would have to discriminate in order to be consistency.<br>
1300	<br>
1301	If in your code rebinding to another object is not an option, then is very
1302	likely that binding for the fist time isn't either. In such case, assignment to
1303	an <i>uninitialized</i> optional<T&> shall be prohibited. It is quite
1304	possible that in such scenario the precondition that the lvalue must be already
1305	initialized exist. If it doesn't, then binding for the first time is OK while
1306	rebinding is not which is IMO
1307	very unlikely.<br>
1308	In such scenario, you can assign the value itself directly, as in:</p>
1309	<pre>assert(!!opt);
1310	*opt=value; </pre>
1311
1312	<HR>
1313
1314	<H2><A NAME="inplace">In-Place Factories</A></H2>
1315	<p>
1316	One of the typical problems with wrappers and containers is that their
1317	interfaces usually provide an operation to initialize or assign the contained
1318	object as a copy of some other object. This not only requires the underlying
1319	type to be <a href="../../utility/CopyConstructible.html">Copy Constructible</a>, but also requires the existence of a fully
1320	constructed object, often temporary, just to follow the copy from:</p>
1321	<pre>struct X
1322	{
1323	X ( int, std:::string ) ;
1324	} ;</pre>
1325	<pre>class W
1326	{
1327	X wrapped_ ;
1328
1329	public:
1330
1331	W ( X const& x ) : wrapped_(x) {}
1332	} ;</pre>
1333	<pre>void foo()
1334	{
1335	// Temporary object created.
1336	W ( X(123,"hello") ) ;
1337	}
1338	</pre>
1339	<p>A solution to this problem is to support direct construction of the contained
1340	object right in the container's storage.<br>
1341	In this scheme, the user only needs to supply the arguments to the constructor
1342	to use in the wrapped object construction.</p>
1343	<pre>class W
1344	{
1345	X wrapped_ ;
1346
1347	public:
1348
1349	W ( X const& x ) : wrapped_(x) {}
1350	W ( int a0, std::string a1) : wrapped_(a0,a1) {}
1351	} ;</pre>
1352	<pre>void foo()
1353	{
1354	// Wrapped object constructed in-place
1355	// No temporary created.
1356	W (123,"hello") ;
1357	}
1358	</pre>
1359	<p>A limitation of this method is that it doesn't scale well to wrapped objects with multiple
1360	constructors nor to generic code were the constructor overloads are unknown.</p>
1361	<p>The solution presented in this library is the family of <b>InPlaceFactories</b> and
1362	<b>TypedInPlaceFactories</b>.<br>
1363	These factories are a family of classes which encapsulate an increasing number of arbitrary
1364	constructor parameters and supply a method to construct an object of a given type using those
1365	parameters at an address specified by the user via placement new.</p>
1366	<p> For example, one member of this family looks like:</p>
1367	<pre>template<class T,class A0, class A1>
1368	class TypedInPlaceFactory2
1369	{
1370	A0 m_a0 ; A1 m_a1 ;
1371
1372	public:
1373
1374	TypedInPlaceFactory2( A0 const& a0, A1 const& a1 ) : m_a0(a0), m_a1(a1) {}
1375
1376	void construct ( void* p ) { new (p) T(m_a0,m_a1) ; }
1377	} ;
1378	</pre>
1379	<p>A wrapper class aware of this can use it as:</p>
1380	<pre>class W
1381	{
1382	X wrapped_ ;
1383
1384	public:
1385
1386	W ( X const& x ) : wrapped_(x) {}
1387	W ( TypedInPlaceFactory2 const& fac ) { fac.construct(&wrapped_) ; }
1388	} ;</pre>
1389	<pre>void foo()
1390	{
1391	// Wrapped object constructed in-place via a TypedInPlaceFactory.
1392	// No temporary created.
1393	W ( TypedInPlaceFactory2<X,int,std::string&rt;(123,"hello")) ;
1394	}
1395	</pre>
1396	<p>The factories are divided in two groups:<ul>
1397	<li><u>TypedInPlaceFactories</u>: those which take the target type as a primary template parameter.</li>
1398	<li><u>InPlaceFactories</u>: those with a template <code>construct(void*)</code> member function taking the target type.</li>
1399	</ul>
1400	<p>Within each group, all the family members differ only in the number of parameters allowed.</p>
1401	<p></p>
1402	<p>This library provides an overloaded set of helper template functions to construct these factories
1403	without requiring unnecessary template parameters:</p>
1404	<pre>template<class A0,...,class AN>
1405	InPlaceFactory<i>N </i><A0,...,AN> <b>in_place</b> ( A0 const& a0, ..., AN const& aN) ;
1406
1407	template<class T,class A0,...,class AN>
1408	TypedInPlaceFactory<i>N </i><T,A0,...,AN> <b>in_place</b> ( T const& a0, A0 const& a0, ..., AN const& aN) ;</pre>
1409
1410	<p>In-place factories can be used generically by the wrapper and user as follows:</p>
1411	<pre>class W
1412	{
1413	X wrapped_ ;
1414
1415	public:
1416
1417	W ( X const& x ) : wrapped_(x) {}
1418
1419	template<class InPlaceFactory></class>
1420	W ( InPlaceFactory const& fac ) { fac.template <X>construct(&wrapped_) ; }
1421
1422	} ;</pre>
1423	<pre>void foo()
1424	{
1425	// Wrapped object constructed in-place via a InPlaceFactory.
1426	// No temporary created.
1427	W ( in_place(123,"hello") ) ;
1428	}
1429	</pre>
1430	<p>The factories are implemented in the headers:
1431	<a href="../../../boost/utility/in_place_factory.hpp">in_place_factory.hpp</a> and
1432	<a href="../../../boost/utility/typed_in_place_factory.hpp">typed_in_place_factory.hpp</a>
1433	</p>
1434
1435	<HR>
1436
1437	<H2><A NAME="bool">A note about optional<bool></A></H2>
1438	<p><code>optional<bool></code> should be used with special caution and consideration.</p>
1439	<p>First, it is functionally similar to a tristate boolean (false,maybe,true) —such as <a href="http://www.boost.org/doc/html/tribool.html">boost::tribool</a>—except that in a tristate boolean,
1440	the <i>maybe</i> state <u>represents a valid value</u>, unlike the corresponding state
1441	of an uninitialized optional<bool>.<br>
1442	It should be carefully considered if an optional<bool> instead of a tribool is really needed</p>
1443	<p>Second, optional<> provides an implicit conversion to bool. This conversion
1444	refers to the initialization state and not to the contained value.<br>
1445	Using optional<bool> can lead to subtle errors due to the implicit bool conversion:</p>
1446	<pre>void foo ( bool v ) ;
1447	void bar()
1448	{
1449	optional<bool> v = try();
1450
1451	// The following intended to pass the <b>value</b> of 'v' to foo():
1452	foo(v);
1453	// But instead, the <i>initialization state</i> is passed
1454	// due to a typo: it should have been foo(<b>*</b>v).
1455	}
1456	</pre>
1457	<p>The only implicit conversion is to bool, and it is <i>safe</i> in the sense that typical
1458	integral promotions don't apply (i.e. if foo() takes an 'int' instead, it won't compile). <HR>
1459
1460	<H2><A NAME="exsafety">Exception Safety Guarantees</A></H2>
1461	<H3><u>Assignment and Reset:</u></H3>
1462	<p>Because of the current implementation (see <A HREF="#impl">Implementation Notes</A>), all
1463	of the assignment methods:</p>
1464	<ul>
1465	<li> <code>optional<T>::operator= ( optional<T> const& ) </code>
1466	</li>
1467	<li> <code>optional<T>::operator= ( T const& ) </code></li>
1468	<li> <code>template<class U> optional<T>::operator= ( optional<U> const& ) </code>
1469	</li>
1470	<li> <code>template<class InPlaceFactory> optional<T>::operator= (
1471	InPlaceFactory const& ) </code></li>
1472	<li> <code>template<class TypedInPlaceFactory> optional<T>::operator= (
1473	TypedInPlaceFactory const& ) </code></li>
1474	<li> <code>optional<T>:::reset ( T const&)</code></li>
1475	</ul>
1476	<p>Can only <i>guarantee</i> the <u>basic exception safety</u>: The lvalue optional is left <u>uninitialized</u>
1477	if an exception is thrown (any previous value is <i>first</i> destroyed using T::~T())</p>
1478	<p>On the other hand, the <i>uninitializing</i> methods:</p>
1479	<ul>
1480	<li><code>optional<T>::operator= ( detail::none_t ) </code></li>
1481	<li><code>optional<T>::reset()</code></li>
1482	</ul>
1483	<p>Provide the no-throw guarantee (assuming a no-throw T::~T())</p>
1484	<p>However, since <code>optional&lt&gt</code> itself doesn't throw any exceptions,
1485	the only source for exceptions here are T's constructor, so if you know the exception guarantees
1486	for T::T ( T const& ), you know that optional's assignment and reset has the same guarantees.</p>
1487	<pre>//
1488	// Case 1: Exception thrown during assignment.
1489	//
1490	T v0(123);
1491	optional&ltT&gt opt0(v0);
1492	try
1493	{
1494	T v1(456);
1495	optional&ltT&gt opt1(v1);
1496	opt0 = opt1 ;
1497
1498	// If no exception was thrown, assignment succeeded.
1499	assert( *opt0 == v1 ) ;
1500	}
1501	catch(...)
1502	{
1503	// If any exception was thrown, 'opt0' is reset to uninitialized.
1504	assert( !opt0 ) ;
1505	}
1506
1507	//
1508	// Case 2: Exception thrown during reset(v)
1509	//
1510	T v0(123);
1511	optional&ltT&gt opt(v0);
1512	try
1513	{
1514	T v1(456);
1515	opt.reset ( v1 ) ;
1516
1517	// If no exception was thrown, reset succeeded.
1518	assert( *opt == v1 ) ;
1519	}
1520	catch(...)
1521	{
1522	// If any exception was thrown, 'opt' is reset to uninitialized.
1523	assert( !opt ) ;
1524	}
1525	</pre>
1526	<H3><u>Swap:</u></H3>
1527	<p><code>void swap( optional<T>&, optional<T>& )</code> has the same exception guarantee as <code>swap(T&,T&)</code> when both optionals are initialized.<br>
1528	If only one of the optionals is initialized, it gives the same <i>basic</i> exception guarantee as <code>optional<T>::reset( T const& )</code> (since <code>optional<T>::reset()</code> doesn't throw).<br>
1529	If none of the optionals is initialized, it has no-throw guarantee since it is a no-op. </p>
1530
1531	<HR>
1532
1533	<H2><A NAME="requirements">Type requirements</A></H2>
1534	<p>In general, T must be <a href="../../utility/CopyConstructible.html">Copy Constructible</a> and have a no-throw destructor. The copy-constructible requirement is not needed
1535	if InPlaceFactories are used.<br>
1536	T <u>is not</u> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html">Default Constructible</a> </p>
1537
1538	<HR>
1539
1540	<H2><A NAME="impl">Implementation Notes</A></H2>
1541	<p>optional<T> is currently implemented
1542	using a custom aligned storage facility built from <code>alignment_of</code> and <code>type_with_alignment</code> (both from Type Traits).
1543	It uses a separate boolean flag to indicate the initialization state.<br>
1544	Placement new with T's copy constructor and T's destructor
1545	are explicitly used to initialize,copy and destroy optional values.<br>
1546	As a result, T's default constructor is effectively by-passed, but the exception
1547	guarantees are basic.<br>
1548	It is planned to replace the current implementation with another with
1549	stronger exception safety, such as a future boost::variant<T,nil_t>. </p>
1550
1551	<HR>
1552
1553	<H2><A NAME="porta">Dependencies and Portability</A></H2>
1554
1555	<p>The implementation uses <code>type_traits/alignment_of.hpp</code> and <code>type_traits/type_with_alignment.hpp</code></p>
1556
1557	<HR>
1558
1559	<H2><A NAME="credits">Acknowledgments</A></H2>
1560	<p>Pre-formal review:</p>
1561	<blockquote>
1562	<p>Peter Dimov suggested the name 'optional', and was the first to point out the
1563	need for aligned storage<br>
1564	Douglas Gregor developed 'type_with_alignment', and later Eric Friedman coded
1565	'aligned_storage', which are the core of the optional class implementation.<br>
1566	Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
1567	and their work influenced the current implementation.<br>
1568	Gennadiy Rozental made extensive and important comments which shaped the design.<br>
1569	Vesa Karvonen and Douglas Gregor made quite useful comparisons between optional,
1570	variant and any; and made other relevant comments. Douglas Gregor and Peter
1571	Dimov commented on comparisons and evaluation in boolean contexts.<br>
1572	Eric Friedman helped understand the issues involved with aligned storage, move/copy
1573	operations and exception safety.<br>
1574	Many others have participated with useful comments: Aleksey Gurotov, Kevlin
1575	Henney, David Abrahams, and others I can't recall. </p>
1576	</blockquote>
1577	<p>Post-formal review:</p>
1578	<blockquote>
1579	<p>William Kempf carefully considered the originally proposed interface and
1580	suggested the new interface which is currently used. He also started and fueled
1581	the discussion about the analogy optional<>/smart pointer and about
1582	relational operators.<br>
1583	Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson focused
1584	on the relational semantics of optional (originally undefined); concluding
1585	with the fact that the pointer-like interface doesn't make it a pointer so
1586	it shall have deep relational operators.<br>
1587	Augustus Saunders also explored the different relational semantics between
1588	optional<> and a pointer and developed the OptionalPointee concept as
1589	an aid against potential conflicts on generic code.<br>
1590	Joel de Guzman noticed that optional<> can be seen as an API on top
1591	of variant<T,nil_t>.<br>
1592	Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
1593	the Maybe type constructor (analogy originally pointed out by David Sankel).<br>
1594	Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey, Rob
1595	Stewart, and others.<br>
1596	Joel de Guzman made the case for the support of references and helped with
1597	the proper semantics.<br>
1598	Mat Marcus shown the virtues of a value-oriented interface, influencing the
1599	current design, and contributed the idea of "none".</p>
1600	</blockquote>
1601	<HR>
1602
1603	<P>Revised April 21, 2005</P>
1604	<p>© Copyright Fernando Luis Cacciola Carballal, 2003,2004,2005</p>
1605	<p> Use, modification, and distribution are subject to the Boost Software
1606	License, Version 1.0. (See accompanying file <a href="../../../LICENSE_1_0.txt">
1607	LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
1608	www.boost.org/LICENSE_1_0.txt</a>)</p>
1609	<P>Developed by <A HREF="mailto:fernando_cacciola@hotmail.com">Fernando Cacciola</A>,
1610	the latest version of this file can be found at <A
1611	HREF="http://www.boost.org">www.boost.org</A>, and the boost
1612	<A HREF="http://www.boost.org/more/mailing_lists.htm#main">discussion lists</A></P>
1613	</BODY>
1614	</HTML>

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