mirror of
https://github.com/boostorg/optional.git
synced 2026-01-23 17:52:13 +00:00
1445 lines
56 KiB
HTML
1445 lines
56 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//SoftQuad Software//DTD HoTMetaL PRO 5.0::19981217::extensions to HTML 4.0//EN" "hmpro5.dtd">
|
|
|
|
<HTML>
|
|
|
|
<HEAD>
|
|
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
|
|
<LINK REL="stylesheet" TYPE="text/css" HREF="../../../boost.css">
|
|
<TITLE>Header </TITLE>
|
|
</HEAD>
|
|
|
|
<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080">
|
|
<H2><IMG SRC="../../../c++boost.gif" WIDTH="276" HEIGHT="86">Header <<A
|
|
HREF="../../../boost/optional.hpp">boost/optional.hpp</A>> </H2>
|
|
|
|
<H2>Contents</H2>
|
|
<DL CLASS="page-index">
|
|
<DT><A HREF="#mot">Motivation</A></DT>
|
|
<DT><A HREF="#dev">Development</A></DT>
|
|
<DT><A HREF="#synopsis">Synopsis</A></DT>
|
|
<DT><A HREF="#semantics">Semantics</A></DT>
|
|
<DT><A HREF="#examples">Examples</A></DT>
|
|
<DT><A HREF="#ref">Optional references</A></DT>
|
|
<DT><A HREF="#inplace">In-Place Factories</A></DT>
|
|
<DT><A HREF="#bool">A note about optional<bool></A></DT>
|
|
<DT><A HREF="#exsafety">Exception Safety Guarantees</A></DT>
|
|
<DT><A HREF="#requirements">Type requirements</A></DT>
|
|
<DT><A HREF="#impl">Implementation Notes</A></DT>
|
|
<DT><A HREF="#porta">Dependencies and Portability</A></DT>
|
|
<DT><A HREF="#credits">Acknowledgment</A></DT>
|
|
</DL>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="mot"></A>Motivation</H2>
|
|
|
|
<P>Consider these functions which should return a value but which might not have
|
|
a value to return:</P>
|
|
<pre>(A) double sqrt(double n );
|
|
(B) char get_async_input();
|
|
(C) point polygon::get_any_point_effectively_inside();</pre>
|
|
<P>There are different approaches to the issue of not having a value to return.</P>
|
|
<P>A typical approach is to consider the existence of a valid return value as
|
|
a postcondition, so that if the function cannot compute the value to return,
|
|
it has either undefined behavior (and can use asssert in a debug build)
|
|
or uses a runtime check and throws an exception if the postcondition is violated.
|
|
This is a reasonable choice for example, for function (A), because the
|
|
lack of a proper return value is directly related to an invalid parameter (out
|
|
of domain argument), so it is appropriate to require the callee to supply only
|
|
parameters in a valid domain for execution to continue normally.</P>
|
|
<P>However, function (B), because of its asynchronous nature, does not fail just
|
|
because it can't find a value to return; so it is incorrect to consider
|
|
such a situation an error and assert or throw an exception. This function must
|
|
return, and somehow, must tell the callee that it is not returning a meaningful
|
|
value.</P>
|
|
<P>A similar situation occurs with function (C): it is conceptually an error to
|
|
ask a <i>null-area</i> polygon to return a point inside itself, but in many
|
|
applications, it is just impractical for performance reasons to treat this as
|
|
an error (because detecting that the polygon has no area might be too expensive
|
|
to be required to be tested previously), and either an arbitrary point (typically
|
|
at infinity) is returned, or some efficient way to tell the callee that there
|
|
is no such point is used.</P>
|
|
<P>There are various mechanisms to let functions communicate that the returned
|
|
value is not valid. One such mechanism, which is quite common since it has zero
|
|
or negligible overhead, is to use a special value which is reserved to communicate
|
|
this. Classical examples of such special values are EOF, string::npos, points
|
|
at infinity, etc...</P>
|
|
<P>When those values exist, i.e. the return type can hold all meaningful values
|
|
<i>plus</i> the <i>signal</i> value, this mechanism is quite appropriate and
|
|
well known. Unfortunately, there are cases when such values do not exist. In
|
|
these cases, the usual alternative is either to use a wider type, such as 'int'
|
|
in place of 'char'; or a compound type, such as std::pair<point,bool>.
|
|
</P>
|
|
<P>Returning a std::pair<T,bool>, thus attaching a boolean flag to the result
|
|
which indicates if the result is meaningful, has the advantage that can be turned
|
|
into a consistent idiom since the first element of the pair can be whatever
|
|
the function would conceptually return. For example, the last two functions
|
|
could have the following interface:</P>
|
|
<pre>std::pair<char,bool> get_async_input();
|
|
std::pair<point,bool> polygon::get_any_point_effectively_inside();</pre>
|
|
<p>These functions use a consistent interface for dealing with possibly inexistent
|
|
results:</p>
|
|
<pre>std::pair<point,bool> p = poly.get_any_point_effectively_inside();
|
|
if ( p.second )
|
|
flood_fill(p.first);
|
|
</pre>
|
|
|
|
<P>However, not only is this quite a burden syntactically, it is also error
|
|
prone since the user can easily use the function result (first element of the
|
|
pair) without ever checking if it has a valid value.</P>
|
|
<P>Clearly, we need a better idiom.</P>
|
|
|
|
<H2><A NAME="dev"></A>Development</H2>
|
|
|
|
<h3><u>The models:</u></h3>
|
|
<P>In C++, we can <i>declare</i> an object (a variable) of type T, and we can give this variable
|
|
an <i>initial value</i> (through an <i>initializer</i>. (c.f. 8.5)).<br>
|
|
When a declaration includes a non-empty initializer (an initial value is given), it is said that
|
|
the object has been <i><b>initialized</b></i>.<br>
|
|
If the declaration uses an empty initializer (no initial value is given),
|
|
and neither default nor value initialization applies, it is said that the object is
|
|
<i><b>uninitialized</b></i>. Its actual value exist but has an
|
|
<i>indeterminate inital value</i> (c.f. 8.5.9).<br>
|
|
<code>optional<T></code> intends to formalize the notion of initialization/no-initialization
|
|
allowing a program to test whether an object has been initialized and stating that access to
|
|
the value of an uninitialized object is undefined behaviour. That is,
|
|
when a variable is declared as optional<T> and no initial value is given,
|
|
the variable is formally uninitialized. A formally uninitialized optional object has conceptually
|
|
no value at all and this situation can be tested at runtime. It is formally <i>undefined behaviour</i>
|
|
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
|
|
treatment of initialization states in optional objects, it is even possible to reset an optional to <i>uninitialized</i>.</P>
|
|
<P>In C++ there is no formal notion of uninitialized objects, which
|
|
means that objects always have an initial value even if indeterminate.<br>
|
|
As discussed on the previous section, this has a drawback because you need additional
|
|
information to tell if an object has been effectively initialized.<br>
|
|
One of the typical ways in which this has been historically
|
|
dealt with is via a special value: EOF,npos,-1, etc... This is equivalent to adding
|
|
the special value to the set of possible values of a given type. This super set of
|
|
T plus some <i>nil_t</i>—were nil_t is some stateless POD—can be modeled in modern
|
|
languages as a <b>discriminated union</b> of <code>T</code> and <code>nil_t</code>.
|
|
Discriminated unions are often called <i>variants</i>. A variant has a <i>current type</i>,
|
|
which in our case is either <code>T</code> or <code>nil_t</code>.<br>
|
|
Using the <a href="../../../doc/html/variant.html">Boost.Variant</a> library, this model can be implemented
|
|
in terms of <code>boost::variant<T,nil_t></code>.<br>
|
|
There is precedence for a discriminated union as a model for an optional value: the
|
|
<a href="http://www.haskell.org/"><u>Haskell</u></a> <b>Maybe</b> builtin type constructor,
|
|
thus a discriminated union <code>T+nil_t</code> serves as a conceptual foundation.</p>
|
|
<p>A <code>variant<T,nil_t></code> follows naturally from the traditional idiom of extending
|
|
the range of possible values adding an additional sentinel value with the special meaning of <i>Nothing. </i>
|
|
However, this additional <i>Nothing</i> value is largely irrelevant for our purpose
|
|
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 neccesary in order to do so.</p>
|
|
<p>The observation made in the last paragraph about the irrelevant nature of the additional <code>nil_t</code> with respect to
|
|
<u>purpose</u> of optional<T> suggests
|
|
an alternative model: a <i>container</i> that either has a value of T or nothing.
|
|
</p>
|
|
<p>As of this writting I don't know of any precedence for a variable-size fixed-capacity (of 1)
|
|
stack-based container model for optional values, yet I believe this is the consequence of
|
|
the lack of practical implementations of such a container rather than an inherent shortcoming
|
|
of the container model.</p>
|
|
<p>In any event, both the discriminated-union or the single-element container models serve as a conceptual
|
|
ground for a class representing optional—i.e. possibly uninitialized—objects.<br>
|
|
For instance, these models show the <i>exact</i> semantics required for a wrapper of optional values:</p>
|
|
<p>Discriminated-union:</p>
|
|
<blockquote>
|
|
<li><b>deep-copy</b> semantics: copies of the variant implies copies of the value.</li>
|
|
<li><b>deep-relational</b> semantics: comparisons between variants matches both current types and values</li>
|
|
<li>If the variant's current type is T, it is modeling an <i>initialized</i> optional.</li>
|
|
<li>If the variant's current type is not T, it is modeling an <i>uninitialized</i> optional.</li>
|
|
<li>Testing if the variant's current type is T models testing if the optional is initialized</li>
|
|
<li>Trying to extract a T from a variant when its current type is not T, models the undefined behaviour
|
|
of trying to access the value of an uninitialized optional</li>
|
|
</blockquote>
|
|
<p>Single-element container:</p>
|
|
<blockquote>
|
|
<li><b>deep-copy</b> semantics: copies of the container implies copies of the value.</li>
|
|
<li><b>deep-relational</b> semantics: comparisons between containers compare container size and if match, contained value</li>
|
|
<li>If the container is not empty (contains an object of type T), it is modeling an <i>initialized</i> optional.</li>
|
|
<li>If the container is empty, it is modeling an <i>uninitialized</i> optional.</li>
|
|
<li>Testing if the container is empty models testing if the optional is initialized</li>
|
|
<li>Trying to extract a T from an empty container models the undefined behaviour
|
|
of trying to access the value of an uninitialized optional</li>
|
|
</blockquote>
|
|
|
|
<h3><u>The semantics:</u></h3>
|
|
<p>Objects of type <code>optional<T></code> are intended to be used in places where objects of type T would
|
|
but which might be uninitialized. Hence, <code>optional<T></code>'s purpose is to formalize the
|
|
additional possibly uninitialized state.<br>
|
|
From the perspective of this role, <code>optional<T></code> can have the same operational semantics of T
|
|
plus the additional semantics corresponding to this special state.<br>
|
|
As such, <code>optional<T></code> could be thought of as a <i>supertype</i> of T. Of course,
|
|
we can't do that in C++, so we need to compose the desired semantics using a different mechanism.<br>
|
|
Doing it the other way around, that is, making <code>optional<T></code> a <i>subtype</i> of T is not only
|
|
conceptually wrong but also impractical: it is not allowed to derive from a non-class type, such as a builtin type.</p>
|
|
|
|
<p>We can draw from the purpose of optional<T> the required basic semantics:</p>
|
|
|
|
<blockquote>
|
|
<p><b>Default Construction:</b> To introduce a formally uninitialized wrapped
|
|
object.</p>
|
|
|
|
<p><b>Direct Value Construction via copy:</b> To introduce a formally
|
|
initialized wrapped object whose value is obtained as a copy of some object.</p>
|
|
|
|
<p><b>Deep Copy Construction:</b> To obtain a different yet equivalent wrapped
|
|
object.</p>
|
|
|
|
<p><b>Direct Value Assignment (upon initialized):</b> To assign the wrapped object a value obtained
|
|
as a copy of some object.</p>
|
|
|
|
<p><b>Direct Value Assignment (upon uninitialized):</b> To initialize the wrapped object
|
|
with a value obtained
|
|
as a copy of some object.</p>
|
|
|
|
<p><b>Assignnment (upon initialized):</b> To assign the wrapped object a value obtained as a copy
|
|
of another wrapper's object.</p>
|
|
|
|
<p><b>Assignnment (upon uninitialized):</b> To initialize the wrapped object
|
|
with value obtained as a copy
|
|
of another wrapper's object.</p>
|
|
|
|
<p><b>Deep Relational Operations (when supported by the type T):</b> To compare
|
|
wrapped object values taking into account the presence of uninitialized
|
|
operands.</p>
|
|
|
|
<p><b>Value access:</b> To unwrap the wrapped object.</p>
|
|
|
|
<p><b>Initialization state query:</b> To determine if the object is formally
|
|
initialized or not.</p>
|
|
|
|
<p><b>Swap:</b> To exchange wrapper's objects. (with whatever exception safety
|
|
guarantiees are provided by T's swap).</p>
|
|
|
|
<p><b>De-initialization:</b> To release the wrapped object (if any) and leave
|
|
the wrapper in the uninitialized state.</p>
|
|
|
|
</blockquote>
|
|
|
|
<p>Additional operations are useful, such as converting constructors and
|
|
converting assignments, in-place construction and assignment, and safe value
|
|
access via a pointer to the wrapped object or null.</p>
|
|
<h3><u>The Interface:</u></h3>
|
|
<p>Since the purpose of optional is to allow us to use objects with a formal
|
|
uninitialized additional state, the interface could try to follow the interface
|
|
of the underlying T type as much as possible. In order to choose the proper
|
|
degree of adoption of the native T interface, the following must be noted: <br>
|
|
Even if all the operations supported by an instance of type T are defined for
|
|
the entire range of values for such a type, an optional<T> extends such a set of
|
|
values with a new value for which most (otherwise valid) operations are not
|
|
defined in terms of T.<br>
|
|
Furthermore, since optional<T> itself is merely a T wrapper (modeling a T
|
|
supertype), any attempt to define such operations upon uninitialized optionals
|
|
will be totally artificial w.r.t. T.<br>
|
|
This library chooses an interface which follows from T's interface only for
|
|
those operations which are well defined (w.r.t the type T) even if any of the
|
|
operands are uninitialized. These operations include: construction,
|
|
copy-construction, assignment, swap and relational operations.<br>
|
|
For the value access operations, which are undefined (w.r.t the type T) when the
|
|
operand is uninitialized, a different interface is choosen (which will be
|
|
explained next).<br>
|
|
Also, the presence of the possibly uninitialized state requires additional
|
|
operations not provided by T itself which are supported by a special interface.</p>
|
|
<h3>Lexically-hinted Value Access in the presence of possibly untitialized
|
|
optional objects: The operators * and -></h3>
|
|
<p>A relevant feature of a pointer is that it can have a <b>null
|
|
pointer value</b>. This is a <i>special</i> value which is used to indicate that the
|
|
pointer is not referring to any object at all. In other words, null pointer
|
|
values convey the notion of inexistent objects.</P>
|
|
<P>This meaning of the null pointer value allowed pointers to became a <i>de facto</i> standard
|
|
for handling optional objects because all you have to do to refer to a value which you
|
|
don't really have is to use a null pointer value of the appropriate type.
|
|
Pointers have been used for decades—from the days of C APIs to modern C++ libraries—to
|
|
<i>refer</i> to optional (that is, possibly inexistent) objects; particularly
|
|
as optional arguments to a function, but also quite often as optional data members.</P>
|
|
<P>The possible presence of a null pointer value makes the operations that access the
|
|
pointee's value possibly undefined, therefore, expressions which use dereference
|
|
and access operators, such as: <code>( *p = 2 )</code> and <code>( p->foo())</code>,
|
|
implicitly convey the notion of optionality, and this information is tied to
|
|
the <i>syntax</i> of the expressions. That is, the presence of operators * and -> tell by
|
|
themselves—without any additional context—that the expression will be undefined unless
|
|
the implied pointee actually exist.</P>
|
|
<P>Such a <i>de facto</i> idiom for referring to optional objects can be formalized in the form of a
|
|
concept: the <a href="../../utility/OptionalPointee.html">OptionalPointee</a> concept.<br>
|
|
This concept captures the syntactic usage of operatos *, -> and conversion to bool to convey
|
|
the notion of optionality.</P>
|
|
<P>However, pointers are good to <u>refer</u> to optional objects, but not particularly good
|
|
to handle the optional objects in all other respects, such as initializing or moving/copying
|
|
them. The problem resides in the shallow-copy of pointer semantics: if you need to
|
|
effectively move or copy the object, pointers alone are not enough. The problem
|
|
is that copies of pointers do not imply copies of pointees. For example, as
|
|
was discussed in the motivation, pointers alone cannot be used to return optional
|
|
objects from a function because the object must move outside from the function and
|
|
into the caller's context.<br>
|
|
A solution to the shallow-copy problem that is often used is to resort to dynamic
|
|
allocation and use a smart pointer to automatically handle the details of this.
|
|
For example, if a function is to optionally return an object X, it can use shared_ptr<X>
|
|
as the return value. However, this requires dynamic allocation of X. If X is
|
|
a builtin or small POD, this technique is very poor in terms of required resources.
|
|
Optional objects are essentially values so it is very convenient to be able to use automatic
|
|
storage and deep-copy semantics to manipulate optional values just as we do with ordinary
|
|
values. Pointers do not have this semantics, so are unappropriate for the initialization and
|
|
transport of optional values, yet are quite convenient for handling the access to the
|
|
possible undefined value because of the idiomatic aid present in the OptionalPointee
|
|
concept incarnated by pointers.
|
|
</p>
|
|
<h4>Optional<T> as a model of OptionalPointee</h4>
|
|
<P>For value access operations optional<> uses operators * and -> to lexically
|
|
warn about the possibliy uninitialized state appealing to the familiar pointer
|
|
semantics w.r.t. to null pointers.<br>
|
|
<u><b>However, it is particularly important to note that optional<> objects are not pointers. optional<>
|
|
is not, and does not model, a pointer</b></u><b>.</b>
|
|
<P>For instance, optional<> has not shallow-copy so does not alias: two different optionals
|
|
never refer to the <i>same</i> value unless T itself is an reference (but my have <i>equivalent</i> values).<br>
|
|
The difference between an optional<T> and a pointer must be kept in mind, particularly
|
|
because the semantics of relational operators are different: since optional<T>
|
|
is a value-wrapper, relational operators are deep: they compare optional values;
|
|
but relational operators for pointers are shallow: they do not compare pointee values.<br>
|
|
As a result, you might be able to replace optional<T> by T* on some situations but
|
|
not always. Specifically, on generic code written for both, you cannot use relational
|
|
operators directly, and must use the template functions
|
|
<a href="../../utility/OptionalPointee.html#equal">equal_pointees()</a> and
|
|
<a href="../../utility/OptionalPointee.html#less">less_pointees()</a> instead.
|
|
<HR>
|
|
|
|
<H2><A NAME="synopsis">Synopsis</A></H2>
|
|
|
|
<PRE>namespace boost {
|
|
|
|
template<class T>
|
|
class optional
|
|
{
|
|
public :
|
|
|
|
<i><u>(If T is of referennce type, the parameters and results by reference are by value)</u></i>
|
|
|
|
optional () ;
|
|
|
|
optional ( detail::none_t ) ;
|
|
|
|
optional ( T const& v ) ;
|
|
|
|
optional ( optional const& rhs ) ;
|
|
|
|
template<class U> explicit optional ( optional<U> const& rhs ) ;
|
|
|
|
template<class InPlaceFactory> explicit optional ( InPlaceFactory const& f ) ;
|
|
|
|
template<class TypedInPlaceFactory> explicit optional ( TypedInPlaceFactory const& f ) ;
|
|
|
|
optional& operator = ( detail::none_t ) ;
|
|
|
|
optional& operator = ( T const& v ) ;
|
|
|
|
optional& operator = ( optional const& rhs ) ;
|
|
|
|
template<class U> optional& operator = ( optional<U> const& rhs ) ;
|
|
|
|
template<class InPlaceFactory> optional& operator = ( InPlaceFactory const& f ) ;
|
|
|
|
template<class TypedInPlaceFactory> optional& operator = ( TypedInPlaceFactory const& f ) ;
|
|
|
|
T const& get() const ;
|
|
T& get() ;
|
|
|
|
T const* operator ->() const ;
|
|
T* operator ->() ;
|
|
|
|
T const& operator *() const ;
|
|
T& operator *() ;
|
|
|
|
T const* get_ptr() const ;
|
|
T* get_ptr() ;
|
|
|
|
operator <i>unspecified-bool-type</i>() const ;
|
|
|
|
bool operator!() const ;
|
|
|
|
<i><u>deprectated methods</u></i>
|
|
|
|
void reset() ; (deprectated)
|
|
void reset ( T const& ) ; (deprectated)
|
|
bool is_initialized() const ; (deprectated)
|
|
|
|
} ;
|
|
|
|
template<class T> inline bool operator == ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline bool operator != ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline bool operator < ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline bool operator > ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline bool operator <= ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline bool operator >= ( optional<T> const& x, optional<T> const& y ) ;
|
|
|
|
template<class T> inline T const& get ( optional<T> const& opt ) ;
|
|
|
|
template<class T> inline T& get ( optional<T> & opt ) ;
|
|
|
|
template<class T> inline T const* get ( optional<T> const* opt ) ;
|
|
|
|
template<class T> inline T* get ( optional<T>* opt ) ;
|
|
|
|
template<class T> inline T const* get_pointer ( optional<T> const& opt ) ;
|
|
|
|
template<class T> inline T* get_pointer ( optional<T> & opt ) ;
|
|
|
|
template<class T> inline void swap( optional<T>& x, optional<T>& y ) ;
|
|
|
|
} // namespace boost
|
|
</PRE>
|
|
|
|
<HR>
|
|
|
|
<h2><A NAME="semantics">Detailed Semantics</a></h2>
|
|
|
|
<p><b><u>NOTES: </u></b></p>
|
|
|
|
<p><b>Because T might be of reference type, in the sequel, those entries whose
|
|
semantic depends on T being of reference type or not will be distinguished using
|
|
the following convention:<br>
|
|
If the entry reads: optional<T (not a ref)>, the description corresponds only to
|
|
the case where T is not of reference type.<br>
|
|
If the entry reads: optional<T&>, the description corresponds only to the case
|
|
where T is of reference type. <br>
|
|
If the entry reads: optional<T>, the description is the same for both cases.</b></p>
|
|
|
|
<p><i>The following section contains various assert() which are used only to
|
|
show the postconditions as sample code. It is not implied that the type T must
|
|
support each particular expression but that if the expression is supported, the
|
|
implied condition holds.</i></p>
|
|
|
|
<hr>
|
|
|
|
<pre>optional<T>::optional();</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Default-Constructs an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> <b>*this</b> is <u>uninitialized</u>.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> T's default constructor <u><i>is not</i></u> called.</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T> def ;
|
|
assert ( !def ) ;</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional<T>::optional( detail::none_t );</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Constructs an <b>optional </b>uninitialized.</p>
|
|
<p><b>Postconditions:</b> <b>*this</b> is <u>uninitialized</u>.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b></p>
|
|
<blockquote>
|
|
<p>T's default constructor <u><i>is not</i></u> called.<br>
|
|
The
|
|
expression <code>boost::none</code> denotes an instance of <code>boost::detail::none_t</code> that can be
|
|
used as the parameter.</p>
|
|
</blockquote>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T> n(none) ;
|
|
assert ( !n ) ;</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional<T <i>(not a ref)</i>>::optional( T const& v )</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Directly-Constructs an <b>optional</b>.</p>
|
|
<!-- TemplateName: general/sy_footer_inc.isml -->
|
|
<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is a <i>copy</i> of 'v'.</p>
|
|
<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
|
|
<p><b>Notes: </b> T::T( T const& ) is called.</p>
|
|
<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
|
|
in that case, this constructor has no effect.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v;
|
|
optional<T> opt(v);
|
|
assert ( *opt == v ) ;</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional<T&>::optional( T ref )</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Directly-Constructs an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is an
|
|
instance of an internal type wrapping the reference 'ref'.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v;
|
|
T& vref = v ;
|
|
optional<T&> opt(vref);
|
|
assert ( *opt == v ) ;
|
|
++ v ; // mutate referee
|
|
assert (*opt == v); </pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional<T <i>(not a ref)</i>>::optional( optional const& rhs );</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of the value of <b>rhs</b>; else <b>*this</b>
|
|
is uninitialized.</p>
|
|
<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
|
|
<p><b>Notes:</b> If <b>rhs</b> is initialized, T::T(T const& ) is called.</p>
|
|
<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
|
|
in that case, this constructor has no effect.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T> uninit ;
|
|
assert (!uninit);
|
|
|
|
optional<T> uinit2 ( uninit ) ;
|
|
assert ( uninit2 == uninit );
|
|
|
|
optional<T> init( T(2) );
|
|
assert ( *init == T(2) ) ;
|
|
|
|
optional<T> init2 ( init ) ;
|
|
assert ( init2 == init ) ;
|
|
</pre>
|
|
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional<T&>::optional( optional const& rhs );</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of the internal wrapper holding the references in <b>rhs</b>; else <b>*this</b>
|
|
is uninitialized.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> If <b>rhs</b> is initialized, the internal wrapper will be
|
|
copied and just like true references, both <b>*this</b> and <b>rhs</b> will
|
|
referr to the same object<b> </b>(will alias).</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T&> uninit ;
|
|
assert (!uninit);
|
|
|
|
optional<T&> uinit2 ( uninit ) ;
|
|
assert ( uninit2 == uninit );
|
|
|
|
T v = 2 ; T& ref = v ;
|
|
optional<T> init(ref);
|
|
assert ( *init == v ) ;
|
|
|
|
optional<T> init2 ( init ) ;
|
|
assert ( *init2 == v ) ;
|
|
</pre>
|
|
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>template<U> explicit optional<T <i>(not a ref)</i>>::optional( optional<U> const& rhs );</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of the value of <b>rhs</b> <i>converted</i>
|
|
to type T; else <b>*this</b> is uninitialized.
|
|
</p>
|
|
<p><b>Throws:</b> Whatever T::T( U const& ) throws.</p>
|
|
<p><b>Notes:</b> T::T( U const& ) is called if <b>rhs</b> is initialized, which requires
|
|
a valid conversion from U to T.
|
|
</p>
|
|
<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( U const& );
|
|
in that case, this constructor has no effect.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
|
|
<pre>optional<double> x(123.4);
|
|
assert ( *x == 123.4 ) ;
|
|
|
|
optional<int> y(x) ;
|
|
assert( *y == 123 ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>template<<i>InPlaceFactory</i>> explicit optional<T <i>(not a ref)</i>>::optional( <i>InPlaceFactory</i> const& f );</pre>
|
|
|
|
<pre>template<<i>TypedInPlaceFactory</i>> explicit optional<T <i>(not a ref)</i>>::optional( <i>TypedInPlaceFactory</i> const& f );</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Constructs an <b>optional</b> with a value of T obtained from
|
|
the factory.</p>
|
|
<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is
|
|
<i>directly given</i> from the factory 'f' (i.e, the value<u> is not copied</u>).</p>
|
|
<p><b>Throws:</b> Whatever the T constructor called by the factory throws.</p>
|
|
<p><b>Notes:</b> See <A HREF="#inplace">In-Place Factories</A></p>
|
|
<p><b>Exception Safety:</b> Exceptions can only be thrown during the call to the
|
|
T constructor used by the factory;
|
|
in that case, this constructor has no effect.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
|
|
<pre>class C { C ( char, double, std::string ) ; } ;
|
|
|
|
C v('A',123.4,"hello");
|
|
|
|
optional<C> x( in_place ('A', 123.4, "hello") ); // InPlaceFactory used
|
|
optional<C> y( in_place<C>('A', 123.4, "hello") ); // TypedInPlaceFactory used
|
|
|
|
assert ( *x == v ) ;
|
|
assert ( *y == v ) ;
|
|
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional& optional<T <i>(not a ref)</i>>::operator= ( T const& rhs ) ;</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Assigns the value 'rhs' to an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of <b>rhs.</b></p>
|
|
<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
|
|
<p><b>Notes:</b> If <b>*this</b> was initialized, it is first reset to uninitialized
|
|
using T::~T(), then T::T(<b>rhs</b>) is called.</p>
|
|
<p><b>Exception Safety:</b> <u>Basic:</u> Exceptions can only be thrown during T::T( T const& );
|
|
in that case, <b>*this</b> is left <u>uninitialized</u>.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T x;
|
|
optional<T> opt(x) ;
|
|
|
|
T y;
|
|
opt = y ;
|
|
assert ( *opt == y ) ;
|
|
// previous value (copy of 'v') destroyed from within 'opt'.
|
|
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>optional& optional<T <i>(not a ref)</i>>::operator= ( optional const& rhs ) ;</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Assigns another <b>optional</b> to an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of the value of <b>rhs</b>; else <b>*this</b>
|
|
is uninitialized.
|
|
</p>
|
|
<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
|
|
<p><b>Notes:</b> If <b>*this</b> was initialized, it is first reset to uninitialized
|
|
using T::~T(), then T::T( T const& ) is called if <b>rhs</b> is initialized.
|
|
</p>
|
|
<p><b>Exception Safety:</b> <u>Basic:</u> Exceptions can only be thrown during T::T( T const& );
|
|
in that case, <b>*this</b> is left <u>uninitialized</u>.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v;
|
|
optional<T> opt(v);
|
|
optional<T> uninit ;
|
|
|
|
opt = uninit ;
|
|
assert ( !opt ) ;
|
|
// previous value (copy of 'v') destroyed from within 'opt'.
|
|
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>template<U> optional& optional<T <i>(not a ref)</i>>::operator= ( optional<U> const& rhs ) ;</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Assigns another <i>convertible</i> <b>optional</b> to an <b>optional</b>.</p>
|
|
<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
|
|
and its value is a <i>copy</i> of the value of <b>rhs</b> <i>converted</i>
|
|
to type T; else <b>*this</b> is uninitialized.
|
|
</p>
|
|
<p><b>Throws:</b> Whatever T::T( U const& ) throws.</p>
|
|
<p><b>Notes:</b> If <b>*this</b> was initialized, it is first reset to uninitialized
|
|
using T::~T(), then T::T( U const& ) is called if <b>rhs</b> is initialized,
|
|
which requires a valid conversion from U to T.
|
|
</p>
|
|
<p><b>Exception Safety:</b> <u>Basic:</u> Exceptions can only be thrown during T::T( U const& );
|
|
in that case, <b>*this</b> is left <u>uninitialized</u>.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v;
|
|
optional<T> opt0(v);
|
|
optional<U> opt1;
|
|
|
|
opt1 = opt0 ;
|
|
assert ( *opt1 == static_cast<U>(v) ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>void optional<T <i>(not a ref)</i>>::reset( T const& v ) ;</pre>
|
|
<blockquote>
|
|
<p><b>Deprecated:</b> same as operator= ( T const& v) ;</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>void optional<T>::reset() ;</pre>
|
|
<blockquote>
|
|
<p><b>Deprecated: </b>Same as operator=( detail::none_t );</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>T const& optional<T <i>(not a ref)</i>>::operator*() const ;
|
|
T& optional<T<i> (not a ref)</i>>::operator*();</pre>
|
|
|
|
<pre>T const& optional<T <i>(not a ref)</i>>::get() const ;
|
|
T& optional<T <i>(not a ref)</i>>::get() ;
|
|
|
|
inline T const& get ( optional<T<i> (not a ref)</i>> const& ) ;
|
|
inline T& get ( optional<T <i>(not a ref)</i>> &) ;
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Requirements: *this</b> is initialized</p>
|
|
<p><b>Returns:</b> A reference to the contained value</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v ;
|
|
optional<T> opt ( v );
|
|
T const& u = *opt;
|
|
assert ( u == v ) ;
|
|
T w ;
|
|
*opt = w ;
|
|
assert ( *opt == w ) ;
|
|
</pre>
|
|
</blockquote>
|
|
<pre></pre>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>T const& optional<T&>::operator*() const ;
|
|
T & optional<T<i>&</i>>::operator*();</pre>
|
|
|
|
<pre>T const& optional<T&>::get() const ;
|
|
T& optional<T&>::get() ;
|
|
|
|
inline T const& get ( optional<T<i>&</i>> const& ) ;
|
|
inline T& get ( optional<T&> &) ;
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Requirements: *this</b> is initialized</p>
|
|
<p><b>Returns:</b> <u>The</u> reference contained.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v ;
|
|
T& vref = v ;
|
|
optional<T&> opt ( vref );
|
|
T const& vref2 = *opt;
|
|
assert ( vref2 == v ) ;
|
|
++ v ;
|
|
assert ( *opt == v ) ;</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>T const* optional<T <i>(not a ref)</i>>::get_ptr() const ;
|
|
T* optional<T <i>(not a ref)</i>>::get_ptr() ;
|
|
|
|
inline T const* get_pointer ( optional<T <i>(not a ref)</i>> const& ) ;
|
|
inline T* get_pointer ( optional<T <i>(not a ref)</i>> &) ;
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> If <b>*this</b> is initialized, a pointer to the contained
|
|
value; else 0 (<i>null</i>).
|
|
</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> The contained value is permanently stored within *this, so
|
|
you should not hold nor delete this pointer
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T v;
|
|
optional<T> opt(v);
|
|
optional<T> const copt(v);
|
|
T* p = opt.get_ptr() ;
|
|
T const* cp = copt.get_ptr();
|
|
assert ( p == get_pointer(opt) );
|
|
assert ( cp == get_pointer(copt) ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>T const* optional<T <i>(not a ref)</i>>::operator ->() const ;
|
|
T* optional<T <i>(not a ref)</i>>::operator ->() ;
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Requirements: *this</b> is initialized.</p>
|
|
<p><b>Returns:</b> A pointer to the contained value.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> The requirement is asserted via BOOST_ASSERT().</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>struct X { int mdata ; } ;
|
|
X x ;
|
|
optional<X> opt (x);
|
|
opt->mdata = 2 ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>optional<T>::operator <i>unspecified-bool-type</i>() const ;</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> An unspecified value which if used on a boolean context is equivalent to (get() != 0)</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<blockquote>
|
|
<pre>optional<T> def ;
|
|
assert ( def == 0 );
|
|
optional<T> opt ( v ) ;
|
|
assert ( opt );
|
|
assert ( opt != 0 );
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
|
|
<pre> bool optional<T>::operator!() ;</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> If <b>*this</b> is uninitialized, <code>true</code>; else <code>false.</code></p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> This operator is provided for those compilers which can't use
|
|
the <i>unspecified-bool-type</i> operator in certain boolean contexts.
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T> opt ;
|
|
assert ( !opt );
|
|
*opt = some_T ;
|
|
|
|
// Notice the "double-bang" idiom here.
|
|
assert ( !!opt ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>bool optional<T>::is_initialized() const ;</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> <i>true</i> is the <b>optional</b> is initialized, <i>false</i>
|
|
otherwise.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<blockquote>
|
|
<pre>optional<T> def ;
|
|
assert ( !def.is_initialized() );
|
|
optional<T> opt ( v ) ;
|
|
assert ( opt.is_initialized() );</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>bool operator == ( optional<T> const& x, optional<T> const& y );</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> If both <b>x</b> and <b>y</b> are initialied, <code>(*x == *y)</code>.
|
|
If only x or y is initialized, <code>false</code>. If both are uninitialized, <code>true</code>.
|
|
</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> Pointers have shallow relational operators while <b>optional</b> has
|
|
deep relational operators. Do not use operator == directly in generic code
|
|
which expect to be given either an optional<T> or a pointer;
|
|
use <a href="../../utility/OptionalPointee.html#equal">equal_pointees()</a> instead
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T x(12);
|
|
T y(12);
|
|
T z(21);
|
|
optional<T> def0 ;
|
|
optional<T> def1 ;
|
|
optional<T> optX(x);
|
|
optional<T> optY(y);
|
|
optional<T> optZ(z);
|
|
|
|
// Identity always hold
|
|
assert ( def0 == def0 );
|
|
assert ( optX == optX );
|
|
|
|
// Both uninitialized compare equal
|
|
assert ( def0 == def1 );
|
|
|
|
// Only one initialized compare unequal.
|
|
assert ( def0 != optX );
|
|
|
|
// Both initialized compare as (*lhs == *rhs)
|
|
assert ( optX == optY ) ;
|
|
assert ( optX != optZ ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>bool operator < ( optional<T> const& x, optional<T> const& y );</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> If <b>y</b> is not initialized, <code>false</code>.
|
|
If <b>y</b> is initialized and <b>x</b> is not initialized, <code>true</code>.
|
|
If both <b>x</b> and <b>y</b> are initialized, <code>(*x < *y)</code>.
|
|
</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> Pointers have shallow relational operators while <b>optional</b> has
|
|
deep relational operators. Do not use operator < directly in generic code
|
|
which expect to be given either an optional<T> or a pointer;
|
|
use <a href="../../utility/OptionalPointee.html#less">less_pointees()</a> instead
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T x(12);
|
|
T y(34);
|
|
optional<T> def ;
|
|
optional<T> optX(x);
|
|
optional<T> optY(y);
|
|
|
|
// Identity always hold
|
|
assert ( !(def < def) );
|
|
assert ( optX == optX );
|
|
|
|
// Both uninitialized compare equal
|
|
assert ( def0 == def1 );
|
|
|
|
// Only one initialized compare unequal.
|
|
assert ( def0 != optX );
|
|
|
|
// Both initialized compare as (*lhs == *rhs)
|
|
assert ( optX == optY ) ;
|
|
assert ( optX != optZ ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>bool operator != ( optional<T> const& x, optional<T> const& y );
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> !( x == y );</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>bool operator > ( optional<T> const& x, optional<T> const& y );
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> !( y < x );</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>bool operator <= ( optional<T> const& x, optional<T> const& y );
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> !( y<x );</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>bool operator >= ( optional<T> const& x, optional<T> const& y );
|
|
</pre>
|
|
<blockquote>
|
|
<p><b>Returns:</b> !( x<y );</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>void swap ( optional<T>& x, optional<T>& y );</pre>
|
|
|
|
<blockquote>
|
|
<p><b>Effect:</b> If both <b>x</b> and <b>y</b> are initialized, calls <code>swap(*x,*y)</code>
|
|
using std::swap.<br>
|
|
If only one is initialized, say x, calls: <code>y.reset(*x); x.reset();</code><br>
|
|
If none is initialized, does nothing.
|
|
</p>
|
|
<p><b>Postconditions:</b> The states of x and y interchanged.</p>
|
|
<p><b>Throws:</b> If both are initialized, whatever swap(T&,T&) throws.
|
|
If only one is initialized, whatever T::T ( T const& ) throws.
|
|
</p>
|
|
<p><b>Notes:</b> If both are initialized, swap(T&,T&) is used <i>unqualified</i>
|
|
but with std::swap introduced in scope.<br>
|
|
If only one is initialized, T::~T() and T::T( T const& ) is called.
|
|
</p>
|
|
<p><b>Exception Safety:</b> If both are initialized, this operation has the exception
|
|
safety guarantees of swap(T&,T&).<br>
|
|
If only one is initialized, it has the same <b>basic</b> guarantee as optional<T>::reset( T const& ).
|
|
</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>T x(12);
|
|
T y(21);
|
|
optional<T> def0 ;
|
|
optional<T> def1 ;
|
|
optional<T> optX(x);
|
|
optional<T> optY(y);
|
|
|
|
boost::swap(def0,def1); // no-op
|
|
|
|
boost::swap(def0,optX);
|
|
assert ( *def0 == x );
|
|
assert ( !optX );
|
|
|
|
boost::swap(def0,optX); // Get back to original values
|
|
|
|
boost::swap(optX,optY);
|
|
assert ( *optX == y );
|
|
assert ( *optY == x );
|
|
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
<HR>
|
|
|
|
<H2><A NAME="examples">Examples</A></H2>
|
|
|
|
<h3>Optional return values</h3>
|
|
<PRE>optional<char> get_async_input()
|
|
{
|
|
if ( !queue.empty() )
|
|
return optional<char>(queue.top());
|
|
else return optional<char>(); // uninitialized
|
|
}
|
|
|
|
void receive_async_message()
|
|
{
|
|
optional<char> rcv ;
|
|
// The safe boolean conversion from 'rcv' is used here.
|
|
while ( (rcv = get_async_input()) && !timeout() )
|
|
output(*rcv);
|
|
}
|
|
</pre>
|
|
|
|
<h3>Optional local variables</h3>
|
|
<pre>optional<string> name ;
|
|
if ( database.open() )
|
|
{
|
|
name.reset ( database.lookup(employer_name) ) ;
|
|
}
|
|
else
|
|
{
|
|
if ( can_ask_user )
|
|
name.reset ( user.ask(employer_name) ) ;
|
|
}
|
|
|
|
if ( name )
|
|
print(*name);
|
|
else print("employer's name not found!");
|
|
</pre>
|
|
|
|
<h3>Optional data members</h3>
|
|
<pre>class figure
|
|
{
|
|
public:
|
|
|
|
figure()
|
|
{
|
|
// data member 'm_clipping_rect' is uninitialized at this point.
|
|
}
|
|
|
|
void clip_in_rect ( rect const& rect )
|
|
{
|
|
....
|
|
m_clipping_rect.reset ( rect ) ; // initialized here.
|
|
}
|
|
|
|
void draw ( canvas& cvs )
|
|
{
|
|
if ( m_clipping_rect )
|
|
do_clipping(*m_clipping_rect);
|
|
|
|
cvs.drawXXX(..);
|
|
}
|
|
|
|
// this can return NULL.
|
|
rect const* get_clipping_rect() { return get_pointer(m_clipping_rect); }
|
|
|
|
private :
|
|
|
|
optional<rect> m_clipping_rect ;
|
|
|
|
};
|
|
</pre>
|
|
<h3>Bypassing expensive unnecesary default construction</h3>
|
|
<pre>class ExpensiveCtor { ... } ;
|
|
class Fred
|
|
{
|
|
Fred() : mLargeVector(10000) {}
|
|
|
|
std::vector< optional<ExpensiveCtor> > mLargeVector ;
|
|
} ;
|
|
</pre>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="ref">Optional references</A></H2>
|
|
<p>This library allow the template parameter T to be of reference type: T&, and
|
|
to some extent, T const&.</p>
|
|
|
|
<p>However, since references are not real objects some restrictions apply and
|
|
some operations are not available in this case:</p>
|
|
|
|
<ul>
|
|
<li>Converting constructors</li>
|
|
<li>Converting assignment</li>
|
|
<li>InPlace construction</li>
|
|
<li>InPlace assignment</li>
|
|
<li>Value-access via pointer</li>
|
|
</ul>
|
|
<p>Also, even though optional<T&> treats it wrapped pseudo-object much as a real
|
|
value, a true real reference is stored so aliasing will ocurr: </p>
|
|
|
|
<ul>
|
|
<li>Copies of optional<T&> will copy the references but all these references
|
|
will nonetheless refeer to the same object.</li>
|
|
<li>Value-access will actually provide access to the referenced object rather
|
|
than the reference itself.</li>
|
|
</ul>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="inplace">In-Place Factories</A></H2>
|
|
<p>
|
|
One of the typical problems with wrappers and containers is that their
|
|
interfaces usually provide an operation to initialize or assign the contained
|
|
object as a copy of some other object. This not only requires the underlying
|
|
type to be <a href="../../utility/CopyConstructible.html">Copy Constructible</a>, but also requires the existence of a fully
|
|
constructed object, often temporary, just to follow the copy from:</p>
|
|
<pre>struct X
|
|
{
|
|
X ( int, std:::string ) ;
|
|
} ;</pre>
|
|
<pre>class W
|
|
{
|
|
X wrapped_ ;
|
|
|
|
public:
|
|
|
|
W ( X const& x ) : wrapped_(x) {}
|
|
} ;</pre>
|
|
<pre>void foo()
|
|
{
|
|
// Temporary object created.
|
|
W ( X(123,"hello") ) ;
|
|
}
|
|
</pre>
|
|
<p>A solution to this problem is to support direct construction of the contained
|
|
object right in the container's storage.<br>
|
|
In this shceme, the user only needs to supply the arguments to the constructor
|
|
to use in the wrapped object construction.</p>
|
|
<pre>class W
|
|
{
|
|
X wrapped_ ;
|
|
|
|
public:
|
|
|
|
W ( X const& x ) : wrapped_(x) {}
|
|
W ( int a0, std::string a1) : wrapped_(a0,a1) {}
|
|
} ;</pre>
|
|
<pre>void foo()
|
|
{
|
|
// Wrapped object constructed in-place
|
|
// No temporary created.
|
|
W (123,"hello") ;
|
|
}
|
|
</pre>
|
|
<p>A limitation of this method is that it doesn't scale well to wrapped objects with multiple
|
|
constructors nor to generic code were the constructor overloads are unknown.</p>
|
|
<p>The solution presented in this library is the familiy of <b>InPlaceFactories</b> and
|
|
<b>TypedInPlaceFactories</b>.<br>
|
|
These factories are a family of classes which encapsulate an increasing number of arbitrary
|
|
constructor parameters and supply a method to construct an object of a given type using those
|
|
parameters at an address specified by the user via placement new.</p>
|
|
<p> For example, one member of this familiy looks like:</p>
|
|
<pre>template<class T,class A0, class A1>
|
|
class TypedInPlaceFactory2
|
|
{
|
|
A0 m_a0 ; A1 m_a1 ;
|
|
|
|
public:
|
|
|
|
TypedInPlaceFactory2( A0 const& a0, A1 const& a1 ) : m_a0(a0), m_a1(a1) {}
|
|
|
|
void construct ( void* p ) { new (p) T(m_a0,m_a1) ; }
|
|
} ;
|
|
</pre>
|
|
<p>A wrapper class aware of this can use it as:</p>
|
|
<pre>class W
|
|
{
|
|
X wrapped_ ;
|
|
|
|
public:
|
|
|
|
W ( X const& x ) : wrapped_(x) {}
|
|
W ( TypedInPlaceFactory2 const& fac ) { fac.construct(&wrapped_) ; }
|
|
} ;</pre>
|
|
<pre>void foo()
|
|
{
|
|
// Wrapped object constructed in-place via a TypedInPlaceFactory.
|
|
// No temporary created.
|
|
W ( TypedInPlaceFactory2<X,int,std::string&rt;(123,"hello")) ;
|
|
}
|
|
</pre>
|
|
<p>The factories are divided in two groups:<ul>
|
|
<li><u>TypedInPlaceFactories</u>: those which take the target type as a primary template parameter.</li>
|
|
<li><u>InPlaceFactories</u>: those with a template <code>construct(void*)</code> member function taking the target type.</li>
|
|
</ul>
|
|
<p>Within each group, all the family members differ only in the number of parameters allowed.</p>
|
|
<p></p>
|
|
<p>This library provides an overloaded set of helper template functions to construct these factories
|
|
without requiring unnecessary template parameters:</p>
|
|
<pre>template<class A0,...,class AN>
|
|
InPlaceFactory<i>N </i><A0,...,AN> <b>in_place</b> ( A0 const& a0, ..., AN const& aN) ;
|
|
|
|
template<class T,class A0,...,class AN>
|
|
TypedInPlaceFactory<i>N </i><T,A0,...,AN> <b>in_place</b> ( T const& a0, A0 const& a0, ..., AN const& aN) ;</pre>
|
|
|
|
<p>In-place factories can be used generically by the wrapper and user as follows:</p>
|
|
<pre>class W
|
|
{
|
|
X wrapped_ ;
|
|
|
|
public:
|
|
|
|
W ( X const& x ) : wrapped_(x) {}
|
|
|
|
template<class InPlaceFactory></class>
|
|
W ( InPlaceFactory const& fac ) { fac.template <X>construct(&wrapped_) ; }
|
|
|
|
} ;</pre>
|
|
<pre>void foo()
|
|
{
|
|
// Wrapped object constructed in-place via a InPlaceFactory.
|
|
// No temporary created.
|
|
W ( in_place(123,"hello") ) ;
|
|
}
|
|
</pre>
|
|
<p>The factories are implemented in the headers:
|
|
<a href="../../../boost/detail/in_place_factory.hpp">in_place_factory.hpp</a> and
|
|
<a href="../../../boost/detail/typed_in_place_factory.hpp">typed_in_place_factory.hpp</a>
|
|
</p>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="bool">A note about optional<bool></A></H2>
|
|
<p><code>optional<bool></code> should be used with special caution and consideration.</p>
|
|
<p>First, it is functionally similar to a tristate boolean (false,maybe,true) —such as <u>boost::tribool</u> (not yet formally in boost)—except that in a tristate boolean,
|
|
the <i>maybe</i> state <u>represents a valid value</u>, unlike the corresponding state
|
|
of an uninitialized optional<bool>.<br>
|
|
It should be carefully considered if an optional bool instead of a tribool is really needed</p>
|
|
<p>Second, optional<> provides an implicit conversion to bool. This conversion
|
|
refers to the initialization state and not to the contained value.<br>
|
|
Using optional<bool> can lead to subtle errors due to the implicit bool conversion:</p>
|
|
<pre>void foo ( bool v ) ;
|
|
void bar()
|
|
{
|
|
optional<bool> v = try();
|
|
|
|
// The following intended to pass the <b>value</b> of 'v' to foo():
|
|
foo(v);
|
|
// But instead, the <i>initialization state</i> is passed
|
|
// due to a typo: it should have been foo(<b>*</b>v).
|
|
}
|
|
</pre>
|
|
<p>The only implicit conversion is to bool, and it is <i>safe</i> in the sense that typical
|
|
integral promotions don't apply (i.e. if foo() takes an 'int' instead, it won't compile). <HR>
|
|
|
|
<H2><A NAME="exsafety">Exception Safety Guarantees</A></H2>
|
|
<H3><u>Assignment and Reset:</u></H3>
|
|
<p>Because of the current implementation (see <A HREF="#impl">Implementation Notes</A>), all
|
|
of the assignment methods:</p>
|
|
<ul>
|
|
<li> <code>optional<T>::operator= ( optional<T> const& ) </code>
|
|
</li>
|
|
<li> <code>optional<T>::operator= ( T const& ) </code></li>
|
|
<li> <code>template<class U> optional<T>::operator= ( optional<U> const& ) </code>
|
|
</li>
|
|
<li> <code>template<class InPlaceFactory> optional<T>::operator= (
|
|
InPlaceFactory const& ) </code></li>
|
|
<li> <code>template<class TypedInPlaceFactory> optional<T>::operator= (
|
|
TypedInPlaceFactory const& ) </code></li>
|
|
<li> <code>optional<T>:::reset ( T const&)</code></li>
|
|
</ul>
|
|
<p>Can only <i>guarantee</i> the <u>basic exception safety</u>: The lvalue optional is left <u>uninitialized</u>
|
|
if an exception is thrown (any previous value is <i>first</i> destroyed using T::~T())</p>
|
|
<p>On the other hand, the <i>uninitializing</i> methods:</p>
|
|
<ul>
|
|
<li><code>optional<T>::operator= ( detail::none_t ) </code></li>
|
|
<li><code>optional<T>::reset()</code></li>
|
|
</ul>
|
|
<p>Provide the no-throw guarantee (assuming a no-throw T::~T())</p>
|
|
<p>However, since <code>optional<></code> itself doesn't throw any exceptions,
|
|
the only source for exceptions here are T's constructor, so if you know the exception guarantees
|
|
for T::T ( T const& ), you know that optional's assignment and reset has the same guarantees.</p>
|
|
<pre>//
|
|
// Case 1: Exception thrown during assignment.
|
|
//
|
|
T v0(123);
|
|
optional<T> opt0(v0);
|
|
try
|
|
{
|
|
T v1(456);
|
|
optional<T> opt1(v1);
|
|
opt0 = opt1 ;
|
|
|
|
// If no exception was thrown, assignment succeeded.
|
|
assert( *opt0 == v1 ) ;
|
|
}
|
|
catch(...)
|
|
{
|
|
// If any exception was thrown, 'opt0' is reset to uninitialized.
|
|
assert( !opt0 ) ;
|
|
}
|
|
|
|
//
|
|
// Case 2: Exception thrown during reset(v)
|
|
//
|
|
T v0(123);
|
|
optional<T> opt(v0);
|
|
try
|
|
{
|
|
T v1(456);
|
|
opt.reset ( v1 ) ;
|
|
|
|
// If no exception was thrown, reset succeeded.
|
|
assert( *opt == v1 ) ;
|
|
}
|
|
catch(...)
|
|
{
|
|
// If any exception was thrown, 'opt' is reset to uninitialized.
|
|
assert( !opt ) ;
|
|
}
|
|
</pre>
|
|
<H3><u>Swap:</u></H3>
|
|
<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>
|
|
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>
|
|
If none of the optionals is initialized, it has no-throw guarantee since it is a no-op. </p>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="requirements">Type requirements</A></H2>
|
|
<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
|
|
if InPlaceFactories are used.<br>
|
|
T <u>is not</u> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html">Default Constructible</a> </p>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="impl">Implementation Notes</A></H2>
|
|
<p>optional<T> is currently implemented
|
|
using a custom aligned storage facility built from <code>alignment_of</code> and <code>type_with_alignment</code> (both from Type Traits).
|
|
It uses a separate boolean flag to indicate the initialization state.<br>
|
|
Placement new with T's copy constructor and T's destructor
|
|
are explicitly used to initialize,copy and destroy optional values.<br>
|
|
As a result, T's default constructor is effectively by-passed, but the exception
|
|
guarantees are basic.<br>
|
|
It is planned to replace the current implementation with another with
|
|
stronger exception safety, such as a future boost::variant<T,nil_t>. </p>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="porta">Dependencies and Portability</A></H2>
|
|
|
|
<p>The implementation uses <code>type_traits/alignment_of.hpp</code> and <code>type_traits/type_with_alignment.hpp</code></p>
|
|
|
|
<HR>
|
|
|
|
<H2><A NAME="credits">Acknowledgments</A></H2>
|
|
<p>Pre-formal review:</p>
|
|
<blockquote>
|
|
<p>Peter Dimov suggested the name 'optional', and was the first to point out the
|
|
need for aligned storage<br>
|
|
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman coded
|
|
'aligned_storage', which are the core of the optional class implementation.<br>
|
|
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
|
and their work influenced the current implementation.<br>
|
|
Gennadiy Rozental made extensive and important comments which shaped the design.<br>
|
|
Vesa Karvonen and Douglas Gregor made quite useful comparisons between optional,
|
|
variant and any; and made other relevant comments. Douglas Gregor and Peter
|
|
Dimov commented on comparisons and evaluation in boolean contexts.<br>
|
|
Eric Friedman helped understand the issues involved with aligned storage, move/copy
|
|
operations and exception safety.<br>
|
|
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
|
Henney, David Abrahams, and others I can't recall. </p>
|
|
</blockquote>
|
|
<p>Post-formal review:</p>
|
|
<blockquote>
|
|
<p>William Kempf carefully considered the originally proposed interface and
|
|
suggested the new interface which is currently used. He also started and fueled
|
|
the discussion about the analogy optional<>/smart pointer and about
|
|
relational operators.<br>
|
|
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson focused
|
|
on the relational semantics of optional (originally undefined); concluding
|
|
with the fact that the pointer-like interface doesn't make it a pointer so
|
|
it shall have deep relational operators.<br>
|
|
Augustus Saunders also explored the different relational semantics between
|
|
optional<> and a pointer and developed the OptionalPointee concept as
|
|
an aid against potential conflicts on generic code.<br>
|
|
Joel de Guzman noticed that optional<> can be seen as an API on top
|
|
of variant<T,nil_t>.<br>
|
|
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
|
the Maybe type constructor (analogy originally pointed out by David Sankel).<br>
|
|
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey, Rob
|
|
Stewart, and others.<br>
|
|
Joel de Guzman made the case for the support of references and helped with
|
|
the proper semantics.<br>
|
|
Mat Marcus shown the virtues of a value-oriented interface, influencing the
|
|
current design, and contributed the idea of "none".</p>
|
|
</blockquote>
|
|
<HR>
|
|
|
|
<P>Revised Jannuary 30, 2004</P>
|
|
<P>© Copyright boost.org 2003. Permission to copy, use, modify, sell and
|
|
distribute this document is granted provided this copyright notice appears in
|
|
all copies. This document is provided "as is" without express or implied
|
|
warranty, and with no claim as to its suitability for any purpose.</P>
|
|
<P>Developed by <A HREF="mailto:fernando_cacciola@hotmail.com">Fernando Cacciola</A>,
|
|
the latest version of this file can be found at <A
|
|
HREF="http://www.boost.org">www.boost.org</A>, and the boost
|
|
<A HREF="http://www.boost.org/more/mailing_lists.htm#main">discussion lists</A></P>
|
|
</BODY>
|
|
</HTML> |