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<HTML>
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<HEAD>
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<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
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<LINK REL="stylesheet" TYPE="text/css" HREF="../../../boost.css">
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<TITLE>Header </TITLE>
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</HEAD>
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<BODY BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#800080">
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<H2><IMG SRC="../../../c++boost.gif" WIDTH="276" HEIGHT="86">Header <<A
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HREF="../../../boost/optional.hpp">boost/optional.hpp</A>> </H2>
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<H2>Contents</H2>
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<DL CLASS="page-index">
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<DT><A HREF="#mot">Motivation</A></DT>
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<DT><A HREF="#dev">Development</A></DT>
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<DT><A HREF="#synopsis">Synopsis</A></DT>
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<DT><A HREF="#semantics">Semantics</A></DT>
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<DT><A HREF="#examples">Examples</A></DT>
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<DT><A HREF="#bool">A note about optional<bool></A></DT>
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<DT><A HREF="#exsafety">Exception Safety Guarantees</A></DT>
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<DT><A HREF="#requirements">Type requirements</A></DT>
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<DT><A HREF="#impl">Implementation Notes</A></DT>
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<DT><A HREF="#porta">Dependencies and Portability</A></DT>
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<DT><A HREF="#credits">Acknowledgment</A></DT>
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</DL>
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<HR>
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<H2><A NAME="mot"></A>Motivation</H2>
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<P>Consider these functions which should return a value but which might not have
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a value to return:</P>
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<pre>(A) double sqrt(double n );
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(B) char get_async_input();
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(C) point polygon::get_any_point_effectively_inside();</pre>
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<P>There are different approaches to the issue of not having a value to return.</P>
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<P>A typical approach is to consider the existence of a valid return value as
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a postcondition, so that if the function cannot compute the value to return,
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it has either undefined behavior (and can use asssert in a debug build)
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or uses a runtime check and throws an exception if the postcondition is violated.
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This is a reasonable choice for example, for function (A), because the
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lack of a proper return value is directly related to an invalid parameter (out
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of domain argument), so it is appropriate to require the callee to supply only
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parameters in a valid domain for execution to continue normally.</P>
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<P>However, function (B), because of its asynchronous nature, does not fail just
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because it can't find a value to return; so it is incorrect to consider
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such a situation an error and assert or throw an exception. This function must
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return, and somehow, must tell the callee that it is not returning a meaningful
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value.</P>
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<P>A similar situation occurs with function (C): it is conceptually an error to
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ask a <i>null-area</i> polygon to return a point inside itself, but in many
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applications, it is just impractical for performance reasons to treat this as
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an error (because detecting that the polygon has no area might be too expensive
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to be required to be tested previously), and either an arbitrary point (typically
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at infinity) is returned, or some efficient way to tell the callee that there
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is no such point is used.</P>
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<P>There are various mechanisms to let functions communicate that the returned
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value is not valid. One such mechanism, which is quite common since it has zero
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or negligible overhead, is to use a special value which is reserved to communicate
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this. Classical examples of such special values are EOF, string::npos, points
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at infinity, etc...</P>
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<P>When those values exist, i.e. the return type can hold all meaningful values
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<i>plus</i> the <i>signal</i> value, this mechanism is quite appropriate and
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well known. Unfortunately, there are cases when such values do not exist. In
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these cases, the usual alternative is either to use a wider type, such as 'int'
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in place of 'char'; or a compound type, such as std::pair<point,bool>.
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</P>
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<P>Returning a std::pair<T,bool>, thus attaching a boolean flag to the result
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which indicates if the result is meaningful, has the advantage that can be turned
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into a consistent idiom since the first element of the pair can be whatever
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the function would conceptually return. For example, the last two functions
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could have the following interface:</P>
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<pre>std::pair<char,bool> get_async_input();
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std::pair<point,bool> polygon::get_any_point_effectively_inside();</pre>
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<p>These functions use a consistent interface for dealing with possibly inexistent
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results:</p>
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<pre>std::pair<point,bool> p = poly.get_any_point_effectively_inside();
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if ( p.second )
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flood_fill(p.first);
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</pre>
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<P>However, not only is this quite a burden syntactically, it is also error
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prone since the user can easily use the function result (first element of the
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pair) without ever checking if it has a valid value.</P>
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<P>Clearly, we need a better idiom.</P>
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<H2><A NAME="dev"></A>Development</H2>
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<h3>The model</h3>
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<P>In C++, we can <i>declare</i> an object (a variable) of type T, and we can give this variable
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an <i>initial value</i> (through an <i>initializer</i>. (c.f. 8.5)).<br>
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When a declaration includes a non-empty initializer (an initial value is given), it is said that
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the object has been <i><b>initialized</b></i>.<br>
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If the declaration uses an empty initializer (no initial value is given),
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and neither default nor value initialization applies, it is said that the object is
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<i><b>uninitialized</b></i>. Its actual value exist but has an
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<i>indeterminate inital value</i> (c.f. 8.5.9).<br>
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<code>optional<T></code> intends to formalize the notion of initialization/no-initialization
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allowing a program to test whether an object has been initialized and stating that access to
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the value of an uninitialized object is undefined behaviour. That is,
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when a variable is declared as optional<T> and no initial value is given,
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the variable is formally uninitialized. A formally uninitialized optional object has conceptually
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no value at all and this situation can be tested at runtime. It is formally <i>undefined behaviour</i>
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to try to access the value of an uninitialized optional. An uninitialized optional can be <i>assigned</i> a value, in which case its
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initialization state changes to initialized. And given the formal treatment of initialization
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states in optional objects, it is even possible to
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reset an optional to <i>uninitialized</i>.</P>
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<P>In C++ there is no formal notion of uninitialized objects, which
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means that objects always have an initial value even if indeterminate.<br>
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As discussed on the previous section, this has a drawback because you need additional
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information to tell if an object has been effectively initialized.<br>
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One of the typical ways in which this has been historically
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dealt with is via a special value: EOF,npos,-1, etc... This is equivalent to adding
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the special value to the set of possible values of a given type.
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On modern languages, this can be modeled with a <b>discriminated
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union</b> of T and something else such as a trivial POD or enum.
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Discriminated unions are often called <i>variants</i>.
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A variant has a <i>current type</i>, which in our case is either T or something else. In C++,
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such a variant would be typically implemented as a template class of the form: <code>variant<T,nil_t></code>
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</P>
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<P>There is precedence for a discriminated union as a model for an optional
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value: the <a href="http://www.haskell.org/"><u>Haskell</u></a> <b>Maybe</b> builtin type constructor.</p>
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<p>A discriminated union, which can be seen as a <b>container</b> which has an object of either
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type T or something else, has <i>exactly</i> the semantics required for a wrapper of optional values:</p>
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<li><b>deep-copy</b> semantics: copies of the variant implies copies of the contained value.</li>
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<li><b>deep-relational</b> semantics: comparisons between variants matches both current types and values</li>
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<li>If the variant's current type is T, it is modeling an <i>initialized</i> optional.</li>
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<li>If the variant's current type is not T, it is modeling an <i>uninitialized</i> optional.</li>
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<li>Testing if the variant's current type is T models testing if the optional is initialized</li>
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<li>Trying to extract a T from a variant when its current type is not T, models the undefined behaviour
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of trying to access the value of an uninitialized optional</li>
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<P>Because of the way a discriminated union is used for this purpose, it only matters
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whether its current type is T or not. We can put a layer on top of the variant hidding the other type
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transforming a container of fixed size 1 into a variable size container which either has
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a T or has nothing. Thus, the variant<T,nil_t> can be seen as if it were a variable-size
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fixed-capacity stack-based container with the following optional-oriented interface:</P>
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<pre>// Uninitialized (internally, current type is nil_t)
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optional<T>::optional();
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// Initialized with 'v' (internally, current type is T)
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optional<T>::optional( T const& v ) ;
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// Back to uninitialized (current type is set to nil_t)
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void optional<T>::reset();
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// Assigns 'v' whether previously initialized or not (current type is set to T)
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void optional<T>::reset( T const& v ) ;
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// Returns 'true' if the optional is initialized, 'false' otherwise.
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bool optional<T>::initialized() ;
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// If the optional is initialized (current type is T), returns a reference to its value.
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// Otherwise (current type is nil_t), the result is undefined.
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T const& optional<T>::ref() const ;
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T& optional<T>::ref() ;
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// If both are initialized, calls swap(T&,T&);
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// If only one is initialized, calls reset(T const&) and reset().
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// If both are uninitalized, do nothing.
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void swap ( optional<T>& lhs, optional<T>& rhs ) ;
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// If both are initialized, compare values.
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// If only one is initialized, they are not equal.
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// If both are uninitalized, they are equal.
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bool operator == ( optional<T> const& lhs, optional<T> const& rhs ) ;
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bool operator != ( optional<T> const& lhs, optional<T> const& rhs ) ;
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</pre>
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<h3>Pointers and optional objects</h3>
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<P>In C++, unlike many other languages, objects can be referenced <i>indirectly</i>
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by means of a <b>pointer</b> (or a reference). Pointers have several nice features,
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two of which are relevant to this development.</p>
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<p>One is that a pointer has its own <i>pointer value</i>, which in effect
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references the object being pointed to: the <b>pointee</b>. Consequently,
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copies of pointers do not involve copies of pointees. This effect results in <i>aliasing</i>:
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different pointers can refer to the same object.
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The particular semantic that a copy of a pointer does not involve
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a copy of the pointee is called <b>shallow-copy</b>, which is oppossed to <b>deep-copy</b> were
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a copy of a wrapper involves a copy of the wrapped object (as with optional<>)<br>
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Since this is the semantic followed by pointers (and references), shallow-copy
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(and therefore aliasing) is implied in <b>pointer semantics</b>.</p>
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<p>The other relevant feature of a pointer is that a pointer can have a <b>null
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pointer value</b>. This is a <i>special</i> value which is used to indicate that the
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pointer is not referring to any object at all. In other words, null pointer
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values convey the notion of inexistent objects.</P>
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<P>This meaning of the null pointer value allowed pointers to became a defacto standard
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for handling optional objects because all you have to do to refer to a value which you
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don't really have is to use a null pointer value of the appropriate type.
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Pointers have been used for decades—from the days of C APIs to modern C++ libraries—to
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<i>refer</i> to optional (that is, possibly inexistent) objects; particularly
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as optional arguments to a function, but also quite often as optional data members.</P>
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<P>The possible presence of a null pointer value makes the operations that access the
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pointee's value possibly undefined, therefore, expressions which use dereference
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and access operators, such as: <code>( *p = 2 )</code> and <code>( p->foo())</code>,
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implicitly convey the notion of optionality, and this information is tied to
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the <i>syntax</i> of the expressions. That is, the presence of operators * and -> tell by
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themselves—without any additional context—that the expression will be undefined unless
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the implied pointee actually exist.<br>
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Furthermore, the existence of the pointee can be tested by a comparison against
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the null pointer value or via a conversion to bool, which allows expressions of
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the form: if ( p != 0 ), or if ( p ) to be used to test for the existence of the pointee.</P>
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<p>Such a defacto idiom for referring to optional objects can be formalized in the form of a
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concept: the <a href="../../utility/OptionalPointee.html">OptionalPointee</a> concept.<br>
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This concept captures the syntactic usage of operatos *, -> and conversion to bool to convey
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the notion of optionality.</p>
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<P>However, pointers are good to <u>refer</u> to optional objects, but not particularly good
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to handle the optional objects in all other respects, such as initializing or moving/copying
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them. The problem resides in the shallow-copy of pointer semantics: if you need to
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effectively move or copy the object, pointers alone are not enough. The problem
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is that copies of pointers do not imply copies of pointees. For example, as
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was discussed in the motivation, pointers alone cannot be used to return optional
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objects from a function because the object must move outside from the function and
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into the caller's context.<br>
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A solution to the shallow-copy problem that is often used is to resort to dynamic
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allocation and use a smart pointer to automatically handle the details of this.
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For example, if a function is to optionally return an object X, it can use shared_ptr<X>
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as the return value. However, this requires dynamic allocation of X. If X is
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a builtin or small POD, this technique is very poor in terms of required resources.
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Optional objects are essentially values so it is very convenient to be able to use automatic
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storage and deep-copy semantics to manipulate optional values just as we do with ordinary
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values. Pointers do not have this semantics, so are unappropriate for the initialization and
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transport of optional values, yet are quite convenient for handling the access to the
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possible undefined value because of the idiomatic aid present in the OptionalPointee
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concept incarnated by pointers. <br>
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Therefore, the final solution which is presented in this library is to shape the
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previously discussed optional—which is a value-based container—as a model
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of the OptionalPointee concept.
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</p>
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<h3>Optional<T> as a model of OptionalPointee</h3>
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<P>The optional<> template class presented with this library is a variation of the
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sketch shown before (as a layer on top of a variant). It features <b>deep-copy</b> and
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<b>deep relational operators</b>, but also models the OptionalPointee concept.
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Instead of the member function 'initialized()' it has a safe conversion to bool,
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and instead of the 'ref()' member function, it has operators*() and ->().<br>
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However, it is particularly important that optional<> objects are not mistaken by pointers,
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they are not. <u><b>optional<> does not model a pointer</b></u>.
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For instance, optional<> has not shallow-copy so does not alias: two different optionals
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never refer to the <i>same</i> value (but my have <i>equivalent</i> values).<br>
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The difference between an optional<T> and a pointer must be kept in mind, particularly
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because the semantics of relational operators are different: since optional<T>
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is a value-wrapper, relational operators are deep: they compare optional values;
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but relational operators for pointers are shallow: they do not compare pointee values.<br>
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As a result, you might be able to replace optional<T> by T* on some situations but
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not always. Specifically, on generic code written for both, you cannot use relational
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operators directly, and must use the template function
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<a href="../../utility/OptionalPointee.html#equal">equal_pointees()</a> instead.
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<HR>
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<H2><A NAME="synopsis">Synopsis</A></H2>
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<PRE>namespace boost {
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template<class T>
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class optional
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{
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public :
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optional () ;
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explicit optional ( T const& v ) ;
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optional ( optional const& rhs ) ;
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template<class U> explicit optional ( optional<U> const& rhs ) ;
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optional& operator = ( optional const& rhs ) ;
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template<class U> optional& operator = ( optional<U> const& rhs ) ;
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T const* get() const ;
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T* get() ;
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T const* operator ->() const ;
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T* operator ->() ;
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T const& operator *() const ;
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T& operator *() ;
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void reset();
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void reset ( T const& ) ;
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operator <i>unspecified-bool-type</i>() const ;
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bool operator!() const ;
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} ;
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template<class T> inline bool operator == ( optional<T> const& x, optional<T> const& y ) ;
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template<class T> inline bool operator != ( optional<T> const& x, optional<T> const& y ) ;
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template<class T> inline T* get_pointer ( optional<T> const& opt ) ;
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template<class T> inline void swap( optional<T>& x, optional<T>& y ) ;
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} // namespace boost
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</PRE>
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<HR>
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<h2><A NAME="semantics">Semantics</a></h2>
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<p><i>Note: the following section contains various assert() which are used only to
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show the postconditions as sample code.
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It is not implied that the type T must support each particular expression
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but that if the expression is supported, the implied condition holds.</i></p>
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<hr>
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<pre>optional<T>::optional();</pre>
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<blockquote>
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<p><b>Effect:</b> Default-Constructs an <b>optional</b>.</p>
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<p><b>Postconditions:</b> <b>*this</b> is <u>uninitialized</u>.</p>
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<p><b>Throws:</b> Nothing.</p>
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<p><b>Notes:</b> T's default constructor <u><i>is not</i></u> called.</p>
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<p><b>Example:</b></p>
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<blockquote>
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<pre>optional<T> def ;
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assert ( !def ) ;</pre>
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</blockquote>
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</blockquote>
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<HR>
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<pre>explicit optional<T>::optional( T const& v )</pre>
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<blockquote>
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<p><b>Effect:</b> Directly-Constructs an <b>optional</b>.</p>
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<p><b>Postconditions:</b> <b>*this</b> is <u>initialized</u> and its value is a <i>copy</i> of 'v'.</p>
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<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
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<p><b>Notes:</b> T::T( T const& ) is called.</p>
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<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
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in that case, this constructor has no effect.
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</p>
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<p><b>Example:</b></p>
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<blockquote>
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<pre>
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T v;
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optional<T> opt(v);
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assert ( *opt == v ) ;
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</pre>
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</blockquote>
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</blockquote>
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<HR>
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<pre>optional<T>::optional( optional const& rhs );</pre>
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<blockquote>
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<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
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<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
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and its value is a <i>copy</i> of the value of <b>rhs</b>; else <b>*this</b>
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is uninitialized.</p>
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<p><b>Throws:</b> Whatever T::T( T const& ) throws.</p>
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<p><b>Notes:</b> T::T( T const& ) is called if <b>rhs</b> is initialized.</p>
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<p><b>Exception Safety:</b> Exceptions can only be thrown during T::T( T const& );
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in that case, this constructor has no effect.
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</p>
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<p><b>Example:</b></p>
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<blockquote>
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<pre>optional<T> uninit ;
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assert (!uninit);
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optional<T> uinit2 ( uninit ) ;
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assert ( uninit2 == uninit );
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optional<T> init( T(2) );
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assert ( *init == T(2) ) ;
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optional<T> init2 ( init ) ;
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assert ( init2 == init ) ;
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</pre>
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</blockquote>
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</blockquote>
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<HR>
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<pre>explicit template<U> optional<T>::optional( optional<U> const& rhs );</pre>
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<blockquote>
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<p><b>Effect:</b> Copy-Constructs an <b>optional</b>.</p>
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<p><b>Postconditions:</b> If <b>rhs</b> is initialized, <b>*this</b> is initialized
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and its value is a <i>copy</i> of the value of <b>rhs</b> <i>converted</i>
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to type T; else <b>*this</b> is uninitialized.
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</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>optional& optional<T>::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>::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>::reset( T const& v ) ;</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Resets the current value.</p>
|
|
<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> If <b>*this</b> was initialized, it is first reset to uninitialized
|
|
using T::~T(), then T::T( T const& ) 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>optional<T> opt ( some_T ) ;
|
|
assert( *opt == some_T );
|
|
opt.reset ( some_other_T ) ;
|
|
assert( *opt == some_other_T );
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
<pre>void optional<T>::reset() ;</pre>
|
|
<blockquote>
|
|
<p><b>Effect:</b> Destroys the current value.</p>
|
|
<p><b>Postconditions: *this</b> is uninitialized.</p>
|
|
<p><b>Throws:</b> Nothing.</p>
|
|
<p><b>Notes:</b> T::~T() is called.</p>
|
|
<p><b>Example:</b></p>
|
|
<blockquote>
|
|
<pre>optional<T> opt ( some_T ) ;
|
|
assert( *opt == some_T );
|
|
opt.reset();
|
|
assert( !opt );
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
<HR>
|
|
|
|
<pre>
|
|
T const* optional<T>::get() const ;
|
|
T* optional<T>::get() ;
|
|
|
|
inline T const* get_pointer ( optional<T> const& ) ;
|
|
inline T* get_pointer ( optional<T>&) ;
|
|
</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() ;
|
|
T const* cp = copt.get();
|
|
assert ( p == get_pointer(opt) );
|
|
assert ( cp == get_pointer(copt) ) ;
|
|
</pre>
|
|
</blockquote>
|
|
</blockquote>
|
|
|
|
|
|
<HR>
|
|
|
|
|
|
<pre>
|
|
T const* optional<T>::operator ->() const ;
|
|
T* optional<T>::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>T const& optional<T>::operator*() const ;
|
|
T& optional<T>::operator*();</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>
|
|
</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 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> !( 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 recieve_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="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>),
|
|
<code> optional<T>::operator=( optional<T> const& )
|
|
and optional<T>::reset( T const& )</code>
|
|
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><code>optional<T>::reset()</code> provides 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 is T's copy 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 ) ;
|
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}
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</pre>
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<H3><u>Swap:</u></H3>
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<p><code>void swap( optional<T>&, optional<T>& )</code>
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has the same exception guarantee as <code>swap(T&,T&)</code> when both optionals are initialized.<br>
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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>
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|
If none of the optionals is initialized, it has no-throw guarantee since it is a no-op.
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</p>
|
|
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<HR>
|
|
|
|
<H2><A NAME="requirements">Type requirements</A></H2>
|
|
<p>T must be <a href="../../utility/CopyConstructible.html">Copy Constructible</a>
|
|
and have a no-throw destructor.<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>
|
|
<p>It has been tested on bcc5.5.1, vc6.0 and gcc2.95.2</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.
|
|
</p>
|
|
</blockquote>
|
|
<HR>
|
|
|
|
<P>Revised January 20, 2003</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 discussion list at
|
|
<A
|
|
HREF="http://www.yahoogroups.com/list/boost">www.yahoogroups.com/list/boost</A>.
|
|
</P>
|
|
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