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Fixed code, updated docs, added emplace()

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Andrzej Krzemienski
2014-06-03 17:36:18 +02:00
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
@ -39,12 +39,12 @@
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
(c.f. 8.5)). When a declaration includes a non-empty initializer (an initial
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
value is given), it is said that the object has been initialized. If the
declaration uses an empty initializer (no initial value is given), and neither
default nor value initialization applies, it is said that the object is
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
has an <span class="emphasis"><em>indeterminate initial value</em></span> (c.f. 8.5.9). <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> intends
has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> intends
to formalize the notion of initialization (or lack of it) allowing a program
to test whether an object has been initialized and stating that access to
the value of an uninitialized object is undefined behavior. That is, when
@ -67,13 +67,12 @@
special value: <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is equivalent to
adding the special value to the set of possible values of a given type. This
super set of <code class="computeroutput"><span class="identifier">T</span></code> plus some
<span class="emphasis"><em>nil_t</em></span>&#8212;were <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless POD-can be modeled in modern languages as a <span class="bold"><strong>discriminated
union</strong></span> of T and nil_t. Discriminated unions are often called <span class="emphasis"><em>variants</em></span>.
A variant has a <span class="emphasis"><em>current type</em></span>, which in our case is either
<code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a>
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>.
<span class="emphasis"><em>nil_t</em></span>&#8212;where <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless POD&#8212;can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a <span class="emphasis"><em>current
type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code>
or <code class="computeroutput"><span class="identifier">nil_t</span></code>. Using the <a href="../../../../variant/index.html" target="_top">Boost.Variant</a> library, this model
can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">&gt;</span></code>.
There is precedent for a discriminated union as a model for an optional value:
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
@ -329,8 +328,8 @@
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-&gt;</span></code>
and conversion to <code class="computeroutput"><span class="keyword">bool</span></code> to convey
the notion of optionality.
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
to convey the notion of optionality.
</p>
<p>
However, pointers are good to <span class="underline">refer</span>
@ -407,7 +406,7 @@
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