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<H2><IMG SRC="../../c++boost.gif" WIDTH="276" HEIGHT="86">Header &lt;<A
HREF="../../boost/utility/aligned_storage.hpp">boost/utility/value_init.hpp</A>&gt;
</H2>
<H2>Contents</H2>
<DL>
<DT><A HREF="#intro">Introduction</A></DT>
</DL>
<UL>
<LI><A HREF="#valueinit">value-initialization</A></LI>
<LI><A HREF="#valueinitsyn">value-initialization syntax</A></LI>
</UL>
<DL CLASS="page-index">
<DT><A HREF="#types">Types</A></DT>
</DL>
<UL>
<LI><A HREF="#val_init"><CODE>value_initialized&lt;&gt;</CODE></A></LI>
</UL>
<HR>
<H2><A NAME="into"></A>Introduction</H2>
<P>The C++ standard document realeased by 1998 contains the definitions of
<CODE>zero-initialization</CODE> and <CODE>default-initialization</CODE>.
<h2><img src="../../c++boost.gif" width="276" height="86">
Header &lt;<a href="../../boost/utility/value_init.hpp">boost/utility/value_init.hpp</a>&gt;
</h2>
<h2>Contents</h2>
<dl>
<dt><a href="#intro">Rationale</a></dt>
<dt><a href="#rationale">Introduction</a></dt>
</dl>
<ul>
<li><a href="#valueinit">value-initialization</a></li>
<li><a href="#valueinitsyn">value-initialization syntax</a></li>
</ul>
<dl class="page-index">
<dt><a href="#types">Types</a></dt>
</dl>
<ul>
<li><a href="#val_init"><code>value_initialized&lt;&gt;</code></a></li>
</ul>
<a href="#acknowledgements">Acknowledgements</a><br>
<br>
<hr>
<h2><a name="rationale"></a>Rationale</h2>
<p>Constructing and initializing objects in a generic way is difficult in
C++. The problem is that there are several different rules that apply
for initialization. Depending on the type, the value of a newly constructed
object can be zero-initialized (logically 0), default-constructed (using
the default constructor), or indeterminate. When writing generic code,
this problem must be addressed. <code>value_initialized</code> provides
a solution with consistent syntax for value initialization of scalar,
union and class types. <br>
</p>
<h2><a name="into"></a>Introduction</h2>
<p>The C++ standard [<a href="#references">1</a>] contains the definitions
of <code>zero-initialization</code> and <code>default-initialization</code>.
Informally, zero-initialization means that the object is given the initial
value 0 (converted to the type) and default-initialization means that POD types
are zero-initialized while class types are initialized with their corresponding
default constructors. A <I>declaration</I> can contain an <I>initializer</I>,
which specifies the object's initial value. The initializer can be just '()',
which determines that the object shall be default-initialized (but see below).
However, if a <I>declaration</I> has no <I>initializer</I> and it is of a
non-const non-static POD type, the initial value is indeterminate:<CITE>(see
8.5 for the accurate definitions)</CITE></P>
<PRE>int x ; // no initializer. x value is indeterminate.
std::string s ; // no initializer, s is default-constructed.
value 0 (converted to the type) and default-initialization means that
POD [<a href="#references">2</a>] types are zero-initialized, while class
types are initialized with their corresponding default constructors. A
<i>declaration</i> can contain an <i>initializer</i>, which specifies the
object's initial value. The initializer can be just '()', which states that
the object shall be default-initialized (but see below). However, if a <i>declaration</i>
has no <i>initializer</i> and it is of a non-<code>const</code>, non-<code>static</code>
POD type, the initial value is indeterminate:<cite>(see &sect;8.5 for the
accurate definitions).</cite></p>
int y = int() ;
// y is initialized using copy-initialization
// but the temporary uses an empty set of parentheses as the initializer,
// so it is default-constructed.
// A default constructed POD type is zero-initialized,
// therefore, y == 0.
<pre>int x ; // no initializer. x value is indeterminate.<br>std::string s ; // no initializer, s is default-constructed.<br><br>int y = int() ; <br>// y is initialized using copy-initialization<br>// but the temporary uses an empty set of parentheses as the initializer,<br>// so it is default-constructed.<br>// A default constructed POD type is zero-initialized,<br>// therefore, y == 0.<br><br>void foo ( std::string ) ;<br>foo ( std::string() ) ; <br>// the temporary string is default constructed <br>// as indicated by the initializer () </pre>
void foo ( std::string ) ;
foo ( std::string() ) ;
// the temporary string is default constructed
// as indicated by the initializer () </PRE>
<h3><a name="valueinit">value-initialization</a></h3>
<H3><A NAME="valueinit">value-initialization</A></H3>
<P>The first Technical Corrigendum for the C++ Standard (TC1), whose darft was
released to the public on Nov, 2001, introduced Core Issue 178 (among many
other issues, of course).</P>
<P> That issue introduced the new concept of <CODE>value-initialization</CODE>
(it also fixed the wording for zero-initialization). Informally,
value-initialization is similar to default-initialization with the exception
that on some cases non static data members and base class sub-objects are also
value-initialized. The difference is that an object which is value-initialized
won't have (or at least it is less likely to have) indeterminate values for
data members and base class sub-objects; unlike the case of an object default
constructed. (see Core Issue 178 for a normative description)</P>
<P>In order to specify value-initialization of an object we need to use the
empty-set initializer: (). </P>
<P><I>(but recall that the released official Std document says that '()'
invokes default-initialization, not value-initialization as it is now)</I></P>
<P>As before, a declaration with no intializer specifies
default-initialization, and a declaration with a non-empty initializer
specifies copy (=xxx) or direct (xxx) initialization. </P>
<PRE>template&lt;class T&gt; void eat(T);
int x ; // indeterminate initial value.
std::string s; // default-initialized.
eat ( int() ) ; // value-initialized
eat ( std::string() ) ; // value-initialied</PRE>
<p>The first <a
href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/cwg_defects.html">Technical
Corrigendum for the C++ Standard</a> (TC1), whose draft was released to
the public in November 2001, introduced <a
href="http://anubis.dkuug.dk/JTC1/SC22/WG21/docs/cwg_defects.html#178">Core
Issue 178</a> (among many other issues, of course).</p>
<H4><A NAME="valueinitsyn">value-initialization</A> syntax</H4>
<P>Value initialization is specified using (). However, the empty set of
parentheses is not permited by the syntax of the initializer because it is
parsed as the declaration of a function taking no arguments: </P>
<PRE>int x() ; // declares function int(*)()
int y ( int() ) ; // decalares function int(*)( int(*)() )</PRE>
<p> That issue introduced the new concept of <code>value-initialization</code>
(it also fixed the wording for zero-initialization). Informally, value-initialization
is similar to default-initialization with the exception that in some cases
non-static data members and base class sub-objects are also value-initialized.
The difference is that an object that is value-initialized won't have
(or at least is less likely to have) indeterminate values for data members
and base class sub-objects; unlike the case of an object default constructed.
(see Core Issue 178 for a normative description).</p>
<P>Thus, the empty () must be put in some other initialization context.</P>
<P>One alternative is to use copy-initialization syntax:</P>
<PRE>int x = int() ;</PRE>
<p>In order to specify value-initialization of an object we need to use the
empty-set initializer: (). </p>
<P>This works perfectly fine for POD types. But for non-POD class types,
copy-initialization searches for a suitable constructor, which could be, for
instance, the copy-constructor (it also searches for a suitable conversion
sequence but this doesn't apply in our context). For an arbitrary unknown type,
using this syntax may not have the value-initialization effect intended because
we don't know if a copy from a default constructed object is exactly the same
as a default constructed object, and the compiler is allowed (in some cases)
but never required to optimize the copy away.</P>
<P>One possible generic solution is to use value-initialization of a non static
data member:</P>
<PRE>template&lt;class T&gt;
struct W
{
// value-initialization of 'data' here.
W() : data() {}
T data ;
} ;
W&lt;int&gt; w ;
// w.data is value-initialized for any type. </PRE>
<p><i>(but recall that the current C++ Standard states that '()' invokes default-initialization,
not value-initialization)</i></p>
<P>This is the solution supplied by the value_initialized&lt;&gt; template
class.</P>
<H2><A NAME="types"></A>Types</H2>
<H2><A NAME="val_init"><CODE>template class
value_initialized&lt;T&gt;</CODE></A></H2>
<PRE>namespace boost {
<p>As before, a declaration with no intializer specifies default-initialization,
and a declaration with a non-empty initializer specifies copy (=xxx) or
direct (xxx) initialization. </p>
template&lt;class T&gt;
class value_initialized
{
public :
<pre>template&lt;class T&gt; void eat(T);<br>int x ; // indeterminate initial value.<br>std::string s; // default-initialized.<br>eat ( int() ) ; // value-initialized<br>eat ( std::string() ) ; // value-initialied</pre>
value_initialized() : x() {}
<h4><a name="valueinitsyn">value-initialization</a> syntax</h4>
operator T&amp;() const { return x ; }
<p>Value initialization is specified using (). However, the empty set of
parentheses is not permitted by the syntax of initializers because it is
parsed as the declaration of a function taking no arguments: </p>
T&amp; data() const { return x ; }
<pre>int x() ; // declares function int(*)()<br>int y ( int() ) ; // decalares function int(*)( int(*)() )</pre>
private :
<p>Thus, the empty () must be put in some other initialization context.</p>
<I>impll-defined</I> x ;
} ;
<p>One alternative is to use copy-initialization syntax:</p>
template&lt;class T&gt;
T const&amp; get ( value_initialized&lt;T&gt; const&amp; x )
{
return x.data() ;
}
template&lt;class T&gt;
T&amp; get ( value_initialized&lt;T&gt;&amp; x )
{
return x.data() ;
}
<pre>int x = int() ;</pre>
} // namespace boost
</PRE>
<p>This works perfectly fine for POD types. But for non-POD class types,
copy-initialization searches for a suitable constructor, which could be,
for instance, the copy-constructor (it also searches for a suitable conversion
sequence but this doesn't apply in this context). For an arbitrary unknown
type, using this syntax may not have the value-initialization effect intended
because we don't know if a copy from a default constructed object is exactly
the same as a default constructed object, and the compiler is allowed (in
some cases), but never required to, optimize the copy away.</p>
<P>An object of this template class is a T-wrapper convertible to
<CODE>'T&amp;'</CODE> whose wrapped object (data member of type T) is
<A HREF="#valueinit">value-initialized</A> upon default-initialization of this
wrapper class: </P>
<PRE>
int zero = 0 ;
value_initialized&lt;int&gt; x ;
assert ( x == zero ) ;
<p>One possible generic solution is to use value-initialization of a non static
data member:</p>
std::string def ;
value_initialized&lt; std::string &gt; y ;
assert ( y == def ) ;
</PRE>
<pre>template&lt;class T&gt; <br>struct W <br>{<br> // value-initialization of 'data' here.<br> W() : data() {}<br> T data ;<br>} ;<br>W&lt;int&gt; w ;<br>// w.data is value-initialized for any type. </pre>
<P>The purpose of this wrapper is to provide a consistent syntax for value
initialization of scalar, union and class types (POD and non-POD) since the
correct syntax for value initialization varies (see <A
HREF="#valueinitsyn">value-initialization syntax</A>)</P>
<P>The wrapped object can be accessed either through the conversion operator
T&amp;, the member function data(), or the non-member friend function get():
</P>
<PRE>void watch(int);
value_initialized&lt;int&gt; x;
<p><code>This is the solution supplied by the value_initialized&lt;&gt; template
class.</code></p>
watch(x) ; // operator T&amp; used.
watch(x.data());
watch( get(x) ) // friend function get() used</PRE>
<h2><a name="types"></a>Types</h2>
<P>Both <CODE>const and non-const</CODE> objects can be wrapped. Non-constant
objects can be modified directly from within the wrapper but constant objects
cannot:</P>
<PRE>value_initialized&lt;int&gt; x ;
static_cast&lt;int&amp;&gt;(x) = 1 ; // OK
get(x) = 1 ; // OK
<h2><a name="val_init"><code>template class value_initialized&lt;T&gt;</code></a></h2>
value_initialized&lt;int const&gt; y ;
static_cast&lt;int&amp;&gt;(y) = 1 ; // ERROR: cannot cast to int&amp;
static_cast&lt;int const&amp;&gt;(y) = 1 ; // ERROR: cannot modify a const value
get(y) = 1 ; // ERROR: cannot modify a const value</PRE>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class value_initialized<br>{<br> public :<br> value_initialized() : x() {}<br> operator T&amp;() const { return x ; }<br> T&amp; data() const { return x ; }<br><br> private :<br> <i>impll-defined</i> x ;<br>} ;<br><br>template&lt;class T&gt;<br>T const&amp; get ( value_initialized&lt;T&gt; const&amp; x )<br>{<br> return x.data() ;<br>}<br><br>template&lt;class T&gt;<br>T&amp; get ( value_initialized&lt;T&gt;&amp; x )<br>{<br> return x.data() ;<br>}<br><br>} // namespace boost<br></pre>
<H3>warning:</H3>
<BLOCKQUOTE> <P>Both the conversion operator and the data() member function are
<CODE>const</CODE> in order to allow access to the wrapped object from a
constant wrapper:</P>
<PRE>void foo(int);
value_initialized&lt;int&gt; const x ;
foo(x);
</PRE>
<p>An object of this template class is a <code>T</code>-wrapper convertible
to <code>'T&amp;'</code> whose wrapped object (data member of type <code>T</code>)
is <a href="#valueinit">value-initialized</a> upon default-initialization
of this wrapper class: </p>
<P>But notice that this conversion operator is to <CODE>T&amp;</CODE> but it is
itself <CODE>const</CODE>. As a consequence, if T is a non-const type, you can
modify the wrapped object even from within a constant wrapper:</P>
<PRE>value_initialized&lt;int&gt; const x_c ;
int&amp; xr = x_c ; // OK, conversion to int&amp; available even though x_c is itself const.
xr = 2 ; </PRE>
<pre>int zero = 0 ;<br>value_initialized&lt;int&gt; x ;<br>assert ( x == zero ) ;<br><br>std::string def ;<br>value_initialized&lt; std::string &gt; y ;<br>assert ( y == def ) ;<br></pre>
<P>The reason for this obscure behaviour is that some commonly used compilers
just don't accept the following valid code:</P>
<PRE>
struct X
{
operator int&amp;() ;
operator int const&amp;() const ;
};
X x ;
(x == 1 ) ; // ERROR HERE!</PRE>
<p>The purpose of this wrapper is to provide a consistent syntax for value
initialization of scalar, union and class types (POD and non-POD) since
the correct syntax for value initialization varies (see <a
href="#valueinitsyn">value-initialization syntax</a>)</p>
<P>These compilers complain about ambiguity between the conversion operators.
<BR>
This is strictly wrong, but the only workaround that I know about is to provide
only one of them, which leads to the obscure behaviour just explained.</P>
</BLOCKQUOTE>
<H3>Recomended practice: the non-member non-friend get() idiom</H3>
<P>The obscure behaviour just warned about being able to modify a non-const
wrapped object from within a constant wrapper can be avoided if access to the
wrapped object is always done through the get() idiom:</P>
<PRE>value_initialized&lt;int&gt; x ;
get(x) = 1 ; // OK
<p>The wrapped object can be accessed either through the conversion operator
<code>T&amp;</code>, the member function <code>data()</code>, or the
non-member function <code>get()</code>: </p>
value_initialized&lt;int const&gt; cx ;
get(x) = 1 ; // ERROR: Cannot modify a const object
<pre>void watch(int);<br>value_initialized&lt;int&gt; x;<br><br>watch(x) ; // operator T&amp; used.<br>watch(x.data());<br>watch( get(x) ) // function get() used</pre>
value_initialized&lt;int&gt; const x_c ;
get(x_c) = 1 ; // ERROR: Cannot modify a const object
<p>Both <code>const</code> and non-<code>const</code> objects can be wrapped.
Mutable objects can be modified directly from within the wrapper but constant
objects cannot:</p>
value_initialized&lt;int const&gt; const cx_c ;
get(cx_c) = 1 ; // ERROR: Cannot modify a const object
</PRE>
<pre>value_initialized&lt;int&gt; x ; <br>static_cast&lt;int&amp;&gt;(x) = 1 ; // OK<br>get(x) = 1 ; // OK<br><br>value_initialized&lt;int const&gt; y ; <br>static_cast&lt;int&amp;&gt;(y) = 1 ; // ERROR: cannot cast to int&amp;<br>static_cast&lt;int const&amp;&gt;(y) = 1 ; // ERROR: cannot modify a const value<br>get(y) = 1 ; // ERROR: cannot modify a const value</pre>
<HR>
<P>Revised 23 August 2002</P>
<P>&copy; Copyright boost.org 2002. Permission to copy, use, modify, sell and
distribute this document is granted provided this copyright notice appears in
all copies. This document is provided &quot;as is&quot; without express or
implied warranty, and with no claim as to its suitability for any purpose.</P>
<P>Developed by <A HREF="mailto:fcacciola@gosierra.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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<h3>Warning:</h3>
<p>Both the conversion operator and the <code>data()</code> member function
are <code>const</code> in order to allow access to the wrapped object
from a constant wrapper:</p>
<pre>void foo(int);<br>value_initialized&lt;int&gt; const x ;<br>foo(x);<br></pre>
<p>But notice that this conversion operator is to <code>T&amp;</code> although
it is itself <code>const</code>. As a consequence, if <code>T</code> is
a non-<code>const</code> type, you can modify the wrapped object even from
within a constant wrapper:</p>
<pre>value_initialized&lt;int&gt; const x_c ;<br>int&amp; xr = x_c ; // OK, conversion to int&amp; available even though x_c is itself const.<br>xr = 2 ; </pre>
<p>The reason for this obscure behavior is that some commonly used compilers
just don't accept the following valid code:</p>
<pre>struct X<br>{<br> operator int&amp;() ;<br> operator int const&amp;() const ; <br>};<br>X x ;<br>(x == 1 ) ; // ERROR HERE!</pre>
<p>These compilers complain about ambiguity between the conversion operators.
This complaint is incorrect, but the only workaround that I know of is
to provide only one of them, which leads to the obscure behavior just explained.<br>
</p>
<h3>Recommended practice: The non-member get() idiom</h3>
<p>The obscure behavior of being able to modify a non-<code>const</code>
wrapped object from within a constant wrapper can be avoided if access to
the wrapped object is always performed with the <code>get()</code> idiom:</p>
<pre>value_initialized&lt;int&gt; x ;<br>get(x) = 1 ; // OK<br><br>value_initialized&lt;int const&gt; cx ;<br>get(x) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized&lt;int&gt; const x_c ;<br>get(x_c) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized&lt;int const&gt; const cx_c ;<br>get(cx_c) = 1 ; // ERROR: Cannot modify a const object<br></pre>
<h3><a name="references">References</a></h3>
[1] The C++ Standard, ISO/IEC 14882:98 <br>
[2] Plain Old Data
<h3><a name="acknowledgements"></a>Acknowledgements</h3>
value_initialized was developed by Fernando Cacciola, with help and
suggestions from David Abrahams and Darin Adler.<br>
Special thanks to Björn Karlsson who carefully edited and completed this documentation.
<pre>&nbsp;</pre>
<hr>
<p>Revised 19 September 2002</p>
<p>&copy; Copyright boost.org 2002. 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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