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Minor "beautifications" of value_init documentation, inc. placing references in order of appearance

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[SVN r42779]
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Niels Dekker 2008-01-14 21:46:20 +00:00
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value_init.htm
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@ -32,7 +32,7 @@
</dl> </dl>
<ul> <ul>
<li><a href="#val_init"><code>value_initialized&lt;&gt;</code></a></li> <li><a href="#val_init"><code>value_initialized&lt;T&gt;</code></a></li>
</ul> </ul>
<a href="#acknowledgements">Acknowledgements</a><br> <a href="#acknowledgements">Acknowledgements</a><br>
@ -46,7 +46,7 @@
for initialization. Depending on the type, the value of a newly constructed for initialization. Depending on the type, the value of a newly constructed
object can be zero-initialized (logically 0), default-constructed (using object can be zero-initialized (logically 0), default-constructed (using
the default constructor), or indeterminate. When writing generic code, the default constructor), or indeterminate. When writing generic code,
this problem must be addressed. <code>value_initialized</code> provides this problem must be addressed. The template <code>value_initialized</code> provides
a solution with consistent syntax for value initialization of scalar, a solution with consistent syntax for value initialization of scalar,
union and class types. union and class types.
Moreover, <code>value_initialized</code> offers a workaround to various Moreover, <code>value_initialized</code> offers a workaround to various
@ -84,7 +84,7 @@ initialized that way). The third form, <code>T3 var3 = {}</code>
initializes an aggregate, typically a "C-style" <code>struct</code> or a "C-style" array. initializes an aggregate, typically a "C-style" <code>struct</code> or a "C-style" array.
However, the syntax is not allowed for a class that has an explicitly declared However, the syntax is not allowed for a class that has an explicitly declared
constructor. (But watch out for an upcoming C++ language change, constructor. (But watch out for an upcoming C++ language change,
by Bjarne Stroustrup et al [<a href="#references">3</a>]!) by Bjarne Stroustrup et al [<a href="#references">1</a>]!)
The fourth form is the most generic form of them, as it The fourth form is the most generic form of them, as it
can be used to initialize arithmetic types, class types, aggregates, pointers, and can be used to initialize arithmetic types, class types, aggregates, pointers, and
other types. The declaration, <code>T4 var4 = T4()</code>, should be read other types. The declaration, <code>T4 var4 = T4()</code>, should be read
@ -98,7 +98,7 @@ be optimized away, C++ still requires the type <code>T4</code> to be
A class may not be CopyConstructible, for example because it may have a A class may not be CopyConstructible, for example because it may have a
private and undefined copy constructor, private and undefined copy constructor,
or because it may be derived from <a href="utility.htm#Class_noncopyable">boost::noncopyable</a>. or because it may be derived from <a href="utility.htm#Class_noncopyable">boost::noncopyable</a>.
Scott Meyers [<a href="#references">4</a>] explains why a class would be defined like that. Scott Meyers [<a href="#references">2</a>] explains why a class would be defined like that.
</p> </p>
<p> <p>
There is another, less obvious disadvantage to the fourth form, <code>T4 var4 = T4()</code>: There is another, less obvious disadvantage to the fourth form, <code>T4 var4 = T4()</code>:
@ -120,11 +120,11 @@ constructed by the following declaration:
</p> </p>
<h2><a name="details"></a>Details</h2> <h2><a name="details"></a>Details</h2>
<p>The C++ standard [<a href="#references">1</a>] contains the definitions <p>The C++ standard [<a href="#references">3</a>] contains the definitions
of <code>zero-initialization</code> and <code>default-initialization</code>. of <code>zero-initialization</code> and <code>default-initialization</code>.
Informally, zero-initialization means that the object is given the initial Informally, zero-initialization means that the object is given the initial
value 0 (converted to the type) and default-initialization means that value 0 (converted to the type) and default-initialization means that
POD [<a href="#references">2</a>] types are zero-initialized, while non-POD class POD [<a href="#references">4</a>] types are zero-initialized, while non-POD class
types are initialized with their corresponding default constructors. A types are initialized with their corresponding default constructors. A
<i>declaration</i> can contain an <i>initializer</i>, which specifies the <i>declaration</i> can contain an <i>initializer</i>, which specifies the
object's initial value. The initializer can be just '()', which states that object's initial value. The initializer can be just '()', which states that
@ -225,7 +225,7 @@ GCC Bug 33916 - Default constructor fails to initialize array members</a>
<br> <br>
<a href="http://qc.codegear.com/wc/qcmain.aspx?d=51854"> <a href="http://qc.codegear.com/wc/qcmain.aspx?d=51854">
Borland Report 51854 - Value-initialization: POD struct should be zero-initialized</a> Borland Report 51854 - Value-initialization: POD struct should be zero-initialized</a>
<br>Reported by Niels Dekker (LKEB, Leiden University Medical Center), 2007-11-09 <br>Reported by Niels Dekker (LKEB, Leiden University Medical Center), 2007-09-11
<br> <br>
</td></tr></table> </td></tr></table>
</p><p> </p><p>
@ -299,23 +299,23 @@ 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> <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> <h3><a name="references">References</a></h3>
[1] The C++ Standard, Second edition (2003), ISO/IEC 14882:2003 <br> [1] Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote
[2] POD stands for "Plain Old Data" <br>
[3] Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote
various papers, proposing to extend the support for brace-enclosed <em>initializer lists</em> various papers, proposing to extend the support for brace-enclosed <em>initializer lists</em>
in the next version of C++. in the next version of C++.
This would allow a variable <code>var</code> of any DefaultConstructible type This would allow a variable <code>var</code> of any DefaultConstructible type
<code>T</code> to be <em>value-initialized</em> by doing <code>T var = {}</code>. <code>T</code> to be <em>value-initialized</em> by doing <code>T var = {}</code>.
The papers are listed at Bjarne's web page, The papers are listed at Bjarne's web page,
<a href="http://www.research.att.com/~bs/WG21.html">My C++ Standards committee papers</a> <br> <a href="http://www.research.att.com/~bs/WG21.html">My C++ Standards committee papers</a> <br>
[4] Scott Meyers, Effective C++, Third Edition, item 6, [2] Scott Meyers, Effective C++, Third Edition, item 6,
<em>Explicitly disallow the use of compiler-generated functions you do not want</em>, <em>Explicitly disallow the use of compiler-generated functions you do not want</em>,
<a href="http://www.aristeia.com/books.html">Scott Meyers: Books and CDs</a> <a href="http://www.aristeia.com/books.html">Scott Meyers: Books and CDs</a> <br>
[3] The C++ Standard, Second edition (2003), ISO/IEC 14882:2003 <br>
[4] POD stands for "Plain Old Data"
<h3><a name="acknowledgements"></a>Acknowledgements</h3> <h3><a name="acknowledgements"></a>Acknowledgements</h3>
value_initialized was developed by Fernando Cacciola, with help and value_initialized was developed by Fernando Cacciola, with help and
suggestions from David Abrahams and Darin Adler.<br> suggestions from David Abrahams and Darin Adler.<br>
Special thanks to Björn Karlsson who carefully edited and completed this documentation. Special thanks to Bj&ouml;rn Karlsson who carefully edited and completed this documentation.
<p>value_initialized was reimplemented by Fernando Cacciola and Niels Dekker <p>value_initialized was reimplemented by Fernando Cacciola and Niels Dekker
for Boost release version 1.35 (2008), offering a workaround to various compiler issues. for Boost release version 1.35 (2008), offering a workaround to various compiler issues.