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| author | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
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| committer | Anas Nashif <anas.nashif@intel.com> | 2012-10-30 12:57:26 -0700 |
| commit | 1a78a62555be32868418fe52f8e330c9d0f95d5a (patch) | |
| tree | d3765a80e7d3b9640ec2e930743630cd6b9fce2b /libs/utility/value_init.htm | |
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diff --git a/libs/utility/value_init.htm b/libs/utility/value_init.htm new file mode 100644 index 0000000000..4ae9177604 --- /dev/null +++ b/libs/utility/value_init.htm @@ -0,0 +1,505 @@ +<html> +<head> + + <meta http-equiv="Content-Type" + content="text/html; charset=iso-8859-1"> + <title>value_initialized</title> + +</head> + <body vlink="#800080" link="#0000ff" text="#000000" bgcolor="#ffffff"> + +<h2><img src="../../boost.png" width="276" height="86"> + Header <<a href="../../boost/utility/value_init.hpp">boost/utility/value_init.hpp</a>> + </h2> + +<h2>Contents</h2> + +<dl> + <dt><a href="#rationale">Rationale</a></dt> + <dt><a href="#intro">Introduction</a></dt> + <dt><a href="#details">Details</a></dt> +</dl> + +<ul> + <li><a href="#valueinit">value-initialization</a></li> + <li><a href="#valueinitsyn">value-initialization syntax</a></li> + <li><a href="#compiler_issues">compiler issues</a></li> + +</ul> + +<dl class="page-index"> + <dt><a href="#types">Types and objects</a></dt> +</dl> + +<ul> + <li><a href="#val_init"><code>template class value_initialized<T></code></a></li> + <li><a href="#initialized"><code>template class initialized<T></code></a></li> + <li><a href="#initialized_value"><code>initialized_value</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. The template <code>value_initialized</code> provides +a solution with consistent syntax for value initialization of scalar, +union and class types. +Moreover, <code>value_initialized</code> offers a workaround to various +compiler issues regarding value-initialization. + +Furthermore, a <code>const</code> object, <code>initialized_value</code> is provided, +to avoid repeating the type name when retrieving the value from a +<code>value_initialized<T></code> object. +<br> + </p> + +<h2><a name="intro"></a>Introduction</h2> + +<p> +There are various ways to initialize a variable, in C++. The following +declarations all <em>may</em> have a local variable initialized to its default +value: +<pre> + T1 var1; + T2 var2 = 0; + T3 var3 = {}; + T4 var4 = T4(); +</pre> +Unfortunately, whether or not any of those declarations correctly +initialize the variable very much depends on its type. The first +declaration is valid for any <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html"> +DefaultConstructible</a> type (by definition). +However, it does not always do an initialization! +It correctly initializes the variable when it's an instance of a +class, and the author of the class has provided a proper default +constructor. On the other hand, the value of <code>var1</code> is <em>indeterminate</em> when +its type is an arithmetic type, like <code>int</code>, <code>float</code>, or <code>char</code>. +An arithmetic variable +is of course initialized properly by the second declaration, <code>T2 +var2 = 0</code>. But this initialization form usually won't work for a +class type (unless the class was especially written to support being +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. +However, the syntax is not allowed for a class that has an explicitly declared +constructor. (But watch out for an upcoming C++ language change, +by Bjarne Stroustrup et al [<a href="#references">1</a>]!) +The fourth form is the most generic form of them, as it +can be used to initialize arithmetic types, class types, aggregates, pointers, and +other types. The declaration, <code>T4 var4 = T4()</code>, should be read +as follows: First a temporary object is created, by <code>T4()</code>. +This object is <a href="#valueinit">value-initialized</a>. Next the temporary +object is copied to the named variable, <code>var4</code>. Afterwards, the temporary +is destroyed. While the copying and the destruction are likely to +be optimized away, C++ still requires the type <code>T4</code> to be +<a href="CopyConstructible.html">CopyConstructible</a>. +(So <code>T4</code> needs to be <em>both</em> DefaultConstructible <em>and</em> CopyConstructible.) +A class may not be CopyConstructible, for example because it may have a +private and undefined copy constructor, +or because it may be derived from <a href="utility.htm#Class_noncopyable">boost::noncopyable</a>. +Scott Meyers [<a href="#references">2</a>] explains why a class would be defined like that. +</p> +<p> +There is another, less obvious disadvantage to the fourth form, <code>T4 var4 = T4()</code>: +It suffers from various <a href="#compiler_issues">compiler issues</a>, causing +a variable to be left uninitialized in some compiler specific cases. +</p> +<p> +The template <a href="#val_init"><code>value_initialized</code></a> +offers a generic way to initialize +an object, like <code>T4 var4 = T4()</code>, but without requiring its type +to be CopyConstructible. And it offers a workaround to those compiler issues +regarding value-initialization as well! It allows getting an initialized +variable of any type; it <em>only</em> requires the type to be DefaultConstructible. +A properly <em>value-initialized</em> object of type <code>T</code> is +constructed by the following declaration: +<pre> + value_initialized<T> var; +</pre> +</p> +<p> +The template <a href="#initialized"><code>initialized</code></a> +offers both value-initialization and direct-initialization. +It is especially useful as a data member type, allowing the very same object +to be either direct-initialized or value-initialized. +</p> +<p> +The <code>const</code> object <a href="#initialized_value"><code>initialized_value</code></a> +allows value-initializing a variable as follows: +<pre> + T var = initialized_value ; +</pre> +This form of initialization is semantically equivalent to <code>T4 var4 = T4()</code>, +but robust against the aforementioned compiler issues. + +</p> + +<h2><a name="details"></a>Details</h2> +<p>The C++ standard [<a href="#references">3</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 [<a href="#references">4</a>] types are zero-initialized, while non-POD 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 value-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 §8.5, [dcl.init], for the + accurate definitions).</cite></p> + +<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> + +<h3><a name="valueinit">value-initialization</a></h3> + +<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> + +<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>In order to specify value-initialization of an object we need to use the + empty-set initializer: (). </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<class T> 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-initialized</pre> + +<h4><a name="valueinitsyn">value-initialization</a> syntax</h4> + +<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> + +<pre>int x() ; // declares function int(*)()</pre> + +<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>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>One possible generic solution is to use value-initialization of a non static +data member:</p> + +<pre>template<class T> <br>struct W <br>{<br> // value-initialization of 'data' here.<br> W() : data() {}<br> T data ;<br>} ;<br>W<int> w ;<br>// w.data is value-initialized for any type. </pre> + +<p>This is the solution as it was supplied by earlier versions of the +<code>value_initialized<T></code> template + class. Unfortunately this approach suffered from various compiler issues.</p> + +<h4><a name="compiler_issues">compiler issues</a> </h4> + +Various compilers haven't yet fully implemented value-initialization. +So when an object should be <em>value-initialized</em> (according to the C++ Standard), +it <em>may</em> in practice still be left uninitialized, because of those +compiler issues! It's hard to make a general statement on what those issues +are like, because they depend on the compiler you are using, its version number, +and the type of object you would like to have value-initialized. +All compilers we have tested so far support value-initialization for arithmetic types properly. +However, various compilers may leave some types of <em>aggregates</em> uninitialized, when they +should be value-initialized. Value-initialization of objects of a pointer-to-member type may also +go wrong on various compilers. +</p> +<p> +At the moment of writing, May 2010, the following reported issues regarding +value-initialization are still there in current compiler releases: +<ul> +<li> +<a href="https://connect.microsoft.com/VisualStudio/feedback/details/100744"> +Microsoft Visual Studio Feedback ID 100744, Value-initialization in new-expression</a> +<br>Reported by Pavel Kuznetsov (MetaCommunications Engineering), 2005 +</li><li> +<a href="http://connect.microsoft.com/VisualStudio/feedback/details/484295"> +Microsoft Visual Studio Feedback ID 484295, VC++ does not value-initialize members of derived classes without user-declared constructor</a> +<br>Reported by Sylvester Hesp, 2009 +</li><li> +<a href="https://connect.microsoft.com/VisualStudio/feedback/details/499606"> +Microsoft Visual Studio Feedback ID 499606, Presence of copy constructor breaks member class initialization</a> +<br>Reported by Alex Vakulenko, 2009 +</li><li> +<a href="http://qc.embarcadero.com/wc/qcmain.aspx?d=83751"> +Embarcadero/C++Builder Report 83751, Value-initialization: arrays should have each element value-initialized</a> +<br>Reported by Niels Dekker (LKEB), 2010 +</li><li> +<a href="http://qc.embarcadero.com/wc/qcmain.aspx?d=83851"> +Embarcadero/C++Builder Report 83851, Value-initialized temporary triggers internal backend error C1798</a> +<br>Reported by Niels Dekker, 2010 +</li><li> +<a href="http://qc.embarcadero.com/wc/qcmain.aspx?d=84279"> +Embarcadero/C++Builder Report 84279, Internal compiler error (F1004), value-initializing member function pointer by "new T()"</a> +<br>Reported by Niels Dekker, 2010 +</li><li> +Sun CR 6947016, Sun 5.10 may fail to value-initialize an object of a non-POD aggregate. +<br>Reported to Steve Clamage by Niels Dekker, 2010 +</li><li> +IBM's XL V10.1 and V11.1 may fail to value-initialize a temporary of a non-POD aggregate. +<br>Reported to Michael Wong by Niels Dekker, 2010 +</li><li> +Intel support issue 589832, Attempt to value-initialize pointer-to-member triggers internal error +on Intel 11.1. +<br>Reported by John Maddock, 2010 +</li> +</ul> +Note that all known GCC issues regarding value-initialization are +fixed with GCC version 4.4, including +<a href="http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30111">GCC Bug 30111</a>. +Clang also has completely implemented value-initialization, as far as we know, +now that <a href="http://llvm.org/bugs/show_bug.cgi?id=7139">Clang Bug 7139</a> is fixed. +</p><p> + +New versions of <code>value_initialized</code> +(Boost release version 1.35 or higher) +offer a workaround to these issues: <code>value_initialized</code> may now clear +its internal data, prior to constructing the object that it contains. It will do +so for those compilers that need to have such a workaround, based on the +<a href="../config/doc/html/boost_config/boost_macro_reference.html#boost_config.boost_macro_reference.macros_that_describe_defects" +>compiler defect macro</a> BOOST_NO_COMPLETE_VALUE_INITIALIZATION. +</p> + +<h2><a name="types"></a>Types and objects</h2> + +<h2><a name="val_init"><code>template class value_initialized<T></code></a></h2> + +<pre>namespace boost {<br><br>template<class T><br>class value_initialized<br>{ +<br> public : +<br> value_initialized() : x() {} +<br> operator T const &() const { return x ; } +<br> operator T&() { return x ; } +<br> T const &data() const { return x ; } +<br> T& data() { return x ; } +<br> void swap( value_initialized<T>& ); +<br> +<br> private : +<br> <i>unspecified</i> x ; +<br>} ; +<br> +<br>template<class T> +<br>T const& get ( value_initialized<T> const& x ) +<br>{ +<br> return x.data() ; +<br>} +<br> +<br>template<class T> +<br>T& get ( value_initialized<T>& x ) +<br>{ +<br> return x.data() ; +<br>} +<br> +<br>} // namespace boost +<br></pre> + +<p>An object of this template class is a <code>T</code>-wrapper convertible + to <code>'T&'</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> + +<pre>int zero = 0 ;<br>value_initialized<int> x ;<br>assert ( x == zero ) ;<br><br>std::string def ;<br>value_initialized< std::string > y ;<br>assert ( y == def ) ;<br></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 + <code>T&</code>, the member function <code>data()</code>, or the +non-member function <code>get()</code>: </p> + +<pre>void watch(int);<br>value_initialized<int> x; +<br><br>watch(x) ; // operator T& used.<br>watch(x.data());<br>watch( get(x) ) // function get() used</pre> + +<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> + +<p>When <code>T</code> is a <em>Swappable</em> type, <code>value_initialized<T></code> + is swappable as well, by calling its <code>swap</code> member function + as well as by calling <code>boost::swap</code>.</p> + +<pre>value_initialized<int> x ; <br>static_cast<int&>(x) = 1 ; // OK<br>get(x) = 1 ; // OK +<br><br>value_initialized<int const> y ; <br>static_cast<int&>(y) = 1 ; // ERROR: cannot cast to int&<br>static_cast<int const&>(y) = 1 ; // ERROR: cannot modify a const value<br>get(y) = 1 ; // ERROR: cannot modify a const value</pre> + +<h3>Warning:</h3> + +<p>The <code>value_initialized</code> implementation of Boost version 1.40.0 and older +allowed <i>non-const</i> access to the wrapped object, from a constant wrapper, +both by its conversion operator and its <code>data()</code> member function. For example:</p> + +<pre>value_initialized<int> const x_c ;<br>int& xr = x_c ; // OK, conversion to int& available even though x_c is itself const. +<br>xr = 2 ; </pre> + +<p>The reason for this obscure behavior was that some compilers + didn't accept the following valid code:</p> + +<pre>struct X<br>{<br> operator int&() ;<br> operator int const&() const ; <br>};<br>X x ;<br>(x == 1 ) ; // ERROR HERE!</pre> + +<p>The current version of <code>value_initialized</code> no longer has this obscure behavior. +As compilers nowadays widely support overloading the conversion operator by having a <code>const</code> and a <code>non-const</code> version, we have decided to fix the issue accordingly. So the current version supports the idea of logical constness. +<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 (as was supported by previous +versions of <code>value_initialized</code>) +can be avoided if access to +the wrapped object is always performed with the <code>get()</code> idiom:</p> + +<pre>value_initialized<int> x ;<br>get(x) = 1 ; // OK<br><br>value_initialized<int const> cx ;<br>get(x) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized<int> const x_c ;<br>get(x_c) = 1 ; // ERROR: Cannot modify a const object<br><br>value_initialized<int const> const cx_c ;<br>get(cx_c) = 1 ; // ERROR: Cannot modify a const object<br></pre> + +<h2><a name="initialized"><code>template class initialized<T></code></a></h2> + +<pre>namespace boost {<br><br>template<class T><br>class initialized<br>{ +<br> public : +<br> initialized() : x() {} +<br> explicit initialized(T const & arg) : x(arg) {} +<br> operator T const &() const; +<br> operator T&(); +<br> T const &data() const; +<br> T& data(); +<br> void swap( value_initialized<T>& ); +<br> +<br> private : +<br> <i>unspecified</i> x ; +<br>} ; +<br> +<br>template<class T> +<br>T const& get ( initialized<T> const& x ); +<br> +<br>template<class T> +<br>T& get ( initialized<T>& x ); +<br> +<br>} // namespace boost +<br></pre> + +The template class <code>boost::initialized<T></code> supports both value-initialization +and direct-initialization, so its interface is a superset of the interface +of <code>value_initialized<T></code>: Its default-constructor +value-initializes the wrapped object just like the default-constructor of +<code>value_initialized<T></code>, but <code>boost::initialized<T></code> +also offers an extra <code>explicit</code> +constructor, which direct-initializes the wrapped object by the specified value. +<p> + +<code>initialized<T></code> is especially useful when the wrapped +object must be either value-initialized or direct-initialized, depending on +runtime conditions. For example, <code>initialized<T></code> could +hold the value of a data member that may be value-initialized by some +constructors, and direct-initialized by others. +On the other hand, if it is known beforehand that the +object must <i>always</i> be value-initialized, <code>value_initialized<T></code> +may be preferable. And if the object must always be +direct-initialized, none of the two wrappers really needs to be used. +</p> + + +<h2><a name="initialized_value"><code>initialized_value</code></a></h2> + +<pre> +namespace boost { +class initialized_value_t +{ + public : + template <class T> operator T() const ; +}; + +initialized_value_t const initialized_value = {} ; + +} // namespace boost +</pre> + +<code>initialized_value</code> provides a convenient way to get +an initialized value: its conversion operator provides an appropriate +<em>value-initialized</em> object for any CopyConstructible type. + +Suppose you need to have an initialized variable of type <code>T</code>. +You could do it as follows: +<pre> + T var = T(); +</pre> +But as mentioned before, this form suffers from various compiler issues. +The template <code>value_initialized</code> offers a workaround: +<pre> + T var = get( value_initialized<T>() ); +</pre> +Unfortunately both forms repeat the type name, which +is rather short now (<code>T</code>), but could of course be +more like <code>Namespace::Template<Arg>::Type</code>. +Instead, one could use <code>initialized_value</code> as follows: +<pre> + T var = initialized_value ; +</pre> + +<h3><a name="references">References</a></h3> + [1] Bjarne Stroustrup, Gabriel Dos Reis, and J. Stephen Adamczyk wrote + various papers, proposing to extend the support for brace-enclosed <em>initializer lists</em> + in the next version of C++. + 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>. + 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> + [2] Scott Meyers, Effective C++, Third Edition, item 6, + <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> <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> + 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. + +<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. + </p> +<p><code>boost::initialized</code> was very much inspired by feedback from Edward Diener and + Jeffrey Hellrung. + </p> +<p>initialized_value was written by Niels Dekker, and added to Boost release version 1.36 (2008). + </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>. + </p> + +<hr> +<p>Revised 30 May 2010</p> + +<p>© Copyright Fernando Cacciola, 2002 - 2010.</p> + +<p>Distributed under the Boost Software License, Version 1.0. See +<a href="http://www.boost.org/LICENSE_1_0.txt">www.boost.org/LICENSE_1_0.txt</a></p> + + <br> + <br> + +</body> +</html> |