generic-library/boost
1
0
Fork 0
Code Issues Pull Requests Releases Wiki Activity
main
boost/libs/utility/value_init.htm
Edouard DUPIN a97e9ae7d4 [DEV] add v1.66.0
2018-01-12 21:47:58 +01:00

515 lines
25 KiB
HTML
Raw Permalink Blame History

<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 &lt;<a href="../../boost/utility/value_init.hpp">boost/utility/value_init.hpp</a>&gt;
</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&lt;T&gt;</code></a></li>
<li><a href="#initialized"><code>template class initialized&lt;T&gt;</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&lt;T&gt;</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&lt;T&gt; 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 &sect;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://www.open-std.org/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://www.open-std.org/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&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-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&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>This is the solution as it was supplied by earlier versions of the
<code>value_initialized&lt;T&gt;</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&lt;T&gt;</code></a></h2>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class value_initialized<br>{
<br> public :
<br> value_initialized() : x() {}
<br> operator T const &amp;() const { return x ; }
<br> operator T&amp;() { return x ; }
<br> T const &amp;data() const { return x ; }
<br> T&amp; data() { return x ; }
<br> void swap( value_initialized&amp; );
<br>
<br> private :
<br> <i>unspecified</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>template&lt;class T&gt;
<br>void swap ( value_initialized&lt;T&gt;&amp; lhs, value_initialized&lt;T&gt;&amp; rhs )
<br>{
<br> lhs.swap(rhs) ;
<br>}
<br>
<br>} // namespace boost
<br></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>
<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 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&amp;</code>, the member function <code>data()</code>, or the
non-member function <code>get()</code>: </p>
<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>
<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&lt;T&gt;</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&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>
<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&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 was that some compilers
didn'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>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&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>
<h2><a name="initialized"><code>template class initialized&lt;T&gt;</code></a></h2>
<pre>namespace boost {<br><br>template&lt;class T&gt;<br>class initialized<br>{
<br> public :
<br> initialized() : x() {}
<br> explicit initialized(T const &amp; arg) : x(arg) {}
<br> operator T const &amp;() const;
<br> operator T&amp;();
<br> T const &amp;data() const;
<br> T&amp; data();
<br> void swap( initialized&amp; );
<br>
<br> private :
<br> <i>unspecified</i> x ;
<br>} ;
<br>
<br>template&lt;class T&gt;
<br>T const&amp; get ( initialized&lt;T&gt; const&amp; x );
<br>
<br>template&lt;class T&gt;
<br>T&amp; get ( initialized&lt;T&gt;&amp; x );
<br>
<br>template&lt;class T&gt;
<br>void swap ( initialized&lt;T&gt;&amp; lhs, initialized&lt;T&gt;&amp; rhs );
<br>
<br>} // namespace boost
<br></pre>
The template class <code>boost::initialized&lt;T&gt;</code> supports both value-initialization
and direct-initialization, so its interface is a superset of the interface
of <code>value_initialized&lt;T&gt;</code>: Its default-constructor
value-initializes the wrapped object just like the default-constructor of
<code>value_initialized&lt;T&gt;</code>, but <code>boost::initialized&lt;T&gt;</code>
also offers an extra <code>explicit</code>
constructor, which direct-initializes the wrapped object by the specified value.
<p>
<code>initialized&lt;T&gt;</code> is especially useful when the wrapped
object must be either value-initialized or direct-initialized, depending on
runtime conditions. For example, <code>initialized&lt;T&gt;</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&lt;T&gt;</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 &lt;class T&gt; 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&lt;T&gt;() );
</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&lt;Arg&gt;::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 Bj&ouml;rn 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>&copy; 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>
Reference in New Issue View Git Blame Copy Permalink