Added boost::initialized<T> as was agreed at http://lists.boost.org/Archives/boost/2010/04/164916.php -- see #3472

[SVN r61883]

include/boost/utility/value_init.hpp

@@ -9,6 +9,7 @@
 // 23 May 2008 (Fixed operator= const issue, added initialized_value) Niels Dekker, Fernando Cacciola
 // 21 Ago 2008 (Added swap) Niels Dekker, Fernando Cacciola
 // 20 Feb 2009 (Fixed logical const-ness issues) Niels Dekker, Fernando Cacciola
+// 03 Apr 2010 (Added initialized<T>, suggested by Jeffrey Hellrung, fixing #3472) Niels Dekker
 //
 #ifndef BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
 #define BOOST_UTILITY_VALUE_INIT_21AGO2002_HPP
@@ -28,10 +29,19 @@
 #include <cstring>
 #include <new>
 
+#ifdef BOOST_MSVC
+#pragma warning(push)
+#if _MSC_VER >= 1310
+// It is safe to ignore the following warning from MSVC 7.1 or higher:
+// "warning C4351: new behavior: elements of array will be default initialized"
+#pragma warning(disable: 4351)
+#endif
+#endif
+
 namespace boost {
 
 template<class T>
-class value_initialized
+class initialized
 {
   private :
     struct wrapper
@@ -40,6 +50,18 @@ class value_initialized
       typename
 #endif 
       remove_const<T>::type data;
+
+      wrapper()
+      :
+      data()
+      {
+      }
+
+      wrapper(T const & arg)
+      :
+      data(arg)
+      {
+      }
     };
 
     mutable
@@ -55,30 +77,26 @@ class value_initialized
 
   public :
 
-    value_initialized()
+    initialized()
     {
+      // Note: the following memset call will become conditional when ticket #3869 is fixed:
+      // https://svn.boost.org/trac/boost/ticket/3869 reported by Aleksey Gurtovoy.
       std::memset(&x, 0, sizeof(x));
-#ifdef BOOST_MSVC
+
-#pragma warning(push)
-#if _MSC_VER >= 1310
-// When using MSVC 7.1 or higher, the following placement new expression may trigger warning C4345:
-// "behavior change: an object of POD type constructed with an initializer of the form ()
-// will be default-initialized".  It is safe to ignore this warning when using value_initialized.
-#pragma warning(disable: 4345)
-#endif
-#endif
       new (wrapper_address()) wrapper();
-#ifdef BOOST_MSVC
-#pragma warning(pop)
-#endif
     }
 
-    value_initialized(value_initialized const & arg)
+    initialized(initialized const & arg)
     {
       new (wrapper_address()) wrapper( static_cast<wrapper const &>(*(arg.wrapper_address())));
     }
 
-    value_initialized & operator=(value_initialized const & arg)
+    explicit initialized(T const & arg)
+    {
+      new (wrapper_address()) wrapper(arg);
+    }
+
+    initialized & operator=(initialized const & arg)
     {
       // Assignment is only allowed when T is non-const.
       BOOST_STATIC_ASSERT( ! is_const<T>::value );
@@ -86,7 +104,7 @@ class value_initialized
       return *this;
     }
 
-    ~value_initialized()
+    ~initialized()
     {
       wrapper_address()->wrapper::~wrapper();
     }
@@ -101,17 +119,76 @@ class value_initialized
       return wrapper_address()->data;
     }
 
-    void swap(value_initialized & arg)
+    void swap(initialized & arg)
     {
       ::boost::swap( this->data(), arg.data() );
     }
 
-    operator T const &() const { return this->data(); }
+    operator T const &() const
+    {
+      return wrapper_address()->data;
+    }
 
-    operator T&() { return this->data(); }
+    operator T&()
+    {
+      return wrapper_address()->data;
+    }
 
 } ;
 
+template<class T>
+T const& get ( initialized<T> const& x )
+{
+  return x.data() ;
+}
+
+template<class T>
+T& get ( initialized<T>& x )
+{
+  return x.data() ;
+}
+
+template<class T>
+void swap ( initialized<T> & lhs, initialized<T> & rhs )
+{
+  lhs.swap(rhs) ;
+}
+
+template<class T>
+class value_initialized
+{
+  private :
+
+    // initialized<T> does value-initialization by default.
+    initialized<T> m_data;
+
+  public :
+
+    T const & data() const
+    {
+      return m_data.data();
+    }
+
+    T& data()
+    {
+      return m_data.data();
+    }
+
+    void swap(value_initialized & arg)
+    {
+      m_data.swap(arg.m_data);
+    }
+
+    operator T const &() const
+    {
+      return m_data;
+    }
+
+    operator T&()
+    {
+      return m_data;
+    }
+} ;
 
 
 template<class T>
@@ -119,6 +196,7 @@ T const& get ( value_initialized<T> const& x )
 {
   return x.data() ;
 }
+
 template<class T>
 T& get ( value_initialized<T>& x )
 {
@@ -138,7 +216,7 @@ class initialized_value_t
     
     template <class T> operator T() const
     {
-      return get( value_initialized<T>() );
+      return initialized<T>().data();
     }
 };
 
@@ -147,5 +225,8 @@ initialized_value_t const initialized_value = {} ;
 
 } // namespace boost
 
+#ifdef BOOST_MSVC
+#pragma warning(pop)
+#endif
 
 #endif

initialized_test.cpp

@@ -0,0 +1,116 @@
+// Copyright 2010, Niels Dekker.
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// Test program for boost::initialized<T>.
+//
+// 2 May 2010 (Created) Niels Dekker
+
+#include <boost/utility/value_init.hpp>
+#include <boost/detail/lightweight_test.hpp>
+
+#include <string>
+
+namespace
+{
+  // Typical use case for boost::initialized<T>: A generic class that 
+  // holds a value of type T, which must be initialized by either 
+  // value-initialization or direct-initialization.
+  template <class T> class key_value_pair
+  {
+    std::string m_key;
+    boost::initialized<T> m_value;
+  public:
+ 
+    // Value-initializes the object held by m_value.
+    key_value_pair() { }
+
+    // Value-initializes the object held by m_value.
+    explicit key_value_pair(const std::string& key)
+    :
+    m_key(key)
+    {
+    }
+
+    // Direct-initializes the object held by m_value.
+    key_value_pair(const std::string& key, const T& value)
+    :
+    m_key(key), m_value(value)
+    {
+    }
+
+    const T& get_value() const
+    {
+      return m_value;
+    }
+  };
+
+
+  // Tells whether the argument is value-initialized.
+  bool is_value_initialized(const int& arg)
+  {
+    return arg == 0;
+  }
+
+
+  // Tells whether the argument is value-initialized.
+  bool is_value_initialized(const std::string& arg)
+  {
+    return arg.empty();
+  }
+
+  struct foo
+  {
+    int data;
+  };
+
+  bool operator==(const foo& lhs, const foo& rhs)
+  {
+    return lhs.data == rhs.data;
+  }
+
+
+  // Tells whether the argument is value-initialized.
+  bool is_value_initialized(const foo& arg)
+  {
+    return arg.data == 0;
+  }
+
+
+  template <class T>
+  void test_key_value_pair(const T& magic_value)
+  {
+    // The value component of a default key_value_pair must be value-initialized.
+    key_value_pair<T> default_key_value_pair;
+    BOOST_TEST( is_value_initialized(default_key_value_pair.get_value() ) );
+
+    // The value component of a key_value_pair that only has its key explicitly specified
+    // must also be value-initialized.
+    BOOST_TEST( is_value_initialized(key_value_pair<T>("key").get_value()) );
+
+    // However, the value component of the following key_value_pair must be 
+    // "magic_value", as it must be direct-initialized.
+    BOOST_TEST( key_value_pair<T>("key", magic_value).get_value() == magic_value );
+  }
+}
+
+
+// Tests boost::initialize for a fundamental type, a type with a
+// user-defined constructor, and a user-defined type without 
+// a user-defined constructor.
+int main()
+{
+
+  const int magic_number = 42;
+  test_key_value_pair(magic_number);
+
+  const std::string magic_string = "magic value";
+  test_key_value_pair(magic_string);
+
+  const foo magic_foo = { 42 };
+  test_key_value_pair(magic_foo);
+
+  return boost::report_errors();
+}

initialized_test_fail1.cpp

@@ -0,0 +1,33 @@
+// Copyright 2010, Niels Dekker.
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// Test program for boost::initialized<T>. Must fail to compile.
+//
+// Initial: 2 May 2010
+
+#include <boost/utility/value_init.hpp>
+
+namespace
+{
+  void direct_initialize_from_int()
+  {
+    // Okay: initialized<T> supports direct-initialization from T.
+    boost::initialized<int> direct_initialized_int(1);
+  }
+
+  void copy_initialize_from_int()
+  {
+    // The following line should not compile, because initialized<T> 
+    // was not intended to supports copy-initialization from T.
+    boost::initialized<int> copy_initialized_int = 1;
+  }
+}
+
+int main()
+{
+  // This should fail to compile, so there is no need to call any function.
+  return 0;
+}

initialized_test_fail2.cpp

@@ -0,0 +1,37 @@
+// Copyright 2010, Niels Dekker.
+//
+// Distributed under the Boost Software License, Version 1.0. (See
+// accompanying file LICENSE_1_0.txt or copy at
+// http://www.boost.org/LICENSE_1_0.txt)
+//
+// Test program for boost::initialized<T>. Must fail to compile.
+//
+// Initial: 2 May 2010
+
+#include <boost/utility/value_init.hpp>
+
+namespace
+{
+  void from_value_initialized_to_initialized()
+  {
+    boost::value_initialized<int> value_initialized_int;
+
+    // Okay: initialized<T> can be initialized by value_initialized<T>.
+    boost::initialized<int> initialized_int(value_initialized_int);
+  }
+
+  void from_initialized_to_value_initialized()
+  {
+    boost::initialized<int> initialized_int(13);
+
+    // The following line should not compile, because initialized<T>
+    // should not be convertible to value_initialized<T>.
+    boost::value_initialized<int> value_initialized_int(initialized_int);
+  }
+}
+
+int main()
+{
+  // This should fail to compile, so there is no need to call any function.
+  return 0;
+}

test/Jamfile.v2

@@ -32,9 +32,12 @@ test-suite utility
         [ compile result_of_test.cpp ]
         [ run ../shared_iterator_test.cpp ]
         [ run ../value_init_test.cpp ]
+        [ run ../initialized_test.cpp ]
         [ compile-fail ../value_init_test_fail1.cpp ]
         [ compile-fail ../value_init_test_fail2.cpp ]
         [ compile-fail ../value_init_test_fail3.cpp ]
+        [ compile-fail ../initialized_test_fail1.cpp ]
+        [ compile-fail ../initialized_test_fail2.cpp ]
         [ run ../verify_test.cpp ]
     ;
 

value_init.htm

@@ -33,6 +33,7 @@
                    
 <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>
@@ -123,6 +124,12 @@ constructed by the following declaration:
 </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>
@@ -340,6 +347,52 @@ 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( value_initialized&lt;T&gt;&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>} // 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>
@@ -399,6 +452,9 @@ Special thanks to Bj&ouml;rn Karlsson who carefully edited and completed this do
 <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>,
@@ -407,9 +463,9 @@ for Boost release version 1.35 (2008), offering a workaround to various compiler
      </p>
                     
 <hr>          
-<p>Revised 03 October 2009</p>
+<p>Revised 1 May 2010</p>
                    
-<p>&copy; Copyright Fernando Cacciola, 2002, 2009.</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>