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enable_if.html
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enable_if.html
@@ -21,7 +21,6 @@
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<BR>
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<BR>
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Copyright 2003 Jaakko Järvi, Jeremiah Willcock, Andrew Lumsdaine.<BR>
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Copyright 2011 Matt Calabrese.<BR>
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<BR>
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<!--TOC section Introduction-->
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@@ -82,7 +81,7 @@ definitions to find this out. Instantiating the latter definition with
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<PRE>int::result_type negate(const int&);
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</PRE>
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where the return type is invalid. If this were an error, adding an unrelated function template
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where the return type is invalid. If this was an error, adding an unrelated function template
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(that was never called) could break otherwise valid code.
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Due to the SFINAE principle the above example is not, however, erroneous.
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The latter definition of <TT>negate</TT> is simply removed from the overload resolution set.<BR>
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@@ -155,7 +154,6 @@ typename enable_if<boost::is_arithmetic<T>, T>::type
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foo(T t) { return t; }
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</PRE>
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<!--TOC section Using <TT>enable_if</TT>-->
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<H2><A NAME="htoc5">3</A> Using <TT>enable_if</TT></H2><!--SEC END -->
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@@ -164,19 +162,8 @@ foo(T t) { return t; }
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The <TT>enable_if</TT> templates are defined in
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<TT>boost/utility/enable_if.hpp</TT>, which is included by <TT>boost/utility.hpp</TT>.<BR>
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<BR>
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With respect to function templates, <TT>enable_if</TT> can be used in multiple different ways:
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<UL>
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<LI>As the return type of an instantiatied function
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<LI>As an extra parameter of an instantiated function
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<LI>As an extra template parameter (useful only in a compiler that supports C++0x default
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arguments for function template parameters, see <A href="#sec:enable_if_0x">Enabling function
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templates in C++0x</a> for details)
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</UL>
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In the previous section, the return type form of <TT>enable_if</TT> was shown. As an example
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of using the form of <TT>enable_if</TT> that works via an extra function parameter, the
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<TT>foo</TT> function in the previous section could also be written
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The <TT>enable_if</TT> template can be used either as the return type, or as an
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extra argument. For example, the <TT>foo</TT> function in the previous section could also be written
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as:
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<PRE>template <class T>
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T foo(T t, typename enable_if<boost::is_arithmetic<T> >::type* dummy = 0);
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@@ -186,80 +173,18 @@ a default value to keep the parameter hidden from client code.
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Note that the second template argument was not given to <TT>enable_if</TT>, as the default
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<TT>void</TT> gives the desired behavior.<BR>
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<BR>
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Which way to write the enabler is largely a matter of taste, but for certain functions, only a
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subset of the options is possible:
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Whether to write the enabler as an argument or within the return type is
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largely a matter of taste, but for certain functions, only one
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alternative is possible:
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<UL><LI>
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Many operators have a fixed number of arguments, thus <TT>enable_if</TT> must be used either in the
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return type or in an extra template parameter.
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<LI>Functions that have a variadic parameter list must use either the return type form or an extra
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template parameter.
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<LI>Constructors do not have a return type so you must use either an extra function parameter or an
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extra template parameter.
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<LI>Constructors that have a variadic parameter list must an extra template parameter.
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<LI>Conversion operators can only be written with an extra template parameter.
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Operators have a fixed number of arguments, thus <TT>enable_if</TT> must be used in the return type.
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<LI>Constructors and destructors do not have a return type; an extra argument is the only option.
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<LI>There does not seem to be a way to specify an enabler for a conversion operator. Converting constructors,
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however, can have enablers as extra default arguments.
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</UL>
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<!--TOC subsection Enabling function templates in C++0x-->
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<A NAME="sec:enable_if_0x"></A>
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<H3><A NAME="htoc7">3.1</A> Enabling function templates in C++0x</H3><!--SEC END -->
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In a compiler which supports C++0x default arguments for function template parameters, you can
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enable and disable function templates by adding an additional template parameter. This approach
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works in all situations where you would use either the return type form of <TT>enable_if</TT> or
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the function parameter form, including operators, constructors, variadic function templates, and
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even overloaded conversion operations.
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As an example:
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<PRE>#include <boost/type_traits/is_arithmetic.hpp>
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#include <boost/type_traits/is_pointer.hpp>
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#include <boost/utility/enable_if.hpp>
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class test
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{
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public:
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// A constructor that works for any argument list of size 10
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template< class... T
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, typename boost::enable_if_c< sizeof...( T ) == 10, int >::type = 0
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>
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test( T&&... );
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// A conversion operation that can convert to any arithmetic type
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template< class T
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, typename boost::enable_if< boost::is_arithmetic< T >, int >::type = 0
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>
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operator T() const;
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// A conversion operation that can convert to any pointer type
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template< class T
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, typename boost::enable_if< boost::is_pointer< T >, int >::type = 0
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>
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operator T() const;
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};
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int main()
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{
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// Works
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test test_( 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 );
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// Fails as expected
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test fail_construction( 1, 2, 3, 4, 5 );
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// Works by calling the conversion operator enabled for arithmetic types
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int arithmetic_object = test_;
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// Works by calling the conversion operator enabled for pointer types
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int* pointer_object = test_;
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// Fails as expected
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struct {} fail_conversion = test_;
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}
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</PRE>
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<!--TOC subsection Enabling template class specializations-->
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<H3><A NAME="htoc7">3.2</A> Enabling template class specializations</H3><!--SEC END -->
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<H3><A NAME="htoc6">3.1</A> Enabling template class specializations</H3><!--SEC END -->
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<A NAME="sec:enable_if_classes"></A>
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Class template specializations can be enabled or disabled with <TT>enable_if</TT>.
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@@ -285,7 +210,7 @@ is the correct value.<BR>
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<BR>
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<!--TOC subsection Overlapping enabler conditions-->
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<H3><A NAME="htoc8">3.3</A> Overlapping enabler conditions</H3><!--SEC END -->
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<H3><A NAME="htoc7">3.2</A> Overlapping enabler conditions</H3><!--SEC END -->
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<A NAME="sec:overlapping_conditions"></A>
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Once the compiler has examined the enabling conditions and included the
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@@ -314,7 +239,7 @@ partial specializations as well.<BR>
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<BR>
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<!--TOC subsection Lazy <TT>enable_if</TT>-->
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<H3><A NAME="htoc9">3.4</A> Lazy <TT>enable_if</TT></H3><!--SEC END -->
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<H3><A NAME="htoc8">3.3</A> Lazy <TT>enable_if</TT></H3><!--SEC END -->
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<A NAME="sec:enable_if_lazy"></A>
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In some cases it is necessary to avoid instantiating part of a
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@@ -360,7 +285,7 @@ above example, <TT>is_multipliable<T, U>::value</TT> defines when
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<BR>
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<!--TOC subsection Compiler workarounds-->
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<H3><A NAME="htoc10">3.5</A> Compiler workarounds</H3><!--SEC END -->
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<H3><A NAME="htoc9">3.4</A> Compiler workarounds</H3><!--SEC END -->
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<A NAME="sec:workarounds"></A>
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Some compilers flag functions as ambiguous if the only distinguishing factor is a different
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@@ -442,9 +367,9 @@ David Vandevoorde and Nicolai M. Josuttis.
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Addison-Wesley, 2002.</DL>
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<hr/>
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<p>Copyright Jaakko Järvi<sup>*</sup>, Jeremiah Willcock<sup>*</sup>, Andrew Lumsdaine<sup>*</sup>, Matt Calabrese<BR>
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<EM>{jajarvi|jewillco|lums}@osl.iu.edu, rivorus@gmail.com</EM><BR>
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<sup>*</sup>Indiana University<BR>
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<p>Copyright Jaakko Järvi, Jeremiah Willcock and Andrew Lumsdaine<BR>
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<EM>{jajarvi|jewillco|lums}@osl.iu.edu</EM><BR>
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Indiana University<BR>
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Open Systems Lab<br/>
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Use, modification and distribution are subject to the
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Boost Software License, Version 1.0.
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@@ -461,4 +386,4 @@ or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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</EM><A HREF="http://pauillac.inria.fr/~maranget/hevea/index.html"><EM>H<FONT SIZE=2><sup>E</sup></FONT>V<FONT SIZE=2><sup>E</sup></FONT>A</EM></A><EM>.
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</EM></BLOCKQUOTE>
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</BODY>
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</HTML>
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</HTML>
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@@ -28,7 +28,7 @@ namespace detail
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inline void current_function_helper()
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{
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#if defined(__GNUC__) || (defined(__MWERKS__) && (__MWERKS__ >= 0x3000)) || (defined(__ICC) && (__ICC >= 600)) || defined(__ghs__)
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#if defined(__GNUC__) || (defined(__MWERKS__) && (__MWERKS__ >= 0x3000)) || (defined(__ICC) && (__ICC >= 600))
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# define BOOST_CURRENT_FUNCTION __PRETTY_FUNCTION__
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@@ -65,4 +65,3 @@ inline void current_function_helper()
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} // namespace boost
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#endif // #ifndef BOOST_CURRENT_FUNCTION_HPP_INCLUDED
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// See http://www.boost.org/libs/utility/operators.htm for documentation.
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// Revision History
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// 16 Dec 10 Limit warning suppression for 4284 to older versions of VC++
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// (Matthew Bradbury, fixes #4432)
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// 07 Aug 08 Added "euclidean" spelling. (Daniel Frey)
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// 03 Apr 08 Make sure "convertible to bool" is sufficient
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// for T::operator<, etc. (Daniel Frey)
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@@ -90,7 +88,7 @@
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# pragma set woff 1234
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#endif
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#if BOOST_WORKAROUND(BOOST_MSVC, < 1600)
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#if defined(BOOST_MSVC)
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# pragma warning( disable : 4284 ) // complaint about return type of
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#endif // operator-> not begin a UDT
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