From b55dc5cc24fefba9a72de2fc3ef5b3a2a5ab6bd3 Mon Sep 17 00:00:00 2001
From: John Maddock <john@johnmaddock.co.uk>
Date: Mon, 15 Dec 2003 12:01:53 +0000
Subject: [PATCH] Added true_type and false_type to aid user-defined
 specialisations.

[SVN r21269]
---
 index.html                   | 37 +++++++++++++++++++++++++++++--
 test/Jamfile                 |  2 ++
 test/udt_specialisations.cpp | 43 ++++++++++++++++++++++++++++++++++++
 3 files changed, 80 insertions(+), 2 deletions(-)
 create mode 100644 test/udt_specialisations.cpp

diff --git a/index.html b/index.html
index 72286c4..4cf5ffe 100644
--- a/index.html
+++ b/index.html
@@ -16,7 +16,7 @@
 <a href="#transformations">Transformations Between Types</a>
 <a href="#synthesized">Synthesizing Types</a>
 <a href="#function_traits">Function Traits</a>
-<a href="#headers">Type traits headers</a>
+<a href="#headers">Type traits headers</a><BR><A href="#specializations">User defined specializations</A>
 <a href="#example">Example Code</a></pre>
       <h2><a name="intro"></a>Introduction</h2>
       <p>The contents of &lt;boost/type_traits.hpp&gt; are declared in namespace boost.</p>
@@ -850,7 +850,40 @@ bool const y = boost::is_convertible&lt;D*,A*&gt;::value;     // error
          boost/type_traits/.&nbsp; So if for example some code requires 
          is_class&lt;&gt;, then just include:</p>
       <pre>&lt;boost/type_traits/is_class.hpp&gt;</pre>
-      <h2><a name="example"></a>Example code</h2>
+      <h2><A name="specializations"></A>User defined specializations</h2>
+      <P>Occationally the end user may need to provide their own specialization for one 
+         of the type traits - typically where intrinsic compiler support is required to 
+         implement a specific trait fully.&nbsp; These specializations should derive 
+         from boost::true_type or boost::false_type as appropriate:</P>
+      <PRE>#  include &lt;boost/type_traits/is_pod.hpp&gt;
+#  include &lt;boost/type_traits/is_class.hpp&gt;
+#  include &lt;boost/type_traits/is_union.hpp&gt;
+
+struct my_pod{};
+struct my_union
+{
+   char c;
+   int i;
+};
+
+namespace boost
+{
+template&lt;&gt;
+struct is_pod&lt;my_pod&gt; 
+   : public true_type{};
+template&lt;&gt;
+struct is_pod&lt;my_union&gt; 
+   : public true_type{};
+template&lt;&gt;
+struct is_union&lt;my_union&gt; 
+   : public true_type{};
+template&lt;&gt;
+struct is_class&lt;my_union&gt; 
+   : public false_type{};
+}
+
+      </PRE>
+      <H2>Example code</H2>
       <p>Type-traits comes with four example programs that illustrate some of the ways 
          in which the type traits templates may be used:</p>
       <h4>Copy_example.cpp</h4>
diff --git a/test/Jamfile b/test/Jamfile
index f6999d0..5658d8f 100644
--- a/test/Jamfile
+++ b/test/Jamfile
@@ -84,8 +84,10 @@ test-suite type_traits :
 [ type-traits-run tricky_is_enum_test.cpp ]
 [ type-traits-run tricky_partial_spec_test.cpp ]
 [ type-traits-run type_with_alignment_test.cpp ]
+[ type-traits-run udt_specialisations.cpp ]
 
 ; # type traits suite
 
 
 
+
diff --git a/test/udt_specialisations.cpp b/test/udt_specialisations.cpp
new file mode 100644
index 0000000..7554f3d
--- /dev/null
+++ b/test/udt_specialisations.cpp
@@ -0,0 +1,43 @@
+
+#include "test.hpp"
+#include "check_integral_constant.hpp"
+#ifdef TEST_STD
+#  include <type_traits>
+#else
+#  include <boost/type_traits/is_pod.hpp>
+#  include <boost/type_traits/is_class.hpp>
+#  include <boost/type_traits/is_union.hpp>
+#endif
+
+struct my_pod{};
+struct my_union
+{
+   char c;
+   int i;
+};
+
+namespace tt
+{
+template<>
+struct is_pod<my_pod> 
+   : public true_type{};
+template<>
+struct is_pod<my_union> 
+   : public true_type{};
+template<>
+struct is_union<my_union> 
+   : public true_type{};
+template<>
+struct is_class<my_union> 
+   : public false_type{};
+}
+
+TT_TEST_BEGIN(is_pod)
+
+BOOST_CHECK_INTEGRAL_CONSTANT(::tt::is_pod<my_pod>::value, true);
+BOOST_CHECK_INTEGRAL_CONSTANT(::tt::is_pod<my_union>::value, true);
+BOOST_CHECK_INTEGRAL_CONSTANT(::tt::is_union<my_union>::value, true);
+BOOST_CHECK_INTEGRAL_CONSTANT(::tt::is_class<my_union>::value, false);
+
+TT_TEST_END
+