Added enable_if library

[SVN r21090]

include/boost/utility/enable_if.hpp

@@ -0,0 +1,69 @@
+// Boost enable_if library
+
+// Copyright 2003 <20> The Trustees of Indiana University.
+
+// Use, modification, and distribution is subject to 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)
+
+//    Authors: Jaakko J<>rvi (jajarvi at osl.iu.edu)
+//             Jeremiah Willcock (jewillco at osl.iu.edu)
+//             Andrew Lumsdaine (lums at osl.iu.edu)
+
+
+#ifndef BOOST_UTILITY_ENABLE_IF_HPP
+#define BOOST_UTILITY_ENABLE_IF_HPP
+
+namespace boost
+{
+ 
+  template <bool B, class T = void>
+  struct enable_if_c {
+    typedef T type;
+  };
+
+  template <class T>
+  struct enable_if_c<false, T> {};
+
+  template <class Cond, class T = void> 
+  struct enable_if : public enable_if_c<Cond::value, T> {};
+
+  template <bool B, class T>
+  struct lazy_enable_if_c {
+    typedef typename T::type type;
+  };
+
+  template <class T>
+  struct lazy_enable_if_c<false, T> {};
+
+  template <class Cond, class T> 
+  struct lazy_enable_if : public lazy_enable_if_c<Cond::value, T> {};
+
+
+  template <bool B, class T = void>
+  struct disable_if_c {
+    typedef T type;
+  };
+
+  template <class T>
+  struct disable_if_c<true, T> {};
+
+  template <class Cond, class T = void> 
+  struct disable_if : public disable_if_c<Cond::value, T> {};
+
+  template <bool B, class T>
+  struct lazy_disable_if_c {
+    typedef typename T::type type;
+  };
+
+  template <class T>
+  struct lazy_disable_if_c<true, T> {};
+
+  template <class Cond, class T> 
+  struct lazy_disable_if : public lazy_disable_if_c<Cond::value, T> {};
+
+
+  
+} // namespace boost
+
+#endif