ceph/src/boost/libs/core/test/eif_lazy.cpp

   1 // Boost enable_if library
   2
   3 // Copyright 2003 (c) The Trustees of Indiana University.
   4
   5 // Use, modification, and distribution is subject to the Boost Software
   6 // License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
   7 // http://www.boost.org/LICENSE_1_0.txt)
   8
   9 //    Authors: Jaakko Jarvi (jajarvi at osl.iu.edu)
  10 //             Jeremiah Willcock (jewillco at osl.iu.edu)
  11 //             Andrew Lumsdaine (lums at osl.iu.edu)
  12
  13 #include <boost/utility/enable_if.hpp>
  14 #include <boost/type_traits/is_same.hpp>
  15 #include <boost/detail/lightweight_test.hpp>
  16
  17 using boost::enable_if_c;
  18 using boost::lazy_enable_if_c;
  19
  20 // This class provides a reduced example of a traits class for
  21 // computing the result of multiplying two types.  The member typedef
  22 // 'type' in this traits class defines the return type of this
  23 // operator.  The return type member is invalid unless both arguments
  24 // for mult_traits are values that mult_traits expects (ints in this
  25 // case).  This kind of situation may arise if a traits class only
  26 // makes sense for some set of types, not all C++ types.
  27
  28 template <class T> struct is_int {
  29   BOOST_STATIC_CONSTANT(bool, value = (boost::is_same<T, int>::value));
  30 };
  31
  32 template <class T, class U>
  33 struct mult_traits {
  34   typedef typename T::does_not_exist type;
  35 };
  36
  37 template <>
  38 struct mult_traits<int, int> {
  39   typedef int type;
  40 };
  41
  42
  43 // Next, a forwarding function mult() is defined.  It is enabled only
  44 // when both arguments are of type int.  The first version, using
  45 // non-lazy enable_if_c does not work.
  46
  47 #if 0
  48 template <class T, class U>
  49 typename enable_if_c<
  50   is_int<T>::value && is_int<U>::value,
  51   typename mult_traits<T, U>::type
  52 >::type
  53 mult(const T& x, const U& y) {return x * y;}
  54 #endif
  55
  56 // A correct version uses lazy_enable_if_c.
  57 // This template removes compiler errors from invalid code used as an
  58 // argument to enable_if_c.
  59
  60 #if 1
  61 template <class T, class U>
  62 typename lazy_enable_if_c<
  63   is_int<T>::value & is_int<U>::value,
  64   mult_traits<T, U>
  65 >::type
  66 mult(const T& x, const U& y) {return x * y;}
  67 #endif
  68
  69 double mult(int i, double d) { return (double)i * d; }
  70
  71 int main()
  72 {
  73
  74
  75   BOOST_TEST(mult(1, 2) == 2);
  76
  77   BOOST_TEST(mult(1, 3.0) == 3.0);
  78
  79   return boost::report_errors();
  80 }
  81