ceph/src/boost/libs/hana/example/core/default.cpp

   1 // Copyright Louis Dionne 2013-2016
   2 // Distributed under the Boost Software License, Version 1.0.
   3 // (See accompanying file LICENSE.md or copy at http://boost.org/LICENSE_1_0.txt)
   4
   5 #include <boost/hana/assert.hpp>
   6 #include <boost/hana/core/default.hpp>
   7 #include <boost/hana/core/tag_of.hpp>
   8
   9 #include <algorithm>
  10 #include <iterator>
  11 #include <sstream>
  12 #include <vector>
  13 namespace hana = boost::hana;
  14
  15
  16 // In the header defining the concept of a Printable
  17 template <typename T>
  18 struct print_impl : hana::default_ {
  19     template <typename Stream, typename X>
  20     static void apply(Stream& out, X const& x)
  21     { out << x; }
  22 };
  23
  24 auto print = [](auto& stream, auto const& x) {
  25     return print_impl<hana::tag_of_t<decltype(x)>>::apply(stream, x);
  26 };
  27
  28 // In some other header
  29 template <typename T>
  30 struct print_impl<std::vector<T>> {
  31     template <typename Stream>
  32     static void apply(Stream& out, std::vector<T> const& xs) {
  33         out << '[';
  34         std::copy(begin(xs), end(xs), std::ostream_iterator<T>{out, ", "});
  35         out << ']';
  36     }
  37 };
  38
  39 static_assert(hana::is_default<print_impl<int>>{}, "");
  40 static_assert(!hana::is_default<print_impl<std::vector<int>>>{}, "");
  41
  42 int main() {
  43     {
  44         std::stringstream s;
  45         print(s, std::vector<int>{1, 2, 3});
  46         BOOST_HANA_RUNTIME_CHECK(s.str() == "[1, 2, 3, ]");
  47     }
  48
  49     {
  50         std::stringstream s;
  51         print(s, "abcd");
  52         BOOST_HANA_RUNTIME_CHECK(s.str() == "abcd");
  53     }
  54 }