ceph/src/boost/libs/hana/example/misc/overload_linearly.cpp

   1 // Copyright Louis Dionne 2013-2017
   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/equal.hpp>
   7 #include <boost/hana/find_if.hpp>
   8 #include <boost/hana/optional.hpp>
   9 #include <boost/hana/transform.hpp>
  10 #include <boost/hana/tuple.hpp>
  11 #include <boost/hana/type.hpp>
  12
  13 #include <string>
  14 namespace hana = boost::hana;
  15
  16
  17 // We have an utility in the Functional module that does pretty much the
  18 // same, but is more compile-time efficient. It is still interesting to
  19 // see this implemented with sequences and the SFINAE combinator.
  20
  21 auto overload_linearly = [](auto ...candidates) {
  22     return [=](auto ...args) {
  23         auto maybe_function = hana::find_if(hana::make_tuple(candidates...), [=](auto f) {
  24             return hana::is_valid(f)(args...);
  25         });
  26         auto result = hana::transform(maybe_function, [=](auto f) {
  27             return f(args...);
  28         });
  29         return result;
  30     };
  31 };
  32
  33 int main() {
  34     auto f = ::overload_linearly(
  35         [](std::string s) { return s + "abcd"; },
  36         [](int i) { return i + 1; },
  37         [](double f) { return f + 2; }
  38     );
  39
  40     BOOST_HANA_RUNTIME_CHECK(f(1) == hana::just(1 + 1));
  41     BOOST_HANA_RUNTIME_CHECK(f(2.3) == hana::just(static_cast<int>(2.3) + 1));
  42 }