ceph/src/boost/boost/poly_collection/detail/is_likely_stateless_lambda.hpp

   1 /* Copyright 2017 Joaquin M Lopez Munoz.
   2  * Distributed under the Boost Software License, Version 1.0.
   3  * (See accompanying file LICENSE_1_0.txt or copy at
   4  * http://www.boost.org/LICENSE_1_0.txt)
   5  *
   6  * See http://www.boost.org/libs/poly_collection for library home page.
   7  */
   8
   9 #ifndef BOOST_POLY_COLLECTION_DETAIL_IS_LIKELY_STATELESS_LAMBDA_HPP
  10 #define BOOST_POLY_COLLECTION_DETAIL_IS_LIKELY_STATELESS_LAMBDA_HPP
  11
  12 #if defined(_MSC_VER)
  13 #pragma once
  14 #endif
  15
  16 #include <type_traits>
  17
  18 namespace boost{
  19
  20 namespace poly_collection{
  21
  22 namespace detail{
  23
  24 /* Stateless lambda expressions have one (and only one) call operator and are
  25  * convertible to a function pointer with the same signature. Non-lambda types
  26  * could satisfy this too, hence the "likely" qualifier.
  27  */
  28
  29 template<typename T>
  30 struct has_one_operator_call_helper
  31 {
  32   template<typename Q> static std::true_type  test(decltype(&Q::operator())*);
  33   template<typename>   static std::false_type test(...);
  34
  35   using type=decltype(test<T>(nullptr));
  36 };
  37
  38 template<typename T>
  39 using has_one_operator_call=typename has_one_operator_call_helper<T>::type;
  40
  41 template<typename T>
  42 struct equivalent_function_pointer
  43 {
  44   template<typename Q,typename R,typename... Args>
  45   static auto helper(R (Q::*)(Args...)const)->R(*)(Args...);
  46   template<typename Q,typename R,typename... Args>
  47   static auto helper(R (Q::*)(Args...))->R(*)(Args...);
  48
  49   using type=decltype(helper(&T::operator()));
  50 };
  51
  52 template<typename T,typename=void>
  53 struct is_likely_stateless_lambda:std::false_type{};
  54
  55 template<typename T>
  56 struct is_likely_stateless_lambda<
  57   T,
  58   typename std::enable_if<has_one_operator_call<T>::value>::type
  59 >:std::is_convertible<
  60   T,
  61   typename equivalent_function_pointer<T>::type
  62 >{};
  63
  64 } /* namespace poly_collection::detail */
  65
  66 } /* namespace poly_collection */
  67
  68 } /* namespace boost */
  69
  70 #endif