// Copyright (C) 2011 - 2012 Andrzej Krzemienski. // // 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) // // The idea and interface is based on Boost.Optional library // authored by Fernando Luis Cacciola Carballal # ifndef ___OPTIONAL_HPP___ # define ___OPTIONAL_HPP___ # include # include # include # include # include # include # include # define TR2_OPTIONAL_REQUIRES(...) typename enable_if<__VA_ARGS__::value, bool>::type = false # if defined __GNUC__ // NOTE: GNUC is also defined for Clang # if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 8) # define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___ # elif (__GNUC__ > 4) # define TR2_OPTIONAL_GCC_4_8_AND_HIGHER___ # endif # # if (__GNUC__ == 4) && (__GNUC_MINOR__ >= 7) # define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___ # elif (__GNUC__ > 4) # define TR2_OPTIONAL_GCC_4_7_AND_HIGHER___ # endif # # if (__GNUC__ == 4) && (__GNUC_MINOR__ == 8) && (__GNUC_PATCHLEVEL__ >= 1) # define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___ # elif (__GNUC__ == 4) && (__GNUC_MINOR__ >= 9) # define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___ # elif (__GNUC__ > 4) # define TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___ # endif # endif # # if defined __clang_major__ # if (__clang_major__ == 3 && __clang_minor__ >= 5) # define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ # elif (__clang_major__ > 3) # define TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ # endif # if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ # define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_ # elif (__clang_major__ == 3 && __clang_minor__ == 4 && __clang_patchlevel__ >= 2) # define TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_ # endif # endif # # if defined _MSC_VER # if (_MSC_VER >= 1900) # define TR2_OPTIONAL_MSVC_2015_AND_HIGHER___ # endif # endif # if defined __clang__ # if (__clang_major__ > 2) || (__clang_major__ == 2) && (__clang_minor__ >= 9) # define OPTIONAL_HAS_THIS_RVALUE_REFS 1 # else # define OPTIONAL_HAS_THIS_RVALUE_REFS 0 # endif # elif defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___ # define OPTIONAL_HAS_THIS_RVALUE_REFS 1 # elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___ # define OPTIONAL_HAS_THIS_RVALUE_REFS 1 # else # define OPTIONAL_HAS_THIS_RVALUE_REFS 0 # endif # if defined TR2_OPTIONAL_GCC_4_8_1_AND_HIGHER___ # define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 1 # define OPTIONAL_CONSTEXPR_INIT_LIST constexpr # else # define OPTIONAL_HAS_CONSTEXPR_INIT_LIST 0 # define OPTIONAL_CONSTEXPR_INIT_LIST # endif # if defined TR2_OPTIONAL_CLANG_3_5_AND_HIGHTER_ && (defined __cplusplus) && (__cplusplus != 201103L) # define OPTIONAL_HAS_MOVE_ACCESSORS 1 # else # define OPTIONAL_HAS_MOVE_ACCESSORS 0 # endif # // In C++11 constexpr implies const, so we need to make non-const members also non-constexpr # if (defined __cplusplus) && (__cplusplus == 201103L) # define OPTIONAL_MUTABLE_CONSTEXPR # else # define OPTIONAL_MUTABLE_CONSTEXPR constexpr # endif namespace std{ namespace experimental{ // BEGIN workaround for missing is_trivially_destructible # if defined TR2_OPTIONAL_GCC_4_8_AND_HIGHER___ // leave it: it is already there # elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_ // leave it: it is already there # elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___ // leave it: it is already there # elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS // leave it: the user doesn't want it # else template using is_trivially_destructible = std::has_trivial_destructor; # endif // END workaround for missing is_trivially_destructible # if (defined TR2_OPTIONAL_GCC_4_7_AND_HIGHER___) // leave it; our metafunctions are already defined. # elif defined TR2_OPTIONAL_CLANG_3_4_2_AND_HIGHER_ // leave it; our metafunctions are already defined. # elif defined TR2_OPTIONAL_MSVC_2015_AND_HIGHER___ // leave it: it is already there # elif defined TR2_OPTIONAL_DISABLE_EMULATION_OF_TYPE_TRAITS // leave it: the user doesn't want it # else // workaround for missing traits in GCC and CLANG template struct is_nothrow_move_constructible { constexpr static bool value = std::is_nothrow_constructible::value; }; template struct is_assignable { template constexpr static bool has_assign(...) { return false; } template () = std::declval(), true)) > // the comma operator is necessary for the cases where operator= returns void constexpr static bool has_assign(bool) { return true; } constexpr static bool value = has_assign(true); }; template struct is_nothrow_move_assignable { template struct has_nothrow_move_assign { constexpr static bool value = false; }; template struct has_nothrow_move_assign { constexpr static bool value = noexcept( std::declval() = std::declval() ); }; constexpr static bool value = has_nothrow_move_assign::value>::value; }; // end workaround # endif // 20.5.4, optional for object types template class optional; // 20.5.5, optional for lvalue reference types template class optional; // workaround: std utility functions aren't constexpr yet template inline constexpr T&& constexpr_forward(typename std::remove_reference::type& t) noexcept { return static_cast(t); } template inline constexpr T&& constexpr_forward(typename std::remove_reference::type&& t) noexcept { static_assert(!std::is_lvalue_reference::value, "!!"); return static_cast(t); } template inline constexpr typename std::remove_reference::type&& constexpr_move(T&& t) noexcept { return static_cast::type&&>(t); } #if defined NDEBUG # define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) (EXPR) #else # define TR2_OPTIONAL_ASSERTED_EXPRESSION(CHECK, EXPR) ((CHECK) ? (EXPR) : ([]{assert(!#CHECK);}(), (EXPR))) #endif namespace detail_ { // static_addressof: a constexpr version of addressof template struct has_overloaded_addressof { template constexpr static bool has_overload(...) { return false; } template ().operator&()) > constexpr static bool has_overload(bool) { return true; } constexpr static bool value = has_overload(true); }; template )> constexpr T* static_addressof(T& ref) { return &ref; } template )> T* static_addressof(T& ref) { return std::addressof(ref); } // the call to convert(b) has return type A and converts b to type A iff b decltype(b) is implicitly convertible to A template U convert(U v) { return v; } } // namespace detail constexpr struct trivial_init_t{} trivial_init{}; // 20.5.6, In-place construction constexpr struct in_place_t{} in_place{}; // 20.5.7, Disengaged state indicator struct nullopt_t { struct init{}; constexpr explicit nullopt_t(init){} }; constexpr nullopt_t nullopt{nullopt_t::init()}; // 20.5.8, class bad_optional_access class bad_optional_access : public logic_error { public: explicit bad_optional_access(const string& what_arg) : logic_error{what_arg} {} explicit bad_optional_access(const char* what_arg) : logic_error{what_arg} {} }; template union storage_t { unsigned char dummy_; T value_; constexpr storage_t( trivial_init_t ) noexcept : dummy_() {}; template constexpr storage_t( Args&&... args ) : value_(constexpr_forward(args)...) {} ~storage_t(){} }; template union constexpr_storage_t { unsigned char dummy_; T value_; constexpr constexpr_storage_t( trivial_init_t ) noexcept : dummy_() {}; template constexpr constexpr_storage_t( Args&&... args ) : value_(constexpr_forward(args)...) {} ~constexpr_storage_t() = default; }; template struct optional_base { bool init_; storage_t storage_; constexpr optional_base() noexcept : init_(false), storage_(trivial_init) {}; explicit constexpr optional_base(const T& v) : init_(true), storage_(v) {} explicit constexpr optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {} template explicit optional_base(in_place_t, Args&&... args) : init_(true), storage_(constexpr_forward(args)...) {} template >)> explicit optional_base(in_place_t, std::initializer_list il, Args&&... args) : init_(true), storage_(il, std::forward(args)...) {} ~optional_base() { if (init_) storage_.value_.T::~T(); } }; template struct constexpr_optional_base { bool init_; constexpr_storage_t storage_; constexpr constexpr_optional_base() noexcept : init_(false), storage_(trivial_init) {}; explicit constexpr constexpr_optional_base(const T& v) : init_(true), storage_(v) {} explicit constexpr constexpr_optional_base(T&& v) : init_(true), storage_(constexpr_move(v)) {} template explicit constexpr constexpr_optional_base(in_place_t, Args&&... args) : init_(true), storage_(constexpr_forward(args)...) {} template >)> OPTIONAL_CONSTEXPR_INIT_LIST explicit constexpr_optional_base(in_place_t, std::initializer_list il, Args&&... args) : init_(true), storage_(il, std::forward(args)...) {} ~constexpr_optional_base() = default; }; template using OptionalBase = typename std::conditional< is_trivially_destructible::value, constexpr_optional_base, optional_base >::type; template class optional : private OptionalBase { static_assert( !std::is_same::type, nullopt_t>::value, "bad T" ); static_assert( !std::is_same::type, in_place_t>::value, "bad T" ); constexpr bool initialized() const noexcept { return OptionalBase::init_; } T* dataptr() { return std::addressof(OptionalBase::storage_.value_); } constexpr const T* dataptr() const { return detail_::static_addressof(OptionalBase::storage_.value_); } # if OPTIONAL_HAS_THIS_RVALUE_REFS == 1 constexpr const T& contained_val() const& { return OptionalBase::storage_.value_; } # if OPTIONAL_HAS_MOVE_ACCESSORS == 1 OPTIONAL_MUTABLE_CONSTEXPR T&& contained_val() && { return std::move(OptionalBase::storage_.value_); } OPTIONAL_MUTABLE_CONSTEXPR T& contained_val() & { return OptionalBase::storage_.value_; } # else T& contained_val() & { return OptionalBase::storage_.value_; } T&& contained_val() && { return std::move(OptionalBase::storage_.value_); } # endif # else constexpr const T& contained_val() const { return OptionalBase::storage_.value_; } T& contained_val() { return OptionalBase::storage_.value_; } # endif void clear() noexcept { if (initialized()) dataptr()->T::~T(); OptionalBase::init_ = false; } template void initialize(Args&&... args) noexcept(noexcept(T(std::forward(args)...))) { assert(!OptionalBase::init_); ::new (static_cast(dataptr())) T(std::forward(args)...); OptionalBase::init_ = true; } template void initialize(std::initializer_list il, Args&&... args) noexcept(noexcept(T(il, std::forward(args)...))) { assert(!OptionalBase::init_); ::new (static_cast(dataptr())) T(il, std::forward(args)...); OptionalBase::init_ = true; } public: typedef T value_type; // 20.5.5.1, constructors constexpr optional() noexcept : OptionalBase() {}; constexpr optional(nullopt_t) noexcept : OptionalBase() {}; optional(const optional& rhs) : OptionalBase() { if (rhs.initialized()) { ::new (static_cast(dataptr())) T(*rhs); OptionalBase::init_ = true; } } optional(optional&& rhs) noexcept(is_nothrow_move_constructible::value) : OptionalBase() { if (rhs.initialized()) { ::new (static_cast(dataptr())) T(std::move(*rhs)); OptionalBase::init_ = true; } } constexpr optional(const T& v) : OptionalBase(v) {} constexpr optional(T&& v) : OptionalBase(constexpr_move(v)) {} template explicit constexpr optional(in_place_t, Args&&... args) : OptionalBase(in_place_t{}, constexpr_forward(args)...) {} template >)> OPTIONAL_CONSTEXPR_INIT_LIST explicit optional(in_place_t, std::initializer_list il, Args&&... args) : OptionalBase(in_place_t{}, il, constexpr_forward(args)...) {} // 20.5.4.2, Destructor ~optional() = default; // 20.5.4.3, assignment optional& operator=(nullopt_t) noexcept { clear(); return *this; } optional& operator=(const optional& rhs) { if (initialized() == true && rhs.initialized() == false) clear(); else if (initialized() == false && rhs.initialized() == true) initialize(*rhs); else if (initialized() == true && rhs.initialized() == true) contained_val() = *rhs; return *this; } optional& operator=(optional&& rhs) noexcept(is_nothrow_move_assignable::value && is_nothrow_move_constructible::value) { if (initialized() == true && rhs.initialized() == false) clear(); else if (initialized() == false && rhs.initialized() == true) initialize(std::move(*rhs)); else if (initialized() == true && rhs.initialized() == true) contained_val() = std::move(*rhs); return *this; } template auto operator=(U&& v) -> typename enable_if < is_same::type, T>::value, optional& >::type { if (initialized()) { contained_val() = std::forward(v); } else { initialize(std::forward(v)); } return *this; } template void emplace(Args&&... args) { clear(); initialize(std::forward(args)...); } template void emplace(initializer_list il, Args&&... args) { clear(); initialize(il, std::forward(args)...); } // 20.5.4.4, Swap void swap(optional& rhs) noexcept(is_nothrow_move_constructible::value && noexcept(swap(declval(), declval()))) { if (initialized() == true && rhs.initialized() == false) { rhs.initialize(std::move(**this)); clear(); } else if (initialized() == false && rhs.initialized() == true) { initialize(std::move(*rhs)); rhs.clear(); } else if (initialized() == true && rhs.initialized() == true) { using std::swap; swap(**this, *rhs); } } // 20.5.4.5, Observers explicit constexpr operator bool() const noexcept { return initialized(); } constexpr T const* operator ->() const { return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), dataptr()); } # if OPTIONAL_HAS_MOVE_ACCESSORS == 1 OPTIONAL_MUTABLE_CONSTEXPR T* operator ->() { assert (initialized()); return dataptr(); } constexpr T const& operator *() const& { return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val()); } OPTIONAL_MUTABLE_CONSTEXPR T& operator *() & { assert (initialized()); return contained_val(); } OPTIONAL_MUTABLE_CONSTEXPR T&& operator *() && { assert (initialized()); return constexpr_move(contained_val()); } constexpr T const& value() const& { return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val()); } OPTIONAL_MUTABLE_CONSTEXPR T& value() & { return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val()); } OPTIONAL_MUTABLE_CONSTEXPR T&& value() && { if (!initialized()) throw bad_optional_access("bad optional access"); return std::move(contained_val()); } # else T* operator ->() { assert (initialized()); return dataptr(); } constexpr T const& operator *() const { return TR2_OPTIONAL_ASSERTED_EXPRESSION(initialized(), contained_val()); } T& operator *() { assert (initialized()); return contained_val(); } constexpr T const& value() const { return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val()); } T& value() { return initialized() ? contained_val() : (throw bad_optional_access("bad optional access"), contained_val()); } # endif # if OPTIONAL_HAS_THIS_RVALUE_REFS == 1 template constexpr T value_or(V&& v) const& { return *this ? **this : detail_::convert(constexpr_forward(v)); } # if OPTIONAL_HAS_MOVE_ACCESSORS == 1 template OPTIONAL_MUTABLE_CONSTEXPR T value_or(V&& v) && { return *this ? constexpr_move(const_cast&>(*this).contained_val()) : detail_::convert(constexpr_forward(v)); } # else template T value_or(V&& v) && { return *this ? constexpr_move(const_cast&>(*this).contained_val()) : detail_::convert(constexpr_forward(v)); } # endif # else template constexpr T value_or(V&& v) const { return *this ? **this : detail_::convert(constexpr_forward(v)); } # endif }; template class optional { static_assert( !std::is_same::value, "bad T" ); static_assert( !std::is_same::value, "bad T" ); T* ref; public: // 20.5.5.1, construction/destruction constexpr optional() noexcept : ref(nullptr) {} constexpr optional(nullopt_t) noexcept : ref(nullptr) {} constexpr optional(T& v) noexcept : ref(detail_::static_addressof(v)) {} optional(T&&) = delete; constexpr optional(const optional& rhs) noexcept : ref(rhs.ref) {} explicit constexpr optional(in_place_t, T& v) noexcept : ref(detail_::static_addressof(v)) {} explicit optional(in_place_t, T&&) = delete; ~optional() = default; // 20.5.5.2, mutation optional& operator=(nullopt_t) noexcept { ref = nullptr; return *this; } // optional& operator=(const optional& rhs) noexcept { // ref = rhs.ref; // return *this; // } // optional& operator=(optional&& rhs) noexcept { // ref = rhs.ref; // return *this; // } template auto operator=(U&& rhs) noexcept -> typename enable_if < is_same::type, optional>::value, optional& >::type { ref = rhs.ref; return *this; } template auto operator=(U&& rhs) noexcept -> typename enable_if < !is_same::type, optional>::value, optional& >::type = delete; void emplace(T& v) noexcept { ref = detail_::static_addressof(v); } void emplace(T&&) = delete; void swap(optional& rhs) noexcept { std::swap(ref, rhs.ref); } // 20.5.5.3, observers constexpr T* operator->() const { return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, ref); } constexpr T& operator*() const { return TR2_OPTIONAL_ASSERTED_EXPRESSION(ref, *ref); } constexpr T& value() const { return ref ? *ref : (throw bad_optional_access("bad optional access"), *ref); } explicit constexpr operator bool() const noexcept { return ref != nullptr; } template constexpr typename decay::type value_or(V&& v) const { return *this ? **this : detail_::convert::type>(constexpr_forward(v)); } }; template class optional { static_assert( sizeof(T) == 0, "optional rvalue references disallowed" ); }; // 20.5.8, Relational operators template constexpr bool operator==(const optional& x, const optional& y) { return bool(x) != bool(y) ? false : bool(x) == false ? true : *x == *y; } template constexpr bool operator!=(const optional& x, const optional& y) { return !(x == y); } template constexpr bool operator<(const optional& x, const optional& y) { return (!y) ? false : (!x) ? true : *x < *y; } template constexpr bool operator>(const optional& x, const optional& y) { return (y < x); } template constexpr bool operator<=(const optional& x, const optional& y) { return !(y < x); } template constexpr bool operator>=(const optional& x, const optional& y) { return !(x < y); } // 20.5.9, Comparison with nullopt template constexpr bool operator==(const optional& x, nullopt_t) noexcept { return (!x); } template constexpr bool operator==(nullopt_t, const optional& x) noexcept { return (!x); } template constexpr bool operator!=(const optional& x, nullopt_t) noexcept { return bool(x); } template constexpr bool operator!=(nullopt_t, const optional& x) noexcept { return bool(x); } template constexpr bool operator<(const optional&, nullopt_t) noexcept { return false; } template constexpr bool operator<(nullopt_t, const optional& x) noexcept { return bool(x); } template constexpr bool operator<=(const optional& x, nullopt_t) noexcept { return (!x); } template constexpr bool operator<=(nullopt_t, const optional&) noexcept { return true; } template constexpr bool operator>(const optional& x, nullopt_t) noexcept { return bool(x); } template constexpr bool operator>(nullopt_t, const optional&) noexcept { return false; } template constexpr bool operator>=(const optional&, nullopt_t) noexcept { return true; } template constexpr bool operator>=(nullopt_t, const optional& x) noexcept { return (!x); } // 20.5.10, Comparison with T template constexpr bool operator==(const optional& x, const T& v) { return bool(x) ? *x == v : false; } template constexpr bool operator==(const T& v, const optional& x) { return bool(x) ? v == *x : false; } template constexpr bool operator!=(const optional& x, const T& v) { return bool(x) ? *x != v : true; } template constexpr bool operator!=(const T& v, const optional& x) { return bool(x) ? v != *x : true; } template constexpr bool operator<(const optional& x, const T& v) { return bool(x) ? *x < v : true; } template constexpr bool operator>(const T& v, const optional& x) { return bool(x) ? v > *x : true; } template constexpr bool operator>(const optional& x, const T& v) { return bool(x) ? *x > v : false; } template constexpr bool operator<(const T& v, const optional& x) { return bool(x) ? v < *x : false; } template constexpr bool operator>=(const optional& x, const T& v) { return bool(x) ? *x >= v : false; } template constexpr bool operator<=(const T& v, const optional& x) { return bool(x) ? v <= *x : false; } template constexpr bool operator<=(const optional& x, const T& v) { return bool(x) ? *x <= v : true; } template constexpr bool operator>=(const T& v, const optional& x) { return bool(x) ? v >= *x : true; } // Comparison of optional with T template constexpr bool operator==(const optional& x, const T& v) { return bool(x) ? *x == v : false; } template constexpr bool operator==(const T& v, const optional& x) { return bool(x) ? v == *x : false; } template constexpr bool operator!=(const optional& x, const T& v) { return bool(x) ? *x != v : true; } template constexpr bool operator!=(const T& v, const optional& x) { return bool(x) ? v != *x : true; } template constexpr bool operator<(const optional& x, const T& v) { return bool(x) ? *x < v : true; } template constexpr bool operator>(const T& v, const optional& x) { return bool(x) ? v > *x : true; } template constexpr bool operator>(const optional& x, const T& v) { return bool(x) ? *x > v : false; } template constexpr bool operator<(const T& v, const optional& x) { return bool(x) ? v < *x : false; } template constexpr bool operator>=(const optional& x, const T& v) { return bool(x) ? *x >= v : false; } template constexpr bool operator<=(const T& v, const optional& x) { return bool(x) ? v <= *x : false; } template constexpr bool operator<=(const optional& x, const T& v) { return bool(x) ? *x <= v : true; } template constexpr bool operator>=(const T& v, const optional& x) { return bool(x) ? v >= *x : true; } // Comparison of optional with T template constexpr bool operator==(const optional& x, const T& v) { return bool(x) ? *x == v : false; } template constexpr bool operator==(const T& v, const optional& x) { return bool(x) ? v == *x : false; } template constexpr bool operator!=(const optional& x, const T& v) { return bool(x) ? *x != v : true; } template constexpr bool operator!=(const T& v, const optional& x) { return bool(x) ? v != *x : true; } template constexpr bool operator<(const optional& x, const T& v) { return bool(x) ? *x < v : true; } template constexpr bool operator>(const T& v, const optional& x) { return bool(x) ? v > *x : true; } template constexpr bool operator>(const optional& x, const T& v) { return bool(x) ? *x > v : false; } template constexpr bool operator<(const T& v, const optional& x) { return bool(x) ? v < *x : false; } template constexpr bool operator>=(const optional& x, const T& v) { return bool(x) ? *x >= v : false; } template constexpr bool operator<=(const T& v, const optional& x) { return bool(x) ? v <= *x : false; } template constexpr bool operator<=(const optional& x, const T& v) { return bool(x) ? *x <= v : true; } template constexpr bool operator>=(const T& v, const optional& x) { return bool(x) ? v >= *x : true; } // 20.5.12, Specialized algorithms template void swap(optional& x, optional& y) noexcept(noexcept(x.swap(y))) { x.swap(y); } template constexpr optional::type> make_optional(T&& v) { return optional::type>(constexpr_forward(v)); } template constexpr optional make_optional(reference_wrapper v) { return optional(v.get()); } } // namespace experimental } // namespace std namespace std { template struct hash> { typedef typename hash::result_type result_type; typedef std::experimental::optional argument_type; constexpr result_type operator()(argument_type const& arg) const { return arg ? std::hash{}(*arg) : result_type{}; } }; template struct hash> { typedef typename hash::result_type result_type; typedef std::experimental::optional argument_type; constexpr result_type operator()(argument_type const& arg) const { return arg ? std::hash{}(*arg) : result_type{}; } }; } # undef TR2_OPTIONAL_REQUIRES # undef TR2_OPTIONAL_ASSERTED_EXPRESSION # endif //___OPTIONAL_HPP___