Boost.Variant tips

#PPTU7BSJBOUUJQT 5XJUUFS!-JOEB@QQ (JUIVC!SIZTE

CPPTUWBSJBOUͱ͸ w ܕ҆શͳڞ༻ମʢVOJPOʣ w /FWFSFNQUZHVBSBOUFF ! ! ! ! !
w IUUQTTJUFTHPPHMFDPNTJUFCPPTUKQUJQTWBSJBOU 1 #include <string>! 2 #include <boost/variant/variant.hpp>! 3 ! 4 int main()! 5 {! 6 // int ͱ string ͷͲͪΒ΋֨ೲՄೳ! 7 boost::variant<int, std::string> v;! 8 ! 9 v = 42;! 10 v = "aaa";! 11 ! 12 return 0;! 13 }

WBSJBOU΁ͷΞΫηεɿHFU 1 boost::variant<int, std::string> v;! 2 ! 3 switch(v.which()) {!
4 // 1ͭ໨ͷཁૉ͕ೖ͍ͬͯΔͱ͖! 5 case 0:! 6 int i = boost::get<int>(v); // ࣦഊ͢Δͱྫ֎! 7 break;! 8 ! 9 // 2ͭ໨ͷཁૉ͕ೖ͍ͬͯΔͱ͖! 10 case 1:! 11 // ϙΠϯλΞΫηε! 12 std::string *d = boost::get<double>(&v)! 13 break;! 14 }

CPPTUHFUͷ໰୊఺ w XIJDI Ͱࣄલʹத਎Λௐ΂ΔˠϚδοΫφϯόʔ w ϙΠϯλόʔδϣϯˠੜϙΠϯλ৮Γͨ͘ͳ͍ w ྫ֎όʔδϣϯˠຖ౓ྫ֎͕ඈͿͷ͸ʜ w HFU5
ͷ5͸WBSJBOU͕อ࣋͞Ε͍ͯͳ͍ܕͰ΋ ίϯύΠϧ௨ͬͯ͠·͏

CPPTUHFUͷվળ 1 ! 2 // boost::optional Ͱฦ͢όʔδϣϯΛͭ͘Δ! 3 template<class T,
class Variant>! 4 boost::optional<Tconst&> get(Variant const& v);! 5 ! 6 boost::variant<int, std::string> v;! 7 ! 8 if (auto maybe_int = get<int>(v)) {! 9 auto const&value = *maybe_int;! 10 }

CPPTUHFUͷվળ 8 template<class T, class... Args>! 9 inline boost::optional<T const&>
get(boost::variant<Args...> const& v)! 10 {! 11 // Args... ͷதʹ T ͕ແ͚Ε͹Τϥʔ! 12 static_assert(! 13 is_included<T, Args...>::value,! 14 "get_as(const&): T is not included in the variant."! 15 );! 16 ! 17 T const* const ptr = boost::get<T>(&v);! 18 if (ptr) {! 19 return {*ptr};! 20 } else {! 21 return boost::none;! 22 }! 23 }

WBSJBOU΁ͷΞΫηεɿWJTJUPS 1 struct to_string! 2 : boost::static_visitor<std::string> {! 3 template<class
T>! 4 std::string operator()(T v)! 5 {! 6 return std::to_string(v);! 7 }! 8 };! 9 ! 10 int main()! 11 {! 12 boost::variant<int, double> v;! 13 auto s1 = ! ! ! ! ! boost::apply_visitor(to_string{}, v);! 14 }

WJTJUPSͷվળɿϥϜμࣜ 1 struct to_string : boost::static_visitor<std::string> {! 2 ! 3
template<class T>! 4 std::string operator()(T v)! 5 {! 6 return std::to_string(v);! 7 }! 8 };! 9 ! 10 int main()! 11 {! 12 boost::variant<int, double> v;! 13 auto s1 = boost::apply_visitor(to_string{}, v);! 14 } 1 int main()! 2 {! 3 boost::variant<int, double> v;! 4 auto s1 = apply_lambda(! 5 [](auto const& v){ return std::to_string(v); }! 6 , v! 7 );! 8 }

WJTJUPSͷվળɿϥϜμࣜ 1 template<class Lambda, class Result>! 2 struct lambda_wrapped_visitor! 3
: public boost::static_visitor<Result>! 4 , public Lambda {! 5 ! 6 lambda_wrapped_visitor(Lambda const& l) noexcept! 7 : Lambda(l)! 8 {}! 9 };! 10 ! 11 template<class Lambda, class Head, class... Tail>! 12 inline auto apply_lambda(Lambda const& l, boost::variant<Head, Tail...> const& variant)! 13 {! 14 return boost::apply_visitor(! 15 detail::lambda_wrapped_visitor<! 16 Lambda,! 17 decltype(std::declval<Lambda>()(std::declval<Head const&>()))! 18 >{l}! 19 , variant);! 20 }

ཁૉҎ্ͷWBSJBOU 1 using holder =! 2 boost::mpl::vector< Type1! 3 ...!
4 , Type40 >;! 5 ! 6 typename boost::make_variant_over<holder>::type v; w σϑΥϧτཁૉ਺ɿʢWBSJBOU5 5 ʜ 5ʣ w ͨͩ͠਺Λ૿΍͗͢͠ΔͱίϯύΠϧ͕࣌ؒͻͲ͘ ͳΔ

UJQT·ͱΊ w CPPTUHFU ͷ໭Γ஋ΛPQUJPOBMʹͨ͠ w WJTJUPSʹϥϜμࣜΛ࢖͑ΔΑ͏ʹ w ཁૉҎ্Λ࣋ͭWBSJBOUͷͭ͘Γ͔ͨ

Boost.Variant tips

Boost.Variant tips

Linda_pp

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#PPTU7BSJBOUUJQT 5XJUUFS!-JOEB@QQ (JUIVC!SIZTE

CPPTUWBSJBOUͱ͸ w ܕ҆શͳڞ༻ମʢVOJPOʣ w /FWFSFNQUZHVBSBOUFF ! ! ! ! !

WBSJBOU΁ͷΞΫηεɿHFU 1 boost::variant<int, std::string> v;! 2 ! 3 switch(v.which()) {!

CPPTUHFUͷ໰୊఺ w XIJDI Ͱࣄલʹத਎Λௐ΂ΔˠϚδοΫφϯόʔ w ϙΠϯλόʔδϣϯˠੜϙΠϯλ৮Γͨ͘ͳ͍ w ྫ֎όʔδϣϯˠຖ౓ྫ֎͕ඈͿͷ͸ʜ w HFU5

CPPTUHFUͷվળ 1 ! 2 // boost::optional Ͱฦ͢όʔδϣϯΛͭ͘Δ! 3 template<class T,

CPPTUHFUͷվળ 8 template<class T, class... Args>! 9 inline boost::optional<T const&>

WBSJBOU΁ͷΞΫηεɿWJTJUPS 1 struct to_string! 2 : boost::static_visitor<std::string> {! 3 template<class

WJTJUPSͷվળɿϥϜμࣜ 1 struct to_string : boost::static_visitor<std::string> {! 2 ! 3

WJTJUPSͷվળɿϥϜμࣜ 1 template<class Lambda, class Result>! 2 struct lambda_wrapped_visitor! 3

ཁૉҎ্ͷWBSJBOU 1 using holder =! 2 boost::mpl::vector< Type1! 3 ...!

UJQT·ͱΊ w CPPTUHFU ͷ໭Γ஋ΛPQUJPOBMʹͨ͠ w WJTJUPSʹϥϜμࣜΛ࢖͑ΔΑ͏ʹ w ཁૉҎ্Λ࣋ͭWBSJBOUͷͭ͘Γ͔ͨ