{"id":149169,"date":"2020-08-18T17:09:27","date_gmt":"2020-08-18T09:09:27","guid":{"rendered":"http:\/\/4563.org\/?p=149169"},"modified":"2020-08-18T17:09:27","modified_gmt":"2020-08-18T09:09:27","slug":"clang%ef%bc%8c-msvc-%e5%8f%af%e4%bb%a5%e7%bc%96%e8%af%91%e9%80%9a%e8%bf%87%ef%bc%8c-gcc-%e4%b8%8d%e8%a1%8c","status":"publish","type":"post","link":"http:\/\/4563.org\/?p=149169","title":{"rendered":"clang\uff0c msvc \u53ef\u4ee5\u7f16\u8bd1\u901a\u8fc7\uff0c gcc \u4e0d\u884c"},"content":{"rendered":"
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clang\uff0c msvc \u53ef\u4ee5\u7f16\u8bd1\u901a\u8fc7\uff0c gcc \u4e0d\u884c <\/h1>\n

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\u8cc7\u6df1\u5927\u4f6c : Tony042 <\/span> <\/i> 11<\/span> <\/div>\n
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\u6211\u53c8\u6765\u95ee\u95ee\u9898\u5566\uff0c\u8fd9\u56de\u8fd8\u662f\u559c\u95fb\u4e50\u89c1\u7684 C++\u95ee\u9898\uff0c\u540c\u6837\u4e00\u4efd\u4ee3\u7801 MSCVC 19.24 \uff0cclang 10 \u5747\u53ef\u7f16\u8bd1\u901a\u8fc7\uff0cg++-10 \u4e0d\u53ef\u7f16\u8bd1\u901a\u8fc7\uff0c\u8fd9\u662f\u4ec0\u4e48\u539f\u56e0\u5462\uff0c\u6ce8\u610f\uff0c\u987b\u52a0\u5165\u53c2\u6570\u8981\u6c42 compiler \u652f\u6301 C++17 \uff08-std=c++17), \u53ef\u5728\u7ebf\u7f16\u8bd1\u7248\u672c\u4ee3\u7801\u94fe\u63a5 https:\/\/godbolt.org\/z\/eWhsne\uff0c\u6e90\u7801\u5982\u4e0b\uff0c \u5176\u4e2d\u5f00\u5934\u90e8\u5206\u7684 type_traits \u5e94\u8be5\u662f\u6ca1\u95ee\u9898\u7684\uff0c\u4e3b\u8981\u95ee\u9898\u96c6\u4e2d\u5728 gcc \u5bf9 fold expression \u7684\u5904\u7406\u4e0a,\u8fd9\u4efd\u4ee3\u7801\u662f\u5bf9 std::variant \u90e8\u5206\u7b80\u5355\u4e0d\u5b8c\u5168\u5b9e\u73b0<\/p>\n

#include <type_traits> #include <utility> #include <new> #include <cassert> #include <exception>  template <bool COND, typename TrueType, typename FalseType> class IfThenElseT { public:     using Type = TrueType; };  template <typename TrueType, typename FalseType> class IfThenElseT<false, TrueType, FalseType> { public:     using Type = FalseType; };  template <bool COND, typename TrueType, typename FalseType> using IfThenElse = typename IfThenElseT<COND, TrueType, FalseType>::Type;  template <typename... Elements> class Typelist { };  template <typename List> class FrontT;  template <typename Head, typename... Tail> class FrontT<Typelist<Head, Tail...>> { public:     using Type = Head; };  template <typename List> using Front = typename FrontT<List>::Type;  template <typename List> class PopFrontT;  template <typename Head, typename... Tail> class PopFrontT<Typelist<Head, Tail...>> { public:     using Type = Typelist<Tail...>; };  template <typename List> using PopFront = typename PopFrontT<List>::Type;  template <typename List> class IsEmpty { public:     static constexpr bool value = false; };  template <> class IsEmpty<Typelist<>> { public:     static constexpr bool value = true; };  template <typename List, typename T, unsigned N = 0, bool Empty = IsEmpty<List>::value> struct FindIndexOf { };  template <typename List, typename T, unsigned N> struct FindIndexOf<List, T, N, false> : public IfThenElse<std::is_same_v<Front<List>, T>,                                                           std::integral_constant<unsigned, N>,                                                           FindIndexOf<PopFront<List>, T, N + 1>> { };  template <typename List, typename T, unsigned N> struct FindIndexOf<List, T, N, true> { };  template <typename List> class LargestTypeT;  template <typename List> class LargestTypeT { private:     using First = Front<List>;     using Rest = typename LargestTypeT<PopFront<List>>::Type;  public:     using Type = IfThenElse<(sizeof(First) >= sizeof(Rest)), First, Rest>; };  template <> class LargestTypeT<Typelist<>> { public:     using Type = char; };  template <typename List> using LargestType = typename LargestTypeT<List>::Type;  template <typename... Types> class VariantStorage { private:     using LargestT = LargestType<Typelist<Types...>>;     alignas(Types...) unsigned char buffer[sizeof(LargestT)];     unsigned char discriminator = 0;  public:     unsigned char getDiscriminator() { return discriminator; }     void setDiscriminator(unsigned char d) { discriminator = d; }     void *getRawBuffer() { return buffer; }     void const *getRawBuffer() const { return buffer; }      template <typename T>     T *getBufferAs() { return std::launder(reinterpret_cast<T *>(buffer)); }      template <typename T>     T const *getBufferAs() const { return std::launder(reinterpret_cast<T const *>(buffer)); } };  template <typename... Types> class Variant;  template <typename T, typename... Types> class VariantChoice { private:     using Derived = Variant<Types...>;     Derived &getDerived() { return *static_cast<Derived *>(this); }     Derived const &getDerived() const { return *static_cast<Derived const *>(this); }  protected:     constexpr static unsigned Discriminator = FindIndexOf<Typelist<Types...>, T>::value + 1;  public:     VariantChoice() = default;     VariantChoice(T const &value);     VariantChoice(T &&value);     bool destroy();     Derived &operator=(T const &value);     Derived &operator=(T &&value); };  template <typename T, typename... Types> VariantChoice<T, Types...>::VariantChoice(T const &value) {     new (getDerived().getRawBuffer()) T(value);     getDerived().setDiscriminator(Discriminator); }  template <typename T, typename... Types> VariantChoice<T, Types...>::VariantChoice(T &&value) {     new (getDerived().getRawBuffer()) T(std::move(value));     getDerived().setDiscriminator(Discriminator); }  template <typename T, typename... Types> bool VariantChoice<T, Types...>::destroy() {     if (getDerived().getDiscriminator() == Discriminator)     {         getDerived().template getBufferAs<T>()->~T();         return true;     }     return false; }  template <typename T, typename... Types> auto VariantChoice<T, Types...>::operator=(T const &value) -> Derived & {     if (getDerived().getDiscriminator() == Discriminator)     {         *getDerived().template getBufferAs<T>() = value;     }     else     {         getDerived().destroy();         new (getDerived().getRawBuffer()) T(value);         getDerived().setDiscriminator(Discriminator);     }     return getDerived(); }  template <typename T, typename... Types> auto VariantChoice<T, Types...>::operator=(T &&value) -> Derived & {     if (getDerived().getDiscriminator() == Discriminator)     {         *getDerived().template getBufferAs<T>() = std::move(value);     }     else     {         getDerived().destroy();         new (getDerived().getRawBuffer()) T(std::move(value));         getDerived().setDiscriminator(Discriminator);     }     return getDerived(); }  class ComputedResultType;  class EmptyVariant : public std::exception { };  template <typename... Types> class Variant : private VariantStorage<Types...>, private VariantChoice<Types, Types...>... {     template <typename T, typename... OtherTypes>     friend class VariantChoice;  public:     using VariantChoice<Types, Types...>::VariantChoice...;  \/\/g++\u62a5\u9519     using VariantChoice<Types, Types...>::operator=...;  \/\/g++\u62a5\u9519      template <typename T>     bool is() const;      template <typename T>     T &get() &;      template <typename T>     T &&get() &&;      template <typename T>     T const &get() const &;      \/\/ template <typename R=ComputedResultType, typename Visitor>     \/\/ VisitResult<R, Visitor      bool empty() const;     void destroy();     ~Variant() { destroy(); }  private: };  template <typename... Types> template <typename T> bool Variant<Types...>::is() const {     return this->getDiscriminator() == VariantChoice<T, Types...>::Discriminator; \/\/g++\u62a5\u9519 }  template <typename... Types> template <typename T> T &Variant<Types...>::get() & {     if (empty())     {         throw EmptyVariant();     }     assert(is<T>());     return *this->template getBufferAs<T>(); }  template <typename... Types> template <typename T> T &&Variant<Types...>::get() && {     if (empty())     {         throw EmptyVariant();     }     assert(is<T>());     return *this->template getBufferAs<T>(); }  template <typename... Types> template <typename T> T const &Variant<Types...>::get() const & {     if (empty())     {         throw EmptyVariant();     }     assert(is<T>());     return *this->template getBufferAs<T>(); }  template <typename... Types> void Variant<Types...>::destroy() {     \/\/ bool results[] = {VariantChoice<Types, Types...>::destroy()...};     (VariantChoice<Types, Types...>::destroy(), ...);  \/\/g++\u62a5\u9519     this->setDiscriminator(0); }  int main() {     Variant<int> v{17};     return 0; } <\/code><\/pre>\n<\/p><\/div>\n
 \u5927\u4f6c\u6709\u8a71\u8aaa<\/b> (15<\/span>)        <\/div>\n
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     \u8cc7\u6df1\u5927\u4f6c : secondwtq <\/span>  <\/div>\n
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                                                                 \u778e\u6539\u4e86\u4e0b\u628a GCC \u6539\u8fc7\u4e86 …
    \u62a5\u9519\u7684\u5730\u65b9\u5168\u52a0\u4e2a :: \uff0c\u53d8\u6210 ::VariantChoice<Types, Types…> \u5c31\u884c\u4e86                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u4e3b<\/span> \u8cc7\u6df1\u5927\u4f6c : Tony042 <\/span>  <\/div>\n
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                                                                 @secondwtq \u8bd5\u4e86\u4e00\u4e0b\u786e\u5b9e\u7f16\u8bd1\u8fc7\u53bb\u5566\uff0c\u4f30\u8ba1\u662f gcc \u5bf9\u6a21\u677f\u7c7b\u7ee7\u627f\u8303\u56f4\u67e5\u8be2\u51fa\u4e86\u504f\u5dee\uff0c\u8c22\u8c22\u6307\u5bfc~                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : secondwtq <\/span>  <\/div>\n
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                                                                 reduce \u4e86\u534a\u5929 case\uff0c\u597d\u50cf\u5220\u4e86 using VariantChoice<Types, Types…>::VariantChoice… \u8fd9\u53e5\u5c31\u6ca1\u4e8b\u4e86<\/p>\n
    \u7136\u540e\u627e\u5230\u4e86\u4fe9 GCC \u7684 bug report\uff1a
    https:\/\/gcc.gnu.org\/bugzilla\/show_bug.cgi?id=94310
    https:\/\/gcc.gnu.org\/bugzilla\/show_bug.cgi?id=79094                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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                                                                 @secondwtq \u8fd9\u4e2a\u6ca1\u6cd5\u5220\uff0c\u5220\u4e86\u4e4b\u540e\u5c31\u5f97\u624b\u5199 Variant \u7684 constructor \u4e86\uff0c\u4f30\u8ba1\u662f gcc \u7684 bug\uff0c\u770b\u6765 gcc bug \u8fd8\u662f\u76f8\u5bf9\u8f83\u591a\u7684                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : 0x11901 <\/span>  <\/div>\n
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                                                                 \u8fd9\u5c31\u662f\u4e3a\u4ec0\u4e48\u80fd\u7528 clang \u6211\u90fd\u5c3d\u91cf\u7528 clang\uff0cgcc \u771f\u7684\u4e0d\u5927\u884c                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : lewis89 <\/span>  <\/div>\n
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                                                                 @0x11901 #5 \u4e3b\u8981\u8fd8\u662f C++ \u7f16\u8bd1\u5668\u592a\u96be\u5b9e\u73b0\u4e86…                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : zsl199512101234 <\/span>  <\/div>\n
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                                                                 @lewis89 \u8fd9\u53e5\u8bdd\u771f\u8d5e                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : wutiantong <\/span>  <\/div>\n
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                                                                 \u867d\u7136\u8bf4\u4e0d\u6e05\u9519\u5728\u54ea\uff0c\u4f46\u6211\u4eec\u5728 Variant \u91cc\u8865\u4e00\u884c\u8fd9\u4e2a\uff1a
    template<typename T> using Choice = VariantChoice<T, Types…>;<\/p>\n
    \u7136\u540e\u628a\u540e\u9762\u7684 VariantChoice<XXX, Types…> \u90fd\u6362\u6210 Choice<XXX>\uff0cgcc \u5c31\u6ca1\u95ee\u9898\u4e86\u3002                                                            <\/p><\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : wutiantong <\/span>  <\/div>\n
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                                                                 \u53e6\u5916\uff0c\u6211\u4e2a\u4eba\u611f\u89c9 clang11 \u7684 Bug \u6709\u70b9\u591a\uff0c\u8fdc\u4e0d\u5982 clang10                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u4e3b<\/span> \u8cc7\u6df1\u5927\u4f6c : Tony042 <\/span>  <\/div>\n
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                                                                 @wutiantong \u989d\uff0c\u8fd9\u4e2a\u4e0d\u80fd\u8fd9\u4e48\u6539\uff0c\u8fd9\u6837\u6539\u5c31\u6269\u5927\u7ee7\u627f\u7684 VariantChoice \u7684 Constructor \u8303\u56f4\u4e86,VariantChoice<T, Types>\u4e2d\u7684 T \u5fc5\u987b\u662f parameter pack Types \u4e2d\u7684\u4e00\u4e2a\u7c7b\u578b\uff0cVariant \u672c\u8d28\u4e0a\u662f\u4e00\u4e2a\u534a\u81ea\u52a8\u7684\u88ab\u8d4b\u503c\u4e3a Types \u4e2d\u4efb\u610f\u7c7b\u578b\u7684\u53d8\u91cf\uff0c\u8fd9\u5c31\u662f\u4e3a\u4ec0\u4e48\u8981\u7528 using VariantChoice<Types, Types…>::operator=…; \uff0c\u7b2c\u4e8c\u4e2a parameter pack \u5728\u5c16\u62ec\u53f7\u91cc\u5c55\u5f00\uff0c\u7b2c\u4e00\u4e2a parameter pack \u5728\u5916\u9762\u5c55\u5f00\uff0c\u7ee7\u627f number=sizeof…(Types)\u4e2a constructor\uff0c\u672c\u8d28\u4e0a Variant \u7ee7\u627f VariantChoice \u7684 constructor \u518d\u901a\u8fc7 VariantStorage \u624b\u52a8\u7ba1\u7406\u5185\u5b58                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : wutiantong <\/span>  <\/div>\n
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                                                                 @Tony042 \u4f60\u597d\u50cf\u6ca1\u6709\u6b63\u786e\u7406\u89e3\u6211\u7684\u610f\u601d\uff0c\u56e0\u4e3a\u6211\u63d0\u5230\u7684\u6539\u52a8\u5e76\u6ca1\u6709\u6539\u53d8\u4efb\u4f55\u4ee3\u7801\u542b\u4e49\u3002\u4e0e\u5176\u6587\u5b57\u89e3\u91ca\u4e0d\u5982\u76f4\u63a5\u770b\u4e00\u4e0b\u4ee3\u7801\uff1a
    https:\/\/godbolt.org\/z\/srW9xo                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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                                                                 @wutiantong \u662f\u6211\u7406\u89e3\u7684\u6709\u504f\u5dee\uff0c\u8fd9\u6837\u6539\u662f\u6ca1\u6709\u6539\u53d8\u4ee3\u7801\u542b\u4e49\u7684\uff0c\u8c22\u8c22\u5566                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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                                                                 @wutiantong emmm \u8fd9\u6837\u6539\uff0cclang \u5c31\u7f16\u8bd1\u4e0d\u8fc7\u53bb\u4e86\u3002\u3002\u3002\u5fc3\u585e                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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                                                                 @wutiantong \u7b97\u4e86\uff0c\u6211\u5199\u5b8f\u9488\u5bf9\u4e0d\u540c\u7f16\u8bd1\u5668\uff0c\u7528\u4e0d\u540c\u4ee3\u7801\u597d\u4e86                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : wutiantong <\/span>  <\/div>\n
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                                                                 @Tony042 \u6211\u627e\u8865\u4e00\u4e0b\uff0c\u628a using base-class constructor \u90a3\u884c\u5355\u72ec\u7684\u6362\u56de\u539f\u6765\u7684\u5199\u6cd5\uff1a
    using VariantChoice<Types, Types…>::VariantChoice…;<\/p>\n
    \u8fd9\u6837 clang \u548c gcc \u5c31\u90fd\u53ef\u4ee5\u4e86\u3002                                                            <\/p><\/div>\n<\/p><\/div>\n<\/li>\n
  • \n","protected":false},"excerpt":{"rendered":"
    clang\uff0c msvc \u53ef\u4ee5\u7f16\u8bd1\u901a…<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":[],"categories":[],"tags":[],"_links":{"self":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts\/149169"}],"collection":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=149169"}],"version-history":[{"count":0,"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts\/149169\/revisions"}],"wp:attachment":[{"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=149169"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=149169"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=149169"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}