{"id":150261,"date":"2020-09-02T19:12:16","date_gmt":"2020-09-02T11:12:16","guid":{"rendered":"http:\/\/4563.org\/?p=150261"},"modified":"2020-09-02T19:12:16","modified_gmt":"2020-09-02T11:12:16","slug":"c%e5%8f%8b%e5%85%83%e5%87%bd%e6%95%b0%e9%97%ae%e9%a2%98","status":"publish","type":"post","link":"http:\/\/4563.org\/?p=150261","title":{"rendered":"C++\u53cb\u5143\u51fd\u6570\u95ee\u9898"},"content":{"rendered":"
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C++\u53cb\u5143\u51fd\u6570\u95ee\u9898 <\/h1>\n

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\u8cc7\u6df1\u5927\u4f6c : Tony042 <\/span> <\/i> 14<\/span> <\/div>\n
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\u5148\u4e0a\u4ee3\u7801\uff0c\u6ce8\u610f get \u51fd\u6570\u548c\u5176\u53cb\u5143\u51fd\u6570<\/p>\n

#include <utility> #include <type_traits>  template <unsigned Height, typename T, bool = std::is_class_v<T> && !std::is_final_v<T>> class TupleElt;  template <unsigned Height, typename T> class TupleElt<Height, T, false> { private:     T value;  public:     TupleElt() = default;     template <typename U>     TupleElt(U &&other) : value(std::forward<U>(other)) {}     T &get() { return value; }     T const &get() const { return value; } };  template <unsigned Height, typename T> class TupleElt<Height, T, true> : private T { public:     TupleElt() = default;     template <typename U>     TupleElt(U &&other) : T(std::forward<U>(other)) {}     T &get() { return *this; }     T const &get() const { return *this; } };  template <unsigned H, typename T> T &getHeight(TupleElt<H, T> &te) {     return te.get(); }  template <typename... Types> class Tuple;  template <unsigned I, typename... Elements> auto get(Tuple<Elements...> &t) -> decltype(getHeight<sizeof...(Elements) - I - 1>(t)) {     return getHeight<sizeof...(Elements) - I - 1>(t); }   template <typename... Types> class Tuple;  template <typename Head, typename... Tail> class Tuple<Head, Tail...> : private TupleElt<sizeof...(Tail), Head>, private Tuple<Tail...> {     template <unsigned I, typename... Elements>     friend auto get(Tuple<Elements...> &t) -> decltype(getHeight<sizeof...(Elements) - I - 1>(t));  \\ \u53cb\u5143\u51fd\u6570\u58f0\u660e  private:     using HeadElt = TupleElt<sizeof...(Tail), Head>;  public:     Head &getHead() { return static_cast<HeadElt *>(this)->get(); }     Head const &getHead() const { return static_cast<HeadElt const *>(this)->get(); }     Tuple<Tail...> &getTail() { return *this; }     Tuple<Tail...> const &getTail() const { return *this; } };  template <> class Tuple<> { };  int main() {     Tuple<char, char> t1;     get<0>(t1);     return 0 } <\/code><\/pre>\n
\u6211\u8ddf\u7740 C++ templates 2nd edition\uff0c \u5b9e\u73b0\u4e86\u4e00\u4e2a\u7b80\u5355\u7684 tuple \u7c7b\u548c get \u51fd\u6570\uff0c\u5176\u4e2d tuple \u7c7b\u91cc\u9762\u7684\u5143\u7d20\u88ab tuple \u7c7b\u79c1\u6709\u7ee7\u627f\uff0cget \u51fd\u6570\u901a\u8fc7\u8ba1\u7b97\u5143\u7d20\u9ad8\u5ea6\uff0c\u6765\u5b9e\u73b0\u76f4\u63a5\u4ece\u6d3e\u751f\u7c7b\u5230\u57fa\u7c7b\u7684\u8f6c\u6362\u6765\u8bbf\u95ee\u5bf9\u5e94\u7684\u5143\u7d20\uff0c\u7531\u4e8e tupleElt \u7c7b\u88ab\u79c1\u6709\u7ee7\u627f\uff0cget \u51fd\u6570\u5fc5\u987b\u5728 tuple \u7c7b\u4e2d\u88ab\u58f0\u660e\u4e3a\u53cb\u5143\u51fd\u6570\u624d\u53ef\u4ee5\u88ab\u8bbf\u95ee\uff08\u4e0a\u8ff0\u4ee3\u7801\u6ce8\u91ca\u5904\uff09\u4f46\u662f\u5373\u4f7f\u88ab\u58f0\u660e\u4e3a\u4e86\u53cb\u5143\u51fd\u6570\uff0cgcc \u8fd8\u662f\u63d0\u793a get \u51fd\u6570\u65e0\u6cd5\u8bbf\u95ee tuple \u7684\u57fa\u7c7b\uff0c\u5bfc\u81f4\u7f16\u8bd1\u9519\u8bef\uff0c\u4f46 MSVC \u53ef\u4ee5\u987a\u5229\u7f16\u8bd1\u901a\u8fc7\uff0c\u662f\u4e3a\u4ec0\u4e48\u5462\uff0c\u5982\u679c\u6709\u9519\u8bef\u8be5\u600e\u4e48\u6539\uff1f<\/p>\n<\/p><\/div>\n
 \u5927\u4f6c\u6709\u8a71\u8aaa<\/b> (10<\/span>)        <\/div>\n
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     \u4e3b<\/span> \u8cc7\u6df1\u5927\u4f6c : Tony042 <\/span>  <\/div>\n
     <\/i>            <\/span> <\/div>\n<\/p><\/div>\n
                                                                 \u521a\u624d\u53c8\u8bd5\u4e86\u4e0b\uff0cclang-10, MSVC 16.7.0 \u90fd\u53ef\u4ee5\u7f16\u8bd1\u901a\u8fc7\uff0cgcc-9, gcc-10 \u90fd\u7f16\u8bd1\u4e0d\u901a\u8fc7…                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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                                                                 \u679c\u7136 C++\u7684\u56de\u7b54\u5c31\u662f\u5c11\u554a\uff0c\u81ea\u9876\u4e00\u4e0b\uff0c\u53e6\u5916\u9898\u4e2d\u4ee3\u7801\u53ef\u4ee5\u901a\u8fc7\u8fd9\u4e2a\u94fe\u63a5\u8c03\u8bd5 https:\/\/godbolt.org\/z\/Wr4arz                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : codehz <\/span>  <\/div>\n
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                                                                 \u867d\u7136\u4e0d\u77e5\u9053\u53d1\u751f\u4e86\u4ec0\u4e48\uff0c\u4f46\u662f\u4f60\u6539\u6210\u8fd9\u6837\u5c31\u80fd\u901a\u8fc7\u7f16\u8bd1 https:\/\/godbolt.org\/z\/bq8sEn
    \u53e6\u5916\u4f60\u8fd9\u4e2a\u65b9\u6cd5\u505a tuple \u5c45\u7136\u8fd8\u9700\u8981 static_cast\uff0c\u8fd9\u80af\u5b9a\u548c\u6b63\u786e\u7684\u505a\u6cd5\u6709\u6240\u504f\u79bb\u3002\u3002\u3002                                                            <\/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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                                                                 @codehz \u540c\u4e0d\u77e5\u9053\u53d1\u751f\u4e86\u4ec0\u4e48\uff0c\u53cb\u5143\u51fd\u6570\u4e0d\u884c\uff0c\u53cb\u5143\u7c7b\u5374\u53ef\u4ee5\uff0cgcc \u8fd9\u884c\u4e3a\u633a\u5947\u602a\u7684\u3002stl \u91cc\u9762\u7684 get \u51fd\u6570\u5c31\u662f\u901a\u8fc7 static_cast \u83b7\u5f97 tuple \u91cc\u5143\u7d20\u7684\u503c\u3002\u300a C++ templates \u300b\u8fd9\u672c\u4e66\u4ee3\u7801\u6709\u70b9\u5751\uff0c\u597d\u591a\u5730\u65b9\u90fd\u6709\u5c0f\u9519\u8bef\uff0c\u6216\u8005\u4e0d\u592a\u7a33\u5b9a                                                            <\/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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                                                                 @codehz anyway \u8c22\u8c22\u8001\u54e5\u7684\u56de\u590d\u554a\uff0c\u57fa\u672c\u4e0a\u6211\u7684\u6bcf\u4e2a\u95ee\u9898\u90fd\u662f\u4f60\u5e2e\u6211\u56de\u7b54\u7684\uff0c\u8c22\u8c22\u5566                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : Wirbelwind <\/span>  <\/div>\n
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                                                                 \u5927\u6982\u662f\u7f16\u8bd1\u5668\u5b9e\u73b0\u95ee\u9898\uff1f<\/p>\n
    \u4f7f\u7528 private \u7c7b\u578b \u7ee7\u627f\u7684\u7c7b\u578b\uff0c\u5c31\u7b97\u662f\u88ab\u7ee7\u627f\u7c7b\u578b\u4e2d\u7684 public \u6210\u5458\u4e5f\u662f\u4e0d\u80fd\u8bbf\u95ee\u7684\u3002<\/p>\n
    \u5199\u4e2a\u7b80\u5316\u7248\u7684 demo \u7684\u8bdd\uff0cclang \u548c msvc \u4e5f\u662f\u4e0d\u7ed9\u8fc7\u7684\u3002<\/p>\n
    \u7b80\u5355\u7684\u89e3\u51b3\u529e\u6cd5\u5c31\u662f private TupleElt \u6539\u6210 public TupleElt                                                            <\/p><\/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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                                                                 @Wirbelwind \u5e94\u8be5\u662f\u7f16\u8bd1\u5668\u5b9e\u73b0\u95ee\u9898\u5427\uff0c\u6211\u67e5\u4e86\u4e0b stl \u6e90\u7801\uff0cmsvc \u7684\u65b9\u5f0f\u662f\u5c06 get \u51fd\u6570\u58f0\u660e\u4e3a\u53cb\u5143\u51fd\u6570\uff0cgcc \u662f\u58f0\u660e\u53cb\u5143\u7c7b\u3002 \u5728\u8fd9\u4e2a case \u91cc\u9762\uff0c\u7531\u4e8e\u6bcf\u4e2a tuple \u7c7b\u7684\u5b9e\u73b0\u90fd\u58f0\u660e\u4e86\u6240\u6709 get \u51fd\u6570\u4e3a\u53cb\u5143\u51fd\u6570\uff0c\u5176\u5b9e\u6309\u7406\u8bf4 private \u4e0d\u4f1a\u5f71\u54cd get \u51fd\u6570\u505a Derived->Base \u8f6c\u6362\u7684                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : constexpr <\/span>  <\/div>\n
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                                                                 \u6211\u7814\u7a76\u4e86\u4e00\u4e0b,\u6211\u89c9\u5f97\u8fd9\u4e2a\u95ee\u9898\u5927\u6982\u8ddf\u53cb\u5143\u6709\u5173,\u4e3a\u6b64\u6211\u5199\u4e86\u4e00\u6bb5\u6781\u77ed\u7684\u4ee3\u7801<\/p>\n
    struct Base { void f() {} };<\/p>\n
    void m(Base*) {}<\/p>\n
    template<typename T>auto v2ex(T* b) -> decltype(m(b));<\/p>\n
    struct Derived : private Base {
     template<typename T> friend auto v2ex(T* d) -> decltype(m(d)); \/\/problem HERE!!!
    };<\/p>\n
    template<typename T> auto v2ex(T* d) -> decltype(m(d)) { d->f(); }<\/p>\n
    int main() {
     Derived d;
     v2ex(&d);
    }<\/p>\n
    \u8fd9\u6bb5\u4ee3\u7801\u80fd\u5728 MSVC, clang \u4e2d\u7f16\u8bd1\u901a\u8fc7\u5e76\u8fd0\u884c, g++\u65e0\u6cd5\u7f16\u8bd1.<\/p>\n
    \u8fd9\u6bb5\u4ee3\u7801\u7684\u95ee\u9898\u5728\u4e8e, \u6a21\u677f\u51fd\u6570 V2EX \u662f\u7c7b Derived \u7684\u53cb\u5143, \u6545\u53ef\u4ee5\u5728 V2EX \u4e2d\u81ea\u7531\u8f6c\u6362\u6210\u7c7b Base. \u4f46\u662f V2EX \u7684\u58f0\u660e\u6709\u8fd9\u4e48\u4e00\u6bb5 “decltype(m(b))” , \u95ee\u9898\u662f\u51fd\u6570 m \u662f\u5426\u5e94\u8be5\u662f\u7c7b Derived \u7684\u53cb\u5143\u5462? \u5982\u679c\u662f\u7684\u8bdd, \u6211\u4f20\u7ed9\u4ed6\u4e00\u4e2a\u7c7b Dervied \u7684\u6307\u9488,\u4ed6\u5e94\u8be5\u5728 V2EX \u4e2d\u80fd\u7545\u901a\u65e0\u963b\u7684\u8f6c\u6362\u6210\u7c7b Base, clang \u548c MSVC \u90fd\u8ba4\u4e3a\u662f\u7684,\u6240\u4ee5\u7f16\u8bd1\u901a\u8fc7, \u800c g++\u8ba4\u4e3a m \u4e0d\u662f Derived \u7684\u53cb\u5143, m \u4e2d\u4e0d\u5141\u8bb8 Derived->Base \u7684\u8f6c\u6362, \u8fd9\u5c31\u662f\u4ed6\u65e0\u6cd5\u7f16\u8bd1\u7684\u539f\u56e0. 
    \u5bf9\u5e94\u4f60\u7684\u4ee3\u7801\u5c31\u662f g++\u4e0d\u8ba4\u4e3a getHeight \u662f Tuple \u7684\u53cb\u5143, \u6240\u4ee5\u5728\u53cb\u5143\u58f0\u660e\u4e2d “getHeight<sizeof…(Elements) – I – 1>(t)”, g++\u4e0d\u5141\u8bb8 Tuple \u7c7b\u578b\u7684\u53c2\u6570 t \u5411\u57fa\u7c7b TupleElt \u7684\u8f6c\u6362!!!<\/p>\n
    \u6709\u51e0\u70b9\u8bf4\u660e\u4e00\u4e0b:
    \u25a0 \u4ee3\u7801\u4e5f\u8bb8\u53ef\u4ee5\u5199\u7684\u66f4\u77ed, \u4f46\u662f\u4e3a\u4e86\u6070\u597d\u5728 MSVC \u548c clang \u4e0b\u7f16\u8bd1\u6210\u529f, \u800c gcc \u4e0d\u6210\u529f\u624d\u5199\u7684\u7a0d\u590d\u6742\u4e86\u4e00\u70b9.
    \u25a0 \u4f60\u7684\u4ee3\u7801\u6bd4\u6211\u8fd9\u6bb5\u4ee3\u7801\u590d\u6742\u7684\u591a, \u800c\u4e14\u6d89\u53ca\u9012\u5f52\u7ee7\u627f, \u591a\u91cd\u7ee7\u627f. \u867d\u7136\u6211\u4e0d\u6562\u80af\u5b9a\u6211\u63d0\u5230\u7684\u8fd9\u4e2a\u95ee\u9898\u5c31\u4e00\u5b9a\u662f\u4f60\u95ee\u9898\u7684\u89e3\u7b54, \u4f46\u4ece g++\u7f16\u8bd1\u5668\u7ed9\u51fa\u7684\u9519\u8bef\u63d0\u793a\u4e2d, \u5927\u81f4\u5c31\u662f\u6211\u63d0\u51fa\u7684\u8fd9\u4e2a\u95ee\u9898.
    \u25a0 \u81f3\u4e8e ISO \u6807\u51c6\u662f\u600e\u4e48\u6837\u7684, \u6211\u4e0d\u6e05\u695a. \u4e0d\u8fc7\u4e5f\u80fd\u770b\u51fa\u4e0d\u540c\u7f16\u8bd1\u5668\u5bf9\u6807\u51c6\u7684\u89e3\u8bfb,\u5b9e\u73b0\u90fd\u6709\u504f\u5dee.                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : constexpr <\/span>  <\/div>\n
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                                                                 @constexpr V2EX \u597d\u50cf\u81ea\u52a8\u53d8\u5927\u5199\u4e86!~                                                            <\/div>\n<\/p><\/div>\n<\/li>\n
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     \u8cc7\u6df1\u5927\u4f6c : Wirbelwind <\/span>  <\/div>\n
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                                                                 @constexpr \u8fd9\u6837\u7406\u89e3\u786e\u5b9e\u4e0d\u9519\uff0c\u4e4b\u524d\u6211\u8bd5\u4e86\u5728 base class \u52a0\u53cb\u5143\u51fd\u6570\uff0c\u4f46\u662f\u51fa\u73b0 ambigous \u95ee\u9898 \u5c31\u4f5c\u7f62\u4e86<\/p>\n
    \u4f46\u662f\u7f16\u8bd1\u5668\u62a5\u7684\u9519\u8bef\u662f inaccessible base of TupleElt\uff0c\u4e5f\u53ef\u80fd\u662f gcc \u4e0d\u5141\u8bb8\u8bbf\u95ee private<\/p>\n
    \u53cb\u5143\u4e00\u5b9a\u7a0b\u5ea6\u4e0a\u7834\u574f\u4e86 oop \u7279\u6027                                                            <\/p><\/div>\n<\/p><\/div>\n<\/li>\n
  • \n","protected":false},"excerpt":{"rendered":"
    C++\u53cb\u5143\u51fd\u6570\u95ee\u9898 \u8cc7\u6df1\u5927\u4f6c : …<\/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\/150261"}],"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=150261"}],"version-history":[{"count":0,"href":"http:\/\/4563.org\/index.php?rest_route=\/wp\/v2\/posts\/150261\/revisions"}],"wp:attachment":[{"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=150261"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=150261"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/4563.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=150261"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}