{"id":1864,"date":"2018-10-23T17:37:23","date_gmt":"2018-10-23T09:37:23","guid":{"rendered":"https:\/\/damogame.cn:15443\/wordpress\/?p=1864"},"modified":"2018-10-25T08:33:48","modified_gmt":"2018-10-25T00:33:48","slug":"c%e7%9a%84%e6%9d%82%e4%b8%83%e6%9d%82%e5%85%ab%ef%bc%9aconstexpr%e4%b8%8e%e7%bc%96%e8%af%91%e6%9c%9f%e8%ae%a1%e7%ae%97","status":"publish","type":"post","link":"https:\/\/i007.cc\/wordpress\/archives\/1864","title":{"rendered":"C++\u7684\u6742\u4e03\u6742\u516b\uff1aconstexpr\u4e0e\u7f16\u8bd1\u671f\u8ba1\u7b97"},"content":{"rendered":"
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\u7248\u6743\u58f0\u660e\uff1a\u672c\u6587\u4e3a\u535a\u4e3b\u539f\u521b\u6587\u7ae0\uff0c\u91c7\u7528\u77e5\u8bc6\u5171\u4eab \u7f72\u540d-\u975e\u5546\u4e1a\u6027\u4f7f\u7528-\u76f8\u540c\u65b9\u5f0f\u5171\u4eab 4.0 \u56fd\u9645 \u8bb8\u53ef\u534f\u8bae\u8fdb\u884c\u8bb8\u53ef\u3002 https:\/\/blog.csdn.net\/tangxiaoyin\/article\/details\/80123024<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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<\/a>\u5f15\u8a00<\/h3>\n

\u793e\u533a\uff08http:\/\/purecpp.org\/<\/a>\uff09\u91cc\u6709\u670b\u53cb\u63d0\u51fa\u4e86\u7f16\u8bd1\u671f\u5206\u5272\u5b57\u7b26\u4e32\u7684\u9700\u6c42\uff0c\u5927\u5bb6\u5728\u7fa4\u91cc\u8fdb\u884c\u4e86\u70ed\u70c8\u7684\u8ba8\u8bba\uff0c\u4e5f\u505a\u4e86\u8bb8\u591a\u5c1d\u8bd5\uff0c\u4f46\u5e76\u6ca1\u6709\u5f97\u51fa\u786e\u5b9a\u7684\u7ed3\u679c\u3002\u672c\u6587\u4f5c\u8005\u8bd5\u56fe\u5bf9C++11\/14\u91cc\u7684\u65b0\u5173\u952e\u5b57constexpr\u8fdb\u884c\u7f16\u8bd1\u671f\u8ba1\u7b97\u7684\u603b\u7ed3\u548c\u63a2\u8ba8\uff0c\u5e76\u5728\u6700\u540e\u7ed3\u5408constexpr\u7ed9\u51faC++\u5728\u7f16\u8bd1\u671f\u5206\u5272\u5b57\u7b26\u4e32\u7684\u65b9\u6cd5\u3002<\/p>\n

<\/a>\u4e00\u3001\u7f16\u8bd1\u671f\u8ba1\u7b97<\/h3>\n

\u6211\u4eec\u5148\u6765\u770b\u4e00\u6bb5\u6c42\u9636\u4e58\uff08factorial\uff09\u7684\u7b97\u6cd5\uff1a<\/p>\n

size_t factorial(size_t n) noexcept\r\n{\r\n    return (n == 0) ? 1 : n * factorial(n - 1);\r\n}<\/pre>\n
\u5f88\u660e\u663e\uff0c\u8fd9\u662f\u4e00\u6bb5\u8fd0\u884c\u671f\u7b97\u6cd5\u3002\u7a0b\u5e8f\u8fd0\u884c\u7684\u65f6\u5019\uff0c\u4f20\u9012\u4e00\u4e2a\u503c\uff0c\u5b83\u53ef\u4ee5\u662f\u4e00\u4e2a\u53d8\u91cf\uff0c\u4e5f\u53ef\u4ee5\u662f\u4e00\u4e2a\u5e38\u91cf\uff1a<\/p>\n
int main(void)\r\n{\r\n    std::cout << factorial(10) << std::endl;\r\n    return 0;\r\n}<\/pre>\n
\u5982\u679c\u7a0b\u5e8f\u4ec5\u4ec5\u50cf\u8fd9\u6837\u4f20\u9012\u5e38\u91cf\uff0c\u6211\u4eec\u53ef\u80fd\u4f1a\u5e0c\u671b\u80fd\u591f\u8ba9\u5b83\u5b8c\u5168\u5728\u7f16\u8bd1\u7684\u65f6\u5019\u5c31\u628a\u7ed3\u679c\u8ba1\u7b97\u51fa\u6765\uff0c\u90a3\u4e48\u4ee3\u7801\u6539\u6210\u8fd9\u6837\u6216\u8bb8\u662f\u4e2a\u4e0d\u9519\u7684\u9009\u62e9\uff1a<\/p>\n
template <size_t N>\r\nstruct factorial\r\n{\r\n    enum : size_t { value = N * factorial<N - 1>::value };\r\n};\r\n\r\ntemplate <>\r\nstruct factorial<0>\r\n{\r\n    enum : size_t { value = 1 };\r\n};\r\n\r\nint main(void)\r\n{\r\n    std::cout << factorial<10>::value << std::endl;\r\n    return 0;\r\n}<\/pre>\n
\u53ea\u662f\u7528\u8d77\u6765\u4f1a\u7a0d\u663e\u9ebb\u70e6\u70b9\uff0c\u4f46\u597d\u5904\u662f\u8fd0\u884c\u671f\u6ca1\u6709\u4efb\u4f55\u65f6\u95f4\u4ee3\u4ef7\u3002<\/p>\n
\u50cf\u4e0a\u9762\u8fd9\u79cd\u8fd0\u7528\u6a21\u677f\u7684\u505a\u6cd5\uff0c\u7b97\u662f\u6700\u7b80\u5355\u7684\u6a21\u677f\u5143\u7f16\u7a0b\u4e86\u3002\u5bf9C++\u6a21\u677f\u6765\u8bf4\uff0c\u7c7b\u578b\u548c\u503c\u662f\u540c\u4e00\u79cd\u4e1c\u897f\uff1b\u540c\u65f6\uff0c\u53c8\u7531\u4e8eC++\u7684\u6a21\u677f\u6709\u4e86\u201cPattern Matching\u201d\uff08\u5373\u7279\u5316\u548c\u504f\u7279\u5316\uff09\uff0c\u540c\u65f6\u53c8\u5141\u8bb8\u6a21\u677f\u7684\u9012\u5f52\u7ed3\u6784\uff08\u89c1\u4e0a\u9762factorial\u4e2d\u4f7f\u7528factorial\u7684\u60c5\u51b5\uff09\uff0c\u4e8e\u662fC++\u7684\u6a21\u677f\u662f\u56fe\u7075\u5b8c\u5168\u7684\u4e00\u79cd\u72ec\u7acb\u4e8eC++\u7684\u8bed\u8a00\u3002\u7406\u8bba\u4e0a\u6765\u8bf4\uff0c\u6211\u4eec\u53ef\u4ee5\u5229\u7528\u5b83\u5728\u7f16\u8bd1\u671f\u5b8c\u6210\u6240\u6709\u8ba1\u7b97\u2014\u2014\u524d\u63d0\u662f\u8fd9\u4e9b\u8ba1\u7b97\u7684\u8f93\u5165\u90fd\u662fliteral\u7684\u3002<\/p>\n
<\/a>\u4e8c\u3001C++11\u4ee5\u540e\u7684\u65b0\u9650\u5b9a\u7b26\uff1aconstexpr<\/h3>\n
\u4eceC++11\u5f00\u59cb\uff0c\u6211\u4eec\u6709\u4e86constexpr specifier\u3002\u5b83\u53ef\u4ee5\u88ab\u7528\u4e8e\u53d8\u91cf\uff0c\u53ca\u51fd\u6570\u4e0a\uff0c\u50cf\u8fd9\u6837\uff1a<\/p>\n
template <size_t N>\r\nstruct t_factorial_\r\n{\r\n    enum : size_t { value = N * t_factorial_<N - 1>::value };\r\n};\r\n\r\ntemplate <>\r\nstruct t_factorial_<0>\r\n{\r\n    enum : size_t { value = 1 };\r\n};\r\n\r\ntemplate <size_t N>\r\nconstexpr auto t_factorial = t_factorial_<N>::value;\r\n\r\nint main(void)\r\n{\r\n    std::cout << t_factorial<10> << std::endl;\r\n    return 0;\r\n}<\/pre>\n
\u5f53\u7136\u4e86\uff0c\u4e0a\u9762\u66f4\u76f4\u63a5\u7684\u7528\u6cd5\u662f\u8fd9\u6837\uff1a<\/p>\n
constexpr size_t c_factorial(size_t n) noexcept\r\n{\r\n    return (n == 0) ? 1 : n * c_factorial(n - 1);\r\n}<\/pre>\n
\u5728C++11\u4e2d\uff0cconstexpr\u8fd8\u6709\u8bf8\u591a\u9650\u5236\uff0c\u4f46\u5230\u4e86C++14\uff0c\u5b83\u4f3c\u4e4e\u6709\u70b9\u8fc7\u4e8e\u5f3a\u5927\u4e86\u3002\u6bd4\u5982\u6211\u4eec\u53ef\u4ee5\u5728\u51fd\u6570\u4e2d\u5199\u591a\u884c\u8bed\u53e5\uff0c\u5b9a\u4e49\u53d8\u91cf\uff0c\u751a\u81f3\u662f\u5faa\u73af\uff1a<\/p>\n
\/\/ runtime version\r\ntemplate <typename T, size_t N>\r\nsize_t r_count(T&& v, const T(&arr)[N]) noexcept\r\n{\r\n    size_t r = 0;\r\n    for (const auto& a : arr) if (v == a) ++r;\r\n    return r;\r\n}\r\n\r\n\/\/ constexpr version\r\ntemplate <typename T, size_t N>\r\nconstexpr size_t c_count(T&& v, const T(&arr)[N]) noexcept\r\n{\r\n    size_t r = 0;\r\n    for (const auto& a : arr) if (v == a) ++r;\r\n    return r;\r\n}<\/pre>\n
\u5c31\u5982\u540c\u6211\u4eec\u5728\u5199\u7684\u53ea\u662f\u4e00\u4e2a\u666e\u901a\u51fd\u6570\uff0c\u4e4b\u540e\u5728\u51fd\u6570\u7684\u6700\u524d\u9762\u52a0\u4e0aconstexpr\u5b83\u9a6c\u4e0a\u5c31\u53ef\u4ee5\u5728\u7f16\u8bd1\u671f\u6267\u884c\u4e86\u3002<\/p>\n
constexpr\u540c\u6837\u5e26\u6765\u4e86\u5f3a\u5927\u7684\u7c7b\u578b\u8ba1\u7b97\u80fd\u529b\u3002\u6211\u4eec\u7b80\u5355\u7684\u6765\u770b\u4e2a\u4f8b\u5b50\uff0c\u5b9e\u73b0\u4e00\u4e2a\u201ctypes_insert\u201d\uff08Reference\uff1aC++\u7684\u6742\u4e03\u6742\u516b\uff1a\u4f7f\u7528\u6a21\u677f\u5143\u7f16\u7a0b\u64cd\u4f5c\u7c7b\u578b\u96c6\u5408<\/a>\uff09\uff1a<\/p>\n
template <typename...>\r\nstruct types {};\r\n\r\ntemplate <typename T, typename... U>\r\nconstexpr auto insert(types<U...>) noexcept\r\n{\r\n    return types<T, U...>{};\r\n}<\/pre>\n
\u53ef\u4ee5\u770b\u5230\uff0c\u5bf9\u4e8e\u8fd9\u79cd\u7b80\u5355\u7684\u7c7b\u578b\u8ba1\u7b97\uff0cconstexpr\u6bd4\u6a21\u677f\u5143\u7684\u5b9e\u73b0\u8981\u6e05\u6670\u5f88\u591a\u3002
\n\u4e0d\u8fc7\uff0c\u7531\u4e8e\u51fd\u6570\u6a21\u677f\u7f3a\u5c11\u504f\u7279\u5316\uff0c\u56e0\u6b64\u9700\u8981\u7f16\u8bd1\u671f\u5206\u652f\u5224\u65ad\u7684\u201ctypes_assign\u201d\u662f\u6ca1\u529e\u6cd5\u76f4\u63a5\u5199\u51fa\u6765\u7684\uff08\u5728\u8fd9\u91cc\u65e0\u6cd5\u77ed\u8def\u6c42\u503c\u7684std::conditional\u5e76\u6ca1\u6709\u4ec0\u4e48\u7528\uff09\u3002\u8981\u77e5\u9053\uff0c\u51fd\u6570\u91cd\u8f7d\u867d\u7136\u5f3a\u5927\uff0c\u4f46\u4ec5\u80fd\u505a\u7f16\u8bd1\u671f\u7c7b\u578b\uff0c\u800c\u4e0d\u662f\u6570\u503c\u7684Pattern Matching\uff0c\u8fd9\u70b9\u662f\u4e0d\u5982\u7c7b\u6a21\u677f\u7684\u504f\u7279\u5316\/\u7279\u5316\u7684\u3002<\/p>\n
\u4e0d\u8fc7\u6211\u4eec\u53ef\u4ee5\u5229\u7528\u7c7b\u6a21\u677f\u7684\u504f\u7279\u5316\u6765\u6a21\u62df\u51fd\u6570\u6a21\u677f\u7684\u504f\u7279\u5316\uff1a<\/p>\n
template <int N, typename T>\r\nstruct impl_\r\n{\r\n    constexpr static auto assign(void) noexcept\r\n    {\r\n        return insert<T>(impl_<N - 1, T>::assign());\r\n    }\r\n};\r\n\r\ntemplate <typename T>\r\nstruct impl_<0, T>\r\n{\r\n    constexpr static auto assign(void) noexcept\r\n    {\r\n        return types<>{};\r\n    }\r\n};\r\n\r\ntemplate <int N, typename T>\r\nconstexpr auto assign(void) noexcept\r\n{\r\n    return impl_<N, T>::assign();\r\n}<\/pre>\n
\u4f46\u662f\u8bf4\u5b9e\u8bdd\uff0c\u6211\u5e76\u4e0d\u559c\u6b22\u8fd9\u6837\uff0c\u8fd9\u79cd\u5199\u6cd5\u4e27\u5931\u4e86\u51fd\u6570\u6a21\u677f\u7684\u7b80\u6d01\u6027\u3002\u4e00\u822c\u6765\u8bf4\uff0c\u5927\u5bb6\u4e5f\u4e0d\u4f1a\u7528constexpr\u505a\u592a\u590d\u6742\u7684\u7c7b\u578b\u8ba1\u7b97\uff0c\u8fd9\u91cc\u53cd\u800c\u7528\u6a21\u677f\u5143\u6765\u505a\u4f1a\u66f4\u52a0\u6e05\u6670\u4e9b\u3002\u4ece\u4e0a\u9762\u53ef\u4ee5\u770b\u51fa\u6765\uff0c\u5728\u505a\u7c7b\u578b\u8ba1\u7b97\u7684\u65f6\u5019\uff0creturn\u8fd4\u56de\u7684\u6570\u503c\u5e76\u4e0d\u662f\u6211\u4eec\u9700\u8981\u7684\uff0c\u800c\u7c7b\u578b\u7ed3\u679c\u4e00\u822c\u4f1a\u7528decltype\u53d6\u51fa\u6765\u3002\u5728\u8fd9\u79cd\u60c5\u51b5\u4e0b\uff0c\u4e0d\u4f7f\u7528constexpr\uff0c\u4ec5\u7528\u666e\u901a\u51fd\u6570\u90fd\u662f\u53ef\u4ee5\u7684\u3002<\/p>\n
\u771f\u6b63\u8ba9\u4eba\u773c\u524d\u4e00\u4eae\u7684\uff0c\u5e94\u8be5\u8fd8\u662f\u4e0a\u9762c_count\u7684\u5199\u6cd5\u3002\u5229\u7528\u6a21\u677f\u5143\u505a\u6570\u503c\u8ba1\u7b97\u5176\u5b9e\u662f\u5b83\u7684\u77ed\u677f\u3002\u64b0\u5199\u590d\u6742\u4e0d\u8bf4\uff0c\u8fd8\u6709\u4e0d\u5c11\u7684\u5c40\u9650\u6027\u3002<\/p>\n
\u6bd4\u5982c_count\u53ef\u4ee5\u8fd9\u6837\u7528\uff1a<\/p>\n
template <size_t N>\r\nstruct Foo { enum : size_t { value = N }; };\r\n\r\nint main(void)\r\n{\r\n    std::cout << Foo<c_count(',', \"1, 2, 3, 4, 5, 6, 7, 8, 9, 0\")>::value << std::endl;\r\n    return 0;\r\n}<\/pre>\n
\u800c\u6a21\u677f\u5143\u5bf9string literal\u8fd9\u7c7b\u6570\u503c\u505a\u8ba1\u7b97\u662f\u6bd4\u8f83\u9ebb\u70e6\u7684\uff0ctemplate non-type arguments\u88ab\u9650\u5236\u4e3a\u5e38\u6574\u6570\uff08\u5305\u62ec\u679a\u4e3e\uff09\uff0c\u6216\u6307\u5411\u5916\u90e8\u94fe\u63a5\u5bf9\u8c61\u7684\u6307\u9488\uff08\u4e25\u683c\u6765\u8bf4\u4e0d\u6b62\u8fd9\u8fd9\u4e9b\u3002Reference\uff1aTemplate parameters and template arguments<\/a>\uff09\u3002<\/p>\n
<\/a>\u4e09\u3001constexpr\u6027\u80fd\u5b9e\u6d4b<\/h3>\n
\u7406\u8bba\u4e0a\u6765\u8bf4\uff0c\u5982\u679cconstexpr\u53d1\u6325\u4f5c\u7528\uff0c\u8fd0\u884c\u65f6\u662f\u6ca1\u6709\u6027\u80fd\u635f\u8017\u7684\u3002\u6211\u4eec\u7528\u4e00\u4e2a\u4f8b\u5b50\u6765\u6d4b\u8bd5\u4e0b\u5b9e\u9645\u7684\u6548\u679c\uff1a<\/p>\n
\/\/ https:\/\/github.com\/mutouyun\/capo\/blob\/master\/capo\/stopwatch.hpp\r\n#include \"stopwatch.hpp\"\r\n\r\nsize_t r_fib(size_t n) noexcept\r\n{\r\n    if (n == 0) return 0;\r\n    if (n == 1) return 1;\r\n    return r_fib(n - 1) + r_fib(n - 2);\r\n}\r\n\r\nconstexpr size_t c_fib(size_t n) noexcept\r\n{\r\n    if (n == 0) return 0;\r\n    if (n == 1) return 1;\r\n    return c_fib(n - 1) + c_fib(n - 2);\r\n}\r\n\r\ntemplate <size_t N> struct t_fib_ { enum : size_t { value = t_fib_<N - 1>::value + t_fib_<N - 2>::value }; };\r\ntemplate <> struct t_fib_<0> { enum : size_t { value = 0 }; };\r\ntemplate <> struct t_fib_<1> { enum : size_t { value = 1 }; };\r\ntemplate <size_t N>\r\nconstexpr auto t_fib = t_fib_<N>::value;\r\n\r\n\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\r\n\r\nint main(void)\r\n{\r\n    constexpr size_t n = 40;\r\n    capo::stopwatch<> sw;\r\n\r\n    sw.start();\r\n    size_t r1 = r_fib(n);\r\n    std::cout << r1 << \": \" << sw.elapsed<std::chrono::milliseconds>() << \"ms\" << std::endl;\r\n\r\n    sw.start();\r\n    size_t r2 = c_fib(n);\r\n    std::cout << r2 << \": \" << sw.elapsed<std::chrono::milliseconds>() << \"ms\" << std::endl;\r\n\r\n    sw.start();\r\n    size_t r3 = t_fib<n>;\r\n    std::cout << r3 << \": \" << sw.elapsed<std::chrono::milliseconds>() << \"ms\" << std::endl;\r\n\r\n    return 0;\r\n}<\/pre>\n
g++ -v\uff1aversion 5.3.1 20160413 (Ubuntu 5.3.1-14ubuntu2)<\/p>\n
\u7f16\u8bd1\u547d\u4ee4\u884c\uff1ag++ -std=c++14 -O3 -I. .\/test.cpp<\/p>\n
\u6d4b\u8bd5\u7ed3\u679c\uff1a<\/p>\n
102334155: 363ms\r\n102334155: 364ms\r\n102334155: 0ms<\/code><\/pre>\n
\u6d4b\u8bd5\u7ed3\u679c\u5e76\u4e0d\u7406\u60f3\u3002\u6309\u7406\u6765\u8bf4\uff0cc_fib\u5e94\u8be5\u548ct_fib\u4e00\u6837\u4e0d\u9700\u8981\u4efb\u4f55\u65f6\u95f4\u624d\u5bf9\u3002\u4e0b\u9762\u6211\u4eec\u5c1d\u8bd5\u4fee\u6539\u4e00\u4e0bc_fib\u7684\u6d4b\u8bd5case\uff1a<\/p>\n
sw.start();\r\nconst size_t r2 = c_fib(n);\r\nstd::cout << r2 << \": \" << sw.elapsed<std::chrono::milliseconds>() << \"ms\" << std::endl;<\/pre>\n
\u6211\u4eec\u53ef\u4ee5\u770b\u5230\uff0c\u9a6c\u4e0a\uff0c\u6d4b\u8bd5\u7684\u7ed3\u679c\u53d1\u751f\u4e86\u53d8\u5316\uff0c\u8017\u65f6\u53d8\u4e3a0ms\u3002<\/p>\n
\u4ece\u5b9e\u6d4b\u4e0a\u6765\u770b\uff0c\u9664\u4e86const\u4e4b\u5916\uff0c\u5982\u679c\u628ar2\u6539\u4e3astatic\u53d8\u91cf\uff08\u5b9a\u4e49\u65f6\u9a6c\u4e0a\u521d\u59cb\u5316\uff0c\u56e0\u6b64c_fib\u53ea\u4f1a\u88ab\u8c03\u7528\u4e00\u6b21\u3002\u82e5\u662f\u5b9a\u4e49\u540e\u518d\u5bf9static\u53d8\u91cf\u8d4b\u503c\uff0c\u5219\u7ed3\u679c\u548c\u666e\u901a\u53d8\u91cf\u4e00\u6837\uff09\uff0c\u6216constexpr\u53d8\u91cf\uff0c\u6216\u8005\u4e0d\u8981r2\uff0c\u76f4\u63a5\u5c06r_fib(n)\u653e\u5728std::cout\u540e\u9762\u8f93\u51fa\u7ed3\u679c\uff0c\u5728\u8fd9\u4e9b\u60c5\u51b5\u90fd\u4f1a\u89e6\u53d1constexpr\u7684\u7f16\u8bd1\u671f\u8ba1\u7b97\u3002<\/p>\n
\u5f53\u7136\u4e86\uff0c\u5982\u679c\u50cf\u8fd9\u6837\u5199\uff1a<\/p>\n
template <size_t N>\r\nstruct Foo { enum : size_t { value = N }; };\r\n\r\nsize_t xx = Foo<c_fib(40)>::value;<\/code><\/pre>\n
\u6216\u8005\u8fd9\u6837\u5199\uff1a<\/p>\n
char cc[c_fib(n)];<\/code><\/pre>\n
\u662f\u4e00\u5b9a\u4f1a\u89e6\u53d1\u7f16\u8bd1\u671f\u8ba1\u7b97\u7684\u3002<\/p>\n
constexpr\u5e76\u4e0d\u4f1a\u5c3d\u53ef\u80fd\u7684\u8ba9expressions\u5728\u7f16\u8bd1\u671f\u6267\u884c\uff0c\u53ea\u662f\u8868\u793aexpressions\u80fd\u5728\u7f16\u8bd1\u671f\u6267\u884c\u3002\u81f3\u4e8e\u771f\u6b63\u6267\u884c\u7684\u65f6\u673a\u662f\u7f16\u8bd1\u671f\u8fd8\u662f\u8fd0\u884c\u671f\uff0c\u4f9d\u8d56\u7f16\u8bd1\u5668\u81ea\u5df1\u7684\u9009\u62e9\u3002\u89c1constexpr specifier (since C++11)<\/a>\uff1a<\/p>\n
The constexpr specifier declares that it is possible to evaluate the value of the function or variable at compile time. Such variables and functions can then be used where only compile time constant expressions are allowed (provided that appropriate function arguments are given).<\/p><\/blockquote>\n
\u53e6\u5916\uff0c\u9700\u8981\u6ce8\u610f\u7684\u4e00\u70b9\u662f\uff0c\u51fd\u6570\u7684\u53c2\u6570\u4e0d\u80fd\u88ab\u5b9a\u4e49\u4e3aconstexpr\u3002\u4e5f\u5c31\u662f\u8bf4\uff0c\u6211\u4eec\u4e0d\u80fd\u5728\u7f16\u8bd1\u671f\u4f7f\u7528constexpr\u51fd\u6570\u7684\u53c2\u6570\uff0c\u54ea\u6015\u6211\u4eec\u7ed9\u5b83\u4f20\u9012\u7684\u53c2\u6570\u786e\u5b9e\u662f\u4e00\u4e2aliteral\uff1a<\/p>\n
constexpr size_t test(size_t n) noexcept\r\n{\r\n    return Foo<n>::value; \/\/ error: \u2018n\u2019 is not a constant expression\r\n}\r\n\r\nconst size_t xx = test(123);<\/pre>\n
\u81ea\u7136\uff0c\u8fd9\u6837\u4e5f\u662f\u4e0d\u884c\u7684\uff1a<\/p>\n
constexpr size_t cube(size_t n) noexcept\r\n{\r\n    constexpr size_t cp = c_power(n, 3); \/\/ error: \u2018n\u2019 is not a constant expression\r\n    return cp;\r\n}<\/pre>\n
\u7b80\u5355\u6765\u8bf4\uff1a<\/p>\n