BYR Achieve · 镜像论坛

看看C++的魅力吧

2013/9/28镜像同步58 回复

C++模板是图灵完备的，以下这个例子是昨天周末晚随性写的，本身并没什么实际价值，也远不足以证明C++模板的能力，但希望通过这个例子，可稍微展示下C++的魅力：编译时间将一个罗马数字字符串转为整数。这个过程全部由编译器完成，出来的结果也是编译常量，你可以用它来声明静态数组大小，没任何问题。本身很鲁棒，对一个非法的输入（含非法字符的）将导致编译错误，对合法的字母但非法的组合（比如IIII)将输出0. （注，罗马数字本身计数能力范围是1-3999） PS:需要C++11支持 #include <iostream> class R2I { const static int DIG; public: constexpr static char IN [] = "MMMDCCXCIV"; //Input goes here const static int OUT; }; //// template <char C> struct ID {}; template <> struct ID <'I'> {static const int I = 0; static const int V = 1;}; template <> struct ID <'V'> {static const int I = 1; static const int V = 5;}; template <> struct ID <'X'> {static const int I = 2; static const int V = 10;}; template <> struct ID <'L'> {static const int I = 3; static const int V = 50;}; template <> struct ID <'C'> {static const int I = 4; static const int V = 100;}; template <> struct ID <'D'> {static const int I = 5; static const int V = 500;}; template <> struct ID <'M'> {static const int I = 6; static const int V = 1000;}; template <bool A, bool B, int I0, int I1> struct DiffHelper{ static const int D = 9;}; template<int I0, int I1> struct DiffHelper <true, true, I0, I1>{ static const int VAL0 = ID<R2I::IN[I0]>::I; static const int VAL1 = ID<R2I::IN[I1]>::I; static const int D = (VAL0 - VAL1 <= -3) ? -3 : (( VAL0 - VAL1 >= 1) ? 1 : (VAL0 - VAL1)); }; template <int I0, int I1> struct Diff {static const int D = DiffHelper<(I0 < sizeof(R2I::IN) - 1), (I1 < sizeof(R2I::IN) - 1), I0, I1>::D;}; //// template <int N, int C, int P> struct R2IHelper {const static int DIG = -4000;}; template <int P, int N> struct R2IHelper<N, 9, P> {const static int DIG = ID<R2I::IN[P]>::V;}; template <int P> struct R2IHelper<9, 1, P> {const static int DIG = ID<R2I::IN[P]>::V * 4/5 + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I) % 2, Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<9, 9, P> {const static int DIG = ID<R2I::IN[P]>::V * 4/5;}; template <int P> struct R2IHelper<6, 1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I) % 2, Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<6, 0, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<2, Diff<P+1, P+2>::D, P+1>::DIG;}; template <int P> struct R2IHelper<4, 1, P> {const static int DIG = ID<R2I::IN[P]>::V * 3/5 + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I + 1) % 2, Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<4, 9, P> {const static int DIG = ID<R2I::IN[P]>::V * 3/5;}; template <int P> struct R2IHelper<3, 1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I) % 2 , Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<2, 1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I) % 2 , Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<2, 0, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<3, Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<1, 1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I==1)*6 + (ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I + 1) % 2, Diff<P+1, P+2>::D, P+1>::DIG;}; template <int P> struct R2IHelper<0, 1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<(ID<R2I::IN[P]>::I - ID<R2I::IN[P+1]>::I) % 2 , Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<0, 0, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<2, Diff<P+1, P+2>::D, P+1>::DIG; }; template <int P> struct R2IHelper<0, -1, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<4, Diff<P, P+2>::D, P+1>::DIG;}; template <int P> struct R2IHelper<0, -2, P> {const static int DIG = ID<R2I::IN[P]>::V + R2IHelper<9, Diff<P, P+2>::D, P+1>::DIG;}; //// const int R2I::DIG = R2IHelper<ID<R2I::IN[0]>::I % 2 , Diff<0, 1>::D, 0>::DIG; const int R2I::OUT = DIG > 0 ? DIG : 0; using namespace std; int main() { char carr[R2I::OUT]; cout<<sizeof(carr)<<endl; }

订阅后，新回复会通过你的通知中心匿名送达。