隨著C++ 11的語法越來越復雜，功能也越來越強。用泛型+返回類型推導的方法來實現常量字符串的處理，能實現宏實現不了的地方，並且在編譯期實現
SO上的牛人太多了，以下是出處
http://stackoverflow.com/questions/15858141/conveniently-declaring-compile-time-strings-in-c/15890335#15890335
1.定義
1 namespace blog {
2 typedef long long hash64;
3 namespace const_expr
4 {
5 constexpr hash64 prime = 0x100000001B3ull;
6 constexpr hash64 basis = 0xCBF29CE484222325ull;
7 }
8 constexpr hash64 make_hash_static(char const* str)
9 {
10 return (*str) ? (*(str + 1)) ? (((*str) * const_expr::prime + const_expr::basis) ^ make_hash_static(str + 1)) : ((*str) * const_expr::prime + const_expr::basis) : 0;
11 }
12
13 constexpr hash64 operator "" _hash(char const* p, size_t)
14 {
15 return make_hash_static(p);
16 }
17 template<char>using charDummy = char;
18 template <int N>
19 constexpr char at(const char* a) { return a[N]; }
20 template<int... dummy>
21 struct F
22 {
23 const char Name[sizeof...(dummy)+1];
24 const hash64 Hash;
25 const int Length;
26 const int Size;
27
28 constexpr F(const char* a) : Name{ at<dummy>(a)..., 0 }, Length(sizeof...(dummy)), Size(sizeof...(dummy)+1), Hash(a[0] * const_expr::prime + const_expr::basis){}
29 constexpr F(hash64 h, charDummy<dummy>... a) : Name{ a..., 0 }, Length(sizeof...(dummy)), Size(sizeof...(dummy)+1), Hash(h){}
30 constexpr F(const F& a) : Name{ a.Name[dummy]..., 0 }, Length(a.Length), Size(a.Size), Hash(a.Hash){}
31
32 template<int... dummyB>
33 constexpr F<dummy..., sizeof...(dummy)+dummyB...> operator + (F<dummyB...> b)const
34 {
35 return{ this->Hash ^ b.Hash,this->Name[dummy]..., b.Name[dummyB]... };
36 }
37 operator const char*()const { return Name; }
38 };
39
40 template<int I>
41 struct get_string
42 {
43 constexpr static auto g(const char* a) -> decltype(get_string<I - 1>::g(a) + F<0>(a + I))
44 {
45 return get_string<I - 1>::g(a) + F<0>(a + I);
46 }
47 };
48
49 template<>
50 struct get_string<0>
51 {
52 constexpr static F<0> g(const char* a)
53 {
54 return{ a };
55 }
56 };
57
58 template<int I>
59 constexpr auto str(const char(&a)[I]) -> decltype(get_string<I - 2>::g(a))
60 {
61 return get_string<I - 2>::g(a);
62 }
63 }
2.使用
1 constexpr auto s = blog::str("abc")+ blog::str(blog_numb2str(12345678)) + blog::str("edf");
2 std::cout << s.Name << std::endl;
3 std::cout << s.Hash << std::endl;
4 std::cout << s.Length << std::endl;
5 std::cout << s.Size << std::endl;
6 using namespace blog;
7 switch (s.Hash) {
8 case "abcedf"_hash:
9 std::cout << "s is " << s.Hash << std::endl;
10 break;
11 case s.Hash + 1:
12 break;
13 case "abc12345678edf"_hash:
14 std::cout << "s is " << s.Name << std::endl;
15 break;
16 }
3.結果
abc2345678edf
535462162733975
14
15
s is abc2345678edf
4.優缺點
