能够在C ++编译时创建和操作字符串有几个有用的应用程序。尽管可以在C ++中创建编译时字符串，但这个过程非常麻烦，因为字符串需要声明为可变字符序列，例如。
using str = sequence<'H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '!'>;
字符串连接，子字符串提取等许多操作可以很容易地实现为对字符序列的操作。 是否可以更方便地声明编译时字符串？如果没有，是否有一个提案可以方便地声明编译时字符串？
理想情况下，我们希望能够按如下方式声明编译时字符串：
// Approach 1
using str1 = sequence<"Hello, world!">;
或使用用户定义的文字
// Approach 2
constexpr auto str2 = "Hello, world!"_s;
其中decltype(str2)将有一个constexpr构造函数。方法1的混乱版本可以实现，利用您可以执行以下操作的事实：
template <unsigned Size, const char Array[Size]>
struct foo;
但是，数组需要有外部链接，所以为了让方法1起作用，我们必须写这样的东西：
/* Implementation of array to sequence goes here. */
constexpr const char str[] = "Hello, world!";
int main()
{
using s = string<13, str>;
return 0;
}
毋庸置疑，这非常不方便。方法2实际上是不可能实现的。如果我们要声明一个（constexpr）文字运算符，那么我们如何指定返回类型？由于我们需要运算符返回可变字符序列，因此我们需要使用const char*参数来指定返回类型：
constexpr auto
operator"" _s(const char* s, size_t n) -> /* Some metafunction using `s` */
这会导致编译错误，因为s不是constexpr。尝试通过执行以下操作来解决这个问题并没有多大帮助。
template <char... Ts>
constexpr sequence<Ts...> operator"" _s() { return {}; }
标准规定此特定的文字运算符表单是为整数和浮点类型保留的。虽然123_s可行，但abc_s却不会。如果我们完全抛弃用户定义的文字，只使用常规constexpr函数会怎样？
template <unsigned Size>
constexpr auto
string(const char (&array)[Size]) -> /* Some metafunction using `array` */
和以前一样，我们遇到的问题是，数组（现在是constexpr函数的参数）本身不再是constexpr类型。
我认为应该可以定义一个C预处理器宏，它接受一个字符串并将字符串的大小作为参数，并返回一个由字符串中的字符组成的序列（使用BOOST_PP_FOR，stringification，array下标等）。但是，我没有时间（或足够的兴趣）来实现这样的宏=）
答案 0 :(得分：117)
我没有看到任何与Scott Schurr's str_const上呈现的C++ Now 2012优雅相符的内容。它确实需要constexpr。
以下是如何使用它以及它可以做什么：
int
main()
{
constexpr str_const my_string = "Hello, world!";
static_assert(my_string.size() == 13, "");
static_assert(my_string[4] == 'o', "");
constexpr str_const my_other_string = my_string;
static_assert(my_string == my_other_string, "");
constexpr str_const world(my_string, 7, 5);
static_assert(world == "world", "");
// constexpr char x = world[5]; // Does not compile because index is out of range!
}
它没有比编译时范围检查更酷！
使用和实现都没有宏。字符串大小没有人为限制。我在这里发布了实现，但我尊重Scott隐含的版权。实施是在他的演示文稿的单个幻灯片上链接到上面。
答案 1 :(得分：40)
我认为应该可以定义一个C预处理器宏 将字符串和字符串的大小作为参数，并返回一个 由字符串中的字符组成的序列（使用 BOOST_PP_FOR，字符串化，数组下标等。 但是，我没有时间（或足够的兴趣）来实现这样的 一个宏
可以在不依赖boost的情况下实现这一点，使用非常简单的宏和一些C ++ 11特性：
（后两者在这里并不是严格要求的）
我们需要能够使用用户提供的从0到N的标记来实例化一个可变参数模板 - 这个工具也很有用，例如将元组扩展为可变参数模板函数的参数（参见问题：How do I expand a tuple into variadic template function's arguments?
"unpacking" a tuple to call a matching function pointer）
namespace variadic_toolbox
{
template<unsigned count,
template<unsigned...> class meta_functor, unsigned... indices>
struct apply_range
{
typedef typename apply_range<count-1, meta_functor, count-1, indices...>::result result;
};
template<template<unsigned...> class meta_functor, unsigned... indices>
struct apply_range<0, meta_functor, indices...>
{
typedef typename meta_functor<indices...>::result result;
};
}
然后使用非类型定义名为string的可变参数模板 参数char：
namespace compile_time
{
template<char... str>
struct string
{
static constexpr const char chars[sizeof...(str)+1] = {str..., '\0'};
};
template<char... str>
constexpr const char string<str...>::chars[sizeof...(str)+1];
}
现在最有趣的部分 - 将字符文字传递给字符串 模板：
namespace compile_time
{
template<typename lambda_str_type>
struct string_builder
{
template<unsigned... indices>
struct produce
{
typedef string<lambda_str_type{}.chars[indices]...> result;
};
};
}
#define CSTRING(string_literal) \
[]{ \
struct constexpr_string_type { const char * chars = string_literal; }; \
return variadic_toolbox::apply_range<sizeof(string_literal)-1, \
compile_time::string_builder<constexpr_string_type>::produce>::result{}; \
}()
简单的串联演示显示了用法：
namespace compile_time
{
template<char... str0, char... str1>
string<str0..., str1...> operator*(string<str0...>, string<str1...>)
{
return {};
}
}
int main()
{
auto str0 = CSTRING("hello");
auto str1 = CSTRING(" world");
std::cout << "runtime concat: " << str_hello.chars << str_world.chars << "\n <=> \n";
std::cout << "compile concat: " << (str_hello * str_world).chars << std::endl;
}
答案 2 :(得分：19)
编辑：正如Howard Hinnant（以及我对OP的评论中的某些内容）指出的那样，您可能不需要将字符串的每个字符作为单个模板参数的类型。 如果你确实需要这个，下面有一个无宏的解决方案。
在编译时尝试使用字符串时，我发现了一个技巧。它需要引入除“模板字符串”之外的其他类型，但在函数内，您可以限制此类型的范围。
它不使用宏，而是使用一些C ++ 11功能。
#include <iostream>
// helper function
constexpr unsigned c_strlen( char const* str, unsigned count = 0 )
{
return ('\0' == str[0]) ? count : c_strlen(str+1, count+1);
}
// helper "function" struct
template < char t_c, char... tt_c >
struct rec_print
{
static void print()
{
std::cout << t_c;
rec_print < tt_c... > :: print ();
}
};
template < char t_c >
struct rec_print < t_c >
{
static void print() { std::cout << t_c; }
};
// destination "template string" type
template < char... tt_c >
struct exploded_string
{
static void print()
{
rec_print < tt_c... > :: print();
}
};
// struct to explode a `char const*` to an `exploded_string` type
template < typename T_StrProvider, unsigned t_len, char... tt_c >
struct explode_impl
{
using result =
typename explode_impl < T_StrProvider, t_len-1,
T_StrProvider::str()[t_len-1],
tt_c... > :: result;
};
template < typename T_StrProvider, char... tt_c >
struct explode_impl < T_StrProvider, 0, tt_c... >
{
using result = exploded_string < tt_c... >;
};
// syntactical sugar
template < typename T_StrProvider >
using explode =
typename explode_impl < T_StrProvider,
c_strlen(T_StrProvider::str()) > :: result;
int main()
{
// the trick is to introduce a type which provides the string, rather than
// storing the string itself
struct my_str_provider
{
constexpr static char const* str() { return "hello world"; }
};
auto my_str = explode < my_str_provider >{}; // as a variable
using My_Str = explode < my_str_provider >; // as a type
my_str.print();
}
答案 3 :(得分：9)
如果您不想使用Boost solution，可以创建一些类似的简单宏：
#define MACRO_GET_1(str, i) \
(sizeof(str) > (i) ? str[(i)] : 0)
#define MACRO_GET_4(str, i) \
MACRO_GET_1(str, i+0), \
MACRO_GET_1(str, i+1), \
MACRO_GET_1(str, i+2), \
MACRO_GET_1(str, i+3)
#define MACRO_GET_16(str, i) \
MACRO_GET_4(str, i+0), \
MACRO_GET_4(str, i+4), \
MACRO_GET_4(str, i+8), \
MACRO_GET_4(str, i+12)
#define MACRO_GET_64(str, i) \
MACRO_GET_16(str, i+0), \
MACRO_GET_16(str, i+16), \
MACRO_GET_16(str, i+32), \
MACRO_GET_16(str, i+48)
#define MACRO_GET_STR(str) MACRO_GET_64(str, 0), 0 //guard for longer strings
using seq = sequence<MACRO_GET_STR("Hello world!")>;
唯一的问题是64个字符的固定大小（加上额外的零）。但它可以根据您的需要轻松更改。
答案 4 :(得分：6)
我认为应该可以定义一个C预处理器宏，它接受一个字符串和字符串的大小作为参数，并返回一个由字符串中的字符组成的序列（使用BOOST_PP_FOR，字符串化，数组下标和等）
Abel Sinkovics和Dave Abrahams撰写文章：Using strings in C++ template metaprograms。
与使用宏+ BOOST_PP_REPEAT 的想法有一些改进 - 它不需要将显式大小传递给宏。简而言之，它基于字符串大小的固定上限和“字符串溢出保护”：
template <int N>
constexpr char at(char const(&s)[N], int i)
{
return i >= N ? '\0' : s[i];
}
加上条件 boost :: mpl :: push_back 。
我改变了我对Yankes解决方案的接受答案，因为它解决了这个特定的问题，并且在没有使用constexpr或复杂的预处理器代码的情况下做得很优雅。
如果您接受尾随零，手写宏循环， 2x 在扩展宏中重复字符串，并且没有Boost - 那么我同意 - 它更好。虽然，Boost只有三行：
#include <boost/preprocessor/repetition/repeat.hpp>
#define GET_STR_AUX(_, i, str) (sizeof(str) > (i) ? str[(i)] : 0),
#define GET_STR(str) BOOST_PP_REPEAT(64,GET_STR_AUX,str) 0
答案 5 :(得分：3)
一位同事质疑我在编译时在内存中连接字符串。它还包括在编译时实例化单个字符串。完整的代码清单如下：
//Arrange strings contiguously in memory at compile-time from string literals.
//All free functions prefixed with "my" to faciliate grepping the symbol tree
//(none of them should show up).
#include <iostream>
using std::size_t;
//wrapper for const char* to "allocate" space for it at compile-time
template<size_t N>
struct String {
//C arrays can only be initialised with a comma-delimited list
//of values in curly braces. Good thing the compiler expands
//parameter packs into comma-delimited lists. Now we just have
//to get a parameter pack of char into the constructor.
template<typename... Args>
constexpr String(Args... args):_str{ args... } { }
const char _str[N];
};
//takes variadic number of chars, creates String object from it.
//i.e. myMakeStringFromChars('f', 'o', 'o', '\0') -> String<4>::_str = "foo"
template<typename... Args>
constexpr auto myMakeStringFromChars(Args... args) -> String<sizeof...(Args)> {
return String<sizeof...(args)>(args...);
}
//This struct is here just because the iteration is going up instead of
//down. The solution was to mix traditional template metaprogramming
//with constexpr to be able to terminate the recursion since the template
//parameter N is needed in order to return the right-sized String<N>.
//This class exists only to dispatch on the recursion being finished or not.
//The default below continues recursion.
template<bool TERMINATE>
struct RecurseOrStop {
template<size_t N, size_t I, typename... Args>
static constexpr String<N> recurseOrStop(const char* str, Args... args);
};
//Specialisation to terminate recursion when all characters have been
//stripped from the string and converted to a variadic template parameter pack.
template<>
struct RecurseOrStop<true> {
template<size_t N, size_t I, typename... Args>
static constexpr String<N> recurseOrStop(const char* str, Args... args);
};
//Actual function to recurse over the string and turn it into a variadic
//parameter list of characters.
//Named differently to avoid infinite recursion.
template<size_t N, size_t I = 0, typename... Args>
constexpr String<N> myRecurseOrStop(const char* str, Args... args) {
//template needed after :: since the compiler needs to distinguish
//between recurseOrStop being a function template with 2 paramaters
//or an enum being compared to N (recurseOrStop < N)
return RecurseOrStop<I == N>::template recurseOrStop<N, I>(str, args...);
}
//implementation of the declaration above
//add a character to the end of the parameter pack and recurse to next character.
template<bool TERMINATE>
template<size_t N, size_t I, typename... Args>
constexpr String<N> RecurseOrStop<TERMINATE>::recurseOrStop(const char* str,
Args... args) {
return myRecurseOrStop<N, I + 1>(str, args..., str[I]);
}
//implementation of the declaration above
//terminate recursion and construct string from full list of characters.
template<size_t N, size_t I, typename... Args>
constexpr String<N> RecurseOrStop<true>::recurseOrStop(const char* str,
Args... args) {
return myMakeStringFromChars(args...);
}
//takes a compile-time static string literal and returns String<N> from it
//this happens by transforming the string literal into a variadic paramater
//pack of char.
//i.e. myMakeString("foo") -> calls myMakeStringFromChars('f', 'o', 'o', '\0');
template<size_t N>
constexpr String<N> myMakeString(const char (&str)[N]) {
return myRecurseOrStop<N>(str);
}
//Simple tuple implementation. The only reason std::tuple isn't being used
//is because its only constexpr constructor is the default constructor.
//We need a constexpr constructor to be able to do compile-time shenanigans,
//and it's easier to roll our own tuple than to edit the standard library code.
//use MyTupleLeaf to construct MyTuple and make sure the order in memory
//is the same as the order of the variadic parameter pack passed to MyTuple.
template<typename T>
struct MyTupleLeaf {
constexpr MyTupleLeaf(T value):_value(value) { }
T _value;
};
//Use MyTupleLeaf implementation to define MyTuple.
//Won't work if used with 2 String<> objects of the same size but this
//is just a toy implementation anyway. Multiple inheritance guarantees
//data in the same order in memory as the variadic parameters.
template<typename... Args>
struct MyTuple: public MyTupleLeaf<Args>... {
constexpr MyTuple(Args... args):MyTupleLeaf<Args>(args)... { }
};
//Helper function akin to std::make_tuple. Needed since functions can deduce
//types from parameter values, but classes can't.
template<typename... Args>
constexpr MyTuple<Args...> myMakeTuple(Args... args) {
return MyTuple<Args...>(args...);
}
//Takes a variadic list of string literals and returns a tuple of String<> objects.
//These will be contiguous in memory. Trailing '\0' adds 1 to the size of each string.
//i.e. ("foo", "foobar") -> (const char (&arg1)[4], const char (&arg2)[7]) params ->
// -> MyTuple<String<4>, String<7>> return value
template<size_t... Sizes>
constexpr auto myMakeStrings(const char (&...args)[Sizes]) -> MyTuple<String<Sizes>...> {
//expands into myMakeTuple(myMakeString(arg1), myMakeString(arg2), ...)
return myMakeTuple(myMakeString(args)...);
}
//Prints tuple of strings
template<typename T> //just to avoid typing the tuple type of the strings param
void printStrings(const T& strings) {
//No std::get or any other helpers for MyTuple, so intead just cast it to
//const char* to explore its layout in memory. We could add iterators to
//myTuple and do "for(auto data: strings)" for ease of use, but the whole
//point of this exercise is the memory layout and nothing makes that clearer
//than the ugly cast below.
const char* const chars = reinterpret_cast<const char*>(&strings);
std::cout << "Printing strings of total size " << sizeof(strings);
std::cout << " bytes:\n";
std::cout << "-------------------------------\n";
for(size_t i = 0; i < sizeof(strings); ++i) {
chars[i] == '\0' ? std::cout << "\n" : std::cout << chars[i];
}
std::cout << "-------------------------------\n";
std::cout << "\n\n";
}
int main() {
{
constexpr auto strings = myMakeStrings("foo", "foobar",
"strings at compile time");
printStrings(strings);
}
{
constexpr auto strings = myMakeStrings("Some more strings",
"just to show Jeff to not try",
"to challenge C++11 again :P",
"with more",
"to show this is variadic");
printStrings(strings);
}
std::cout << "Running 'objdump -t |grep my' should show that none of the\n";
std::cout << "functions defined in this file (except printStrings()) are in\n";
std::cout << "the executable. All computations are done by the compiler at\n";
std::cout << "compile-time. printStrings() executes at run-time.\n";
}
答案 6 :(得分：2)
似乎没有人喜欢我的另一个答案： - ＆lt;。所以这里我展示了如何将str_const转换为实际类型：
#include <iostream>
#include <utility>
// constexpr string with const member functions
class str_const {
private:
const char* const p_;
const std::size_t sz_;
public:
template<std::size_t N>
constexpr str_const(const char(&a)[N]) : // ctor
p_(a), sz_(N-1) {}
constexpr char operator[](std::size_t n) const {
return n < sz_ ? p_[n] :
throw std::out_of_range("");
}
constexpr std::size_t size() const { return sz_; } // size()
};
template <char... letters>
struct string_t{
static char const * c_str() {
static constexpr char string[]={letters...,'\0'};
return string;
}
};
template<str_const const& str,std::size_t... I>
auto constexpr expand(std::index_sequence<I...>){
return string_t<str[I]...>{};
}
template<str_const const& str>
using string_const_to_type = decltype(expand<str>(std::make_index_sequence<str.size()>{}));
constexpr str_const hello{"Hello World"};
using hello_t = string_const_to_type<hello>;
int main()
{
// char c = hello_t{}; // Compile error to print type
std::cout << hello_t::c_str();
return 0;
}
使用clang编译++ -stdlib = libc ++ -std = c ++ 14（clang 3.7）
答案 7 :(得分：2)
在玩增强版hana地图时，我遇到了这个帖子。由于答案没有解决我的问题，我找到了一个不同的解决方案，我想在这里添加，因为它可能对其他人有帮助。
我的问题是当使用带有hana字符串的boost hana映射时，编译器仍然生成了一些运行时代码（见下文）。原因显然是在编译时查询地图必须是constexpr。这是不可能的，因为BOOST_HANA_STRING宏生成一个lambda，它不能在constexpr上下文中使用。另一方面，地图需要具有不同内容的字符串为不同类型。
由于此主题中的解决方案要么使用lambda，要么不为不同的内容提供不同的类型，我发现以下方法很有帮助。它也避免了hacky str<'a', 'b', 'c'>语法。
基本思想是让Scott Schurr的版本str_const模仿人物的哈希值。它是c++14，但c++11函数的递归实现应该可以crc32。（参见here）。
// str_const from https://github.com/boostcon/cppnow_presentations_2012/blob/master/wed/schurr_cpp11_tools_for_class_authors.pdf?raw=true
#include <string>
template<unsigned Hash> ////// <- This is the difference...
class str_const2 { // constexpr string
private:
const char* const p_;
const std::size_t sz_;
public:
template<std::size_t N>
constexpr str_const2(const char(&a)[N]) : // ctor
p_(a), sz_(N - 1) {}
constexpr char operator[](std::size_t n) const { // []
return n < sz_ ? p_[n] :
throw std::out_of_range("");
}
constexpr std::size_t size() const { return sz_; } // size()
constexpr const char* const data() const {
return p_;
}
};
// Crc32 hash function. Non-recursive version of https://stackoverflow.com/a/23683218/8494588
static constexpr unsigned int crc_table[256] = {
0x00000000, 0x77073096, 0xee0e612c, 0x990951ba, 0x076dc419, 0x706af48f,
0xe963a535, 0x9e6495a3, 0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91, 0x1db71064, 0x6ab020f2,
0xf3b97148, 0x84be41de, 0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec, 0x14015c4f, 0x63066cd9,
0xfa0f3d63, 0x8d080df5, 0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b, 0x35b5a8fa, 0x42b2986c,
0xdbbbc9d6, 0xacbcf940, 0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116, 0x21b4f4b5, 0x56b3c423,
0xcfba9599, 0xb8bda50f, 0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d, 0x76dc4190, 0x01db7106,
0x98d220bc, 0xefd5102a, 0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818, 0x7f6a0dbb, 0x086d3d2d,
0x91646c97, 0xe6635c01, 0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457, 0x65b0d9c6, 0x12b7e950,
0x8bbeb8ea, 0xfcb9887c, 0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2, 0x4adfa541, 0x3dd895d7,
0xa4d1c46d, 0xd3d6f4fb, 0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9, 0x5005713c, 0x270241aa,
0xbe0b1010, 0xc90c2086, 0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4, 0x59b33d17, 0x2eb40d81,
0xb7bd5c3b, 0xc0ba6cad, 0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683, 0xe3630b12, 0x94643b84,
0x0d6d6a3e, 0x7a6a5aa8, 0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe, 0xf762575d, 0x806567cb,
0x196c3671, 0x6e6b06e7, 0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5, 0xd6d6a3e8, 0xa1d1937e,
0x38d8c2c4, 0x4fdff252, 0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60, 0xdf60efc3, 0xa867df55,
0x316e8eef, 0x4669be79, 0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f, 0xc5ba3bbe, 0xb2bd0b28,
0x2bb45a92, 0x5cb36a04, 0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a, 0x9c0906a9, 0xeb0e363f,
0x72076785, 0x05005713, 0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21, 0x86d3d2d4, 0xf1d4e242,
0x68ddb3f8, 0x1fda836e, 0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c, 0x8f659eff, 0xf862ae69,
0x616bffd3, 0x166ccf45, 0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db, 0xaed16a4a, 0xd9d65adc,
0x40df0b66, 0x37d83bf0, 0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6, 0xbad03605, 0xcdd70693,
0x54de5729, 0x23d967bf, 0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
};
template<size_t N>
constexpr auto crc32(const char(&str)[N])
{
unsigned int prev_crc = 0xFFFFFFFF;
for (auto idx = 0; idx < sizeof(str) - 1; ++idx)
prev_crc = (prev_crc >> 8) ^ crc_table[(prev_crc ^ str[idx]) & 0xFF];
return prev_crc ^ 0xFFFFFFFF;
}
// Conveniently create a str_const2
#define CSTRING(text) str_const2 < crc32( text ) >( text )
// Conveniently create a hana type_c<str_const2> for use in map
#define CSTRING_TYPE(text) hana::type_c<decltype(str_const2 < crc32( text ) >( text ))>
用法：
#include <boost/hana.hpp>
#include <boost/hana/map.hpp>
#include <boost/hana/pair.hpp>
#include <boost/hana/type.hpp>
namespace hana = boost::hana;
int main() {
constexpr auto s2 = CSTRING("blah");
constexpr auto X = hana::make_map(
hana::make_pair(CSTRING_TYPE("aa"), 1)
);
constexpr auto X2 = hana::insert(X, hana::make_pair(CSTRING_TYPE("aab"), 2));
constexpr auto ret = X2[(CSTRING_TYPE("aab"))];
return ret;
}
使用clang-cl 5.0得到的汇编代码是：
012A1370 mov eax,2
012A1375 ret
答案 8 :(得分：2)
kacey用于创建唯一编译时类型的解决方案可以通过微小修改也可以与C ++ 11一起使用：
template <char... Chars>
struct string_t {};
namespace detail {
template <typename Str,unsigned int N,char... Chars>
struct make_string_t : make_string_t<Str,N-1,Str().chars[N-1],Chars...> {};
template <typename Str,char... Chars>
struct make_string_t<Str,0,Chars...> { typedef string_t<Chars...> type; };
} // namespace detail
#define CSTR(str) []{ \
struct Str { const char *chars = str; }; \
return detail::make_string_t<Str,sizeof(str)>::type(); \
}()
使用：
template <typename String>
void test(String) {
// ... String = string_t<'H','e','l','l','o','\0'>
}
test(CSTR("Hello"));
答案 9 :(得分：2)
基于来自Howard Hinnant的想法，您可以创建将两个文字一起添加的文字类。
template<int>
using charDummy = char;
template<int... dummy>
struct F
{
const char table[sizeof...(dummy) + 1];
constexpr F(const char* a) : table{ str_at<dummy>(a)..., 0}
{
}
constexpr F(charDummy<dummy>... a) : table{ a..., 0}
{
}
constexpr F(const F& a) : table{ a.table[dummy]..., 0}
{
}
template<int... dummyB>
constexpr F<dummy..., sizeof...(dummy)+dummyB...> operator+(F<dummyB...> b)
{
return { this->table[dummy]..., b.table[dummyB]... };
}
};
template<int I>
struct get_string
{
constexpr static auto g(const char* a) -> decltype( get_string<I-1>::g(a) + F<0>(a + I))
{
return get_string<I-1>::g(a) + F<0>(a + I);
}
};
template<>
struct get_string<0>
{
constexpr static F<0> g(const char* a)
{
return {a};
}
};
template<int I>
constexpr auto make_string(const char (&a)[I]) -> decltype( get_string<I-2>::g(a) )
{
return get_string<I-2>::g(a);
}
constexpr auto a = make_string("abc");
constexpr auto b = a+ make_string("def"); // b.table == "abcdef"
答案 10 :(得分：1)
你的方法＃1是正确的。
但是，数组需要有外部链接，所以要使方法1起作用，我们必须编写如下内容： constexpr const char str [] =“Hello，world！”;
不，不正确。这与clang和gcc编译。我希望它的标准c ++ 11，但我不是语言知识。
#include <iostream>
template <char... letters>
struct string_t{
static char const * c_str() {
static constexpr char string[]={letters...,'\0'};
return string;
}
};
// just live with it, but only once
using Hello_World_t = string_t<'H','e','l','l','o',' ','w','o','r','l','d','!'>;
template <typename Name>
void print()
{
//String as template parameter
std::cout << Name::c_str();
}
int main() {
std::cout << Hello_World_t::c_str() << std::endl;
print<Hello_World_t>();
return 0;
}
我真正喜欢的c ++ 17将是以下相同的（完成方法＃1）
// for template <char...>
<"Text"> == <'T','e','x','t'>
模板化用户定义文字的标准中已存在非常相似的内容，因为void-pointer也会提及，但仅适用于数字。 在此之前，另一个小技巧是使用覆盖编辑模式+复制和粘贴
string_t<' ',' ',' ',' ',' ',' ',' ',' ',' ',' ',' ',' '>;
如果你不介意这个宏，那么（从Yankes回答中稍微修改一下）：
#define MACRO_GET_1(str, i) \
(sizeof(str) > (i) ? str[(i)] : 0)
#define MACRO_GET_4(str, i) \
MACRO_GET_1(str, i+0), \
MACRO_GET_1(str, i+1), \
MACRO_GET_1(str, i+2), \
MACRO_GET_1(str, i+3)
#define MACRO_GET_16(str, i) \
MACRO_GET_4(str, i+0), \
MACRO_GET_4(str, i+4), \
MACRO_GET_4(str, i+8), \
MACRO_GET_4(str, i+12)
#define MACRO_GET_64(str, i) \
MACRO_GET_16(str, i+0), \
MACRO_GET_16(str, i+16), \
MACRO_GET_16(str, i+32), \
MACRO_GET_16(str, i+48)
//CT_STR means Compile-Time_String
#define CT_STR(str) string_t<MACRO_GET_64(#str, 0), 0 >//guard for longer strings
print<CT_STR(Hello World!)>();
答案 11 :(得分：0)
我想对@ user1115339的answer进行两个非常小的改进。我在答案的注释中提到了它们，但为方便起见，我将在此处放置一个复制粘贴解决方案。
唯一的区别是FIXED_CSTRING宏，它允许使用类模板中的字符串以及作为索引运算符的参数（如果您具有例如编译时映射，则很有用）。
namespace variadic_toolbox
{
template<unsigned count,
template<unsigned...> class meta_functor, unsigned... indices>
struct apply_range
{
typedef typename apply_range<count-1, meta_functor, count-1, indices...>::result result;
};
template<template<unsigned...> class meta_functor, unsigned... indices>
struct apply_range<0, meta_functor, indices...>
{
typedef typename meta_functor<indices...>::result result;
};
}
namespace compile_time
{
template<char... str>
struct string
{
static constexpr const char chars[sizeof...(str)+1] = {str..., '\0'};
};
template<char... str>
constexpr const char string<str...>::chars[sizeof...(str)+1];
template<typename lambda_str_type>
struct string_builder
{
template<unsigned... indices>
struct produce
{
typedef string<lambda_str_type{}.chars[indices]...> result;
};
};
}
#define CSTRING(string_literal) \
[]{ \
struct constexpr_string_type { const char * chars = string_literal; }; \
return variadic_toolbox::apply_range<sizeof(string_literal)-1, \
compile_time::string_builder<constexpr_string_type>::produce>::result{}; \
}()
#define FIXED_CSTRING(string_literal) \
([]{ \
struct constexpr_string_type { const char * chars = string_literal; }; \
return typename variadic_toolbox::apply_range<sizeof(string_literal)-1, \
compile_time::string_builder<constexpr_string_type>::template produce>::result{}; \
}())
struct A {
auto test() {
return FIXED_CSTRING("blah"); // works
// return CSTRING("blah"); // works too
}
template<typename X>
auto operator[](X) {
return 42;
}
};
template<typename T>
struct B {
auto test() {
// return CSTRING("blah");// does not compile
return FIXED_CSTRING("blah"); // works
}
};
int main() {
A a;
//return a[CSTRING("blah")]; // fails with error: two consecutive ' [ ' shall only introduce an attribute before ' [ ' token
return a[FIXED_CSTRING("blah")];
}
答案 12 :(得分：0)
我自己的实现基于Boost.Hana字符串（带有可变字符的模板类）中的方法，但是仅使用C++11标准和constexpr函数并严格检查编译时间（会如果不是编译时间表达式，则为编译时间错误）。可以使用普通的原始C字符串（而不是精美的{'a', 'b', 'c' }（通过宏）构造）。
测试： https://sourceforge.net/p/tacklelib/tacklelib/HEAD/tree/trunk/src/tests/unit/test_tmpl_string.cpp
用法示例：
const auto s0 = TACKLE_TMPL_STRING(0, "012"); // "012"
const char c1_s0 = UTILITY_CONSTEXPR_GET(s0, 1); // '1'
const auto s1 = TACKLE_TMPL_STRING(0, "__012", 2); // "012"
const char c1_s1 = UTILITY_CONSTEXPR_GET(s1, 1); // '1'
const auto s2 = TACKLE_TMPL_STRING(0, "__012__", 2, 3); // "012"
const char c1_s2 = UTILITY_CONSTEXPR_GET(s2, 1); // '1'
// TACKLE_TMPL_STRING(0, "012") and TACKLE_TMPL_STRING(1, "012")
// - semantically having different addresses.
// So id can be used to generate new static array class field to store
// a string bytes at different address.
// Can be overloaded in functions with another type to express the compiletimeness between functions:
template <uint64_t id, typename CharT, CharT... tchars>
const overload_resolution_1 & test_overload_resolution(const tackle::tmpl_basic_string<id, CharT, tchars...> &);
template <typename CharT>
const overload_resolution_2 & test_overload_resolution(const tackle::constexpr_basic_string<CharT> &);
// , where `constexpr_basic_string` is another approach which loses
// the compiletimeness between function signature and body border,
// because even in a `constexpr` function the compile time argument
// looses the compiletimeness nature and becomes a runtime one.
有关constexpr函数的编译时间边界的详细信息：https://www.boost.org/doc/libs/1_65_0/libs/hana/doc/html/index.html#tutorial-appendix-constexpr
有关其他用法的详细信息，请参见测试。
整个项目目前处于实验阶段。
答案 13 :(得分：0)
在带有辅助宏功能的C ++ 17中，创建编译时间字符串很容易：
template <char... Cs>
struct ConstexprString
{
static constexpr int size = sizeof...( Cs );
static constexpr char buffer[size] = { Cs... };
};
template <char... C1, char... C2>
constexpr bool operator==( const ConstexprString<C1...>& lhs, const ConstexprString<C2...>& rhs )
{
if( lhs.size != rhs.size )
return false;
return std::is_same_v<std::integer_sequence<char, C1...>, std::integer_sequence<char, C2...>>;
}
template <typename F, std::size_t... Is>
constexpr auto ConstexprStringBuilder( F f, std::index_sequence<Is...> )
{
return ConstexprString<f( Is )...>{};
}
#define CONSTEXPR_STRING( x ) \
ConstexprStringBuilder( []( std::size_t i ) constexpr { return x[i]; }, \
std::make_index_sequence<sizeof(x)>{} )
这是一个用法示例：
auto n = CONSTEXPR_STRING( "ab" );
auto m = CONSTEXPR_STRING( "ab" );
static_assert(n == m);
答案 14 :(得分：0)
@smilingthax 的解决方案可以通过使用 std::index_sequence 来缩短：
template<char...>
struct Str {};
template<class T, size_t... Is>
[[nodiscard]] constexpr auto helper(std::index_sequence<Is...>) {
return Str<T{}.chars[Is]...>{};
}
#define STR(str) \
[] { \
struct Temp { \
const char* chars = str; \
}; \
return helper<Temp>(std::make_index_sequence<sizeof(str) - 1>{}); \
}()
甚至更短：
template<char...>
struct Str {};
#define STR(str) \
[]<size_t... Is>(std::index_sequence<Is...>) { \
return Str<str[Is]...>{}; \
} \
(std::make_index_sequence<sizeof(str) - 1>{})
答案 15 :(得分：0)
改编自 #QuarticCat 的回答
template <char...>
struct Str
{
};
#define STRNAME(str) _constexpr_string_type_helper_##str
#define STR(str) \
auto STRNAME(str) = []<size_t... Is>(std::index_sequence<Is...>) \
{ \
constexpr char chars[] = #str; \
return Str<chars[Is]...>{}; \
} \
(std::make_index_sequence<sizeof(#str) - 1>{}); \
decltype(STRNAME(str))
答案 16 :(得分：0)
非 lambda 版本，使用 std::min 和 sizeof。
购买字符串长度限制到256。
这可用于未评估的上下文，例如 decltype 或 sizeof。
我使用了邮票宏来减少代码大小。
#include <type_traits>
#include <utility>
template <char...>
struct Str
{
};
namespace char_mpl
{
constexpr auto first(char val, char...)
{
return val;
}
constexpr auto second(char, char val, char...)
{
return val;
}
template <class S1, class S2>
struct Concat;
template <char... lefts, char... rights>
struct Concat<Str<lefts...>, Str<rights...>>
{
using type = Str<lefts..., rights...>;
};
template <size_t right_count, class Right>
struct Take;
template <template <char...> class Right, char... vals>
struct Take<0, Right<vals...>>
{
using type = Str<>;
};
template <template <char...> class Right, char... vals>
struct Take<1, Right<vals...>>
{
using type = Str<first(vals...)>;
};
template <template <char...> class Right, char... vals>
struct Take<2, Right<vals...>>
{
using type = Str<first(vals...), second(vals...)>;
};
template <size_t lhs, size_t rhs>
concept greater = lhs > rhs;
// this may be improved for speed.
template <size_t n, char left, char... vals>
requires greater<n, 2> struct Take<n, Str<left, vals...>>
{
using type =
Concat<Str<left>, //
typename Take<n - 1, Str<vals...>>::type//
>::type;
};
};// namespace char_mpl
template <int length, char... vals>
struct RawStr
{
constexpr auto ch(char c, int i)
{
return c;
}
constexpr static auto to_str()
{
return
typename char_mpl::Take<length,
Str<vals...>>::type{};
}
};
#define STAMP4(n, STR, stamper) \
stamper(n, STR) stamper(n + 1, STR) \
stamper(n + 2, STR) stamper(n + 3, STR)
#define STAMP16(n, STR, stamper) \
STAMP4(n, STR, stamper) \
STAMP4(n + 4, STR, stamper) \
STAMP4(n + 8, STR, stamper) \
STAMP4(n + 12, STR, stamper)
#define STAMP64(n, STR, stamper) \
STAMP16(n, STR, stamper) \
STAMP16(n + 16, STR, stamper) \
STAMP16(n + 32, STR, stamper) \
STAMP16(n + 48, STR, stamper)
#define STAMP256(n, STR, stamper) \
STAMP64(n, STR, stamper) \
STAMP64(n + 64, STR, stamper) \
STAMP64(n + 128, STR, stamper) \
STAMP64(n + 192, STR, stamper)
#define STAMP(n, STR, stamper) stamper(STAMP##n, STR, n)
#define CH(STR, i) STR[std::min<size_t>(sizeof(STR) - 1, i)]
#define CSTR_STAMPER_CASE(n, STR) CH(STR, n),
#define CSTR_STAMPER(stamper, STR, n) \
RawStr<sizeof(STR) - 1, \
stamper(0, STR, CSTR_STAMPER_CASE) \
CH(STR, 256)>
#define CSTR(STR) (STAMP(256, STR, CSTR_STAMPER){}).to_str()
int main()
{
constexpr auto s = CSTR("12345");
decltype(CSTR("123123"));
sizeof(CSTR("123123"));
static_assert(
std::is_same_v<
Str<'1'>,
std::remove_cvref_t<decltype(CSTR("1"))>>);
static_assert(
std::is_same_v<
Str<'1', '2'>,
std::remove_cvref_t<decltype(CSTR("12"))>>);
static_assert(
std::is_same_v<
Str<'1', '2', '3', '4', '5'>,
std::remove_cvref_t<decltype(CSTR("12345"))>>);
}