Conveniently Declaring Compile-Time Strings in C++

I believe it should be possible to define a C preprocessor macro that takes a string and the size of the string as arguments, and returns a sequence consisting of the characters in the string (using BOOST_PP_FOR, stringification, array subscripts, and the like). However, I do not have the time (or enough interest) to implement such a macro

it is possible to implement this without relying on boost, using very simple macro and some of C++11 features:

1. lambdas variadic
2. templates
3. generalized constant expressions
4. non-static data member initializers
5. uniform initialization

(the latter two are not strictly required here)

1. we need to be able to instantiate a variadic template with user supplied indicies from 0 to N - a tool also useful for example to expand tuple into variadic template function's argument (see questions: 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; }; } 

2. then define a variadic template called string with non-type parameter 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]; } 

3. now the most interesting part - to pass character literals into string template:

   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{}; \ }() 

a simple concatenation demonstration shows the usage:

 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; } 

https://ideone.com/8Ft2xu

Edit: as Howard Hinnant (and me somewhat in my comment to the OP) pointed out, you might not need a type with every single character of the string as a single template argument. If you do need this, there's a macro-free solution below.

There's a trick I found while trying to work with strings at compile time. It requires to introduce another type besides the "template string", but within functions, you can limit the scope of this type.

It doesn't use macros but rather some C++11 features.

#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); } // destination "template string" type template < char... chars > struct exploded_string { static void print() { char const str[] = { chars... }; std::cout.write(str, sizeof(str)); } }; // struct to explode a `char const*` to an `exploded_string` type template < typename StrProvider, unsigned len, char... chars > struct explode_impl { using result = typename explode_impl < StrProvider, len-1, StrProvider::str()[len-1], chars... > :: result; }; // recursion end template < typename StrProvider, char... chars > struct explode_impl < StrProvider, 0, chars... > { using result = exploded_string < chars... >; }; // syntactical sugar template < typename StrProvider > using explode = typename explode_impl < StrProvider, c_strlen(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(); } 

If you don't want to use the Boost solution you can create simple macros that will do something similar:

#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!")>; 

The only problem is the fixed size of 64 chars (plus additional zero). But it can easily be changed depending on your needs.

Here's a succinct C++14 solution to creating a std::tuple<char...> for each compile-time string passed.

#include <tuple> #include <utility> namespace detail { template <std::size_t ... indices> decltype(auto) build_string(const char * str, std::index_sequence<indices...>) { return std::make_tuple(str[indices]...); } } template <std::size_t N> constexpr decltype(auto) make_string(const char(&str)[N]) { return detail::build_string(str, std::make_index_sequence<N>()); } auto HelloStrObject = make_string("hello"); 

And here's one for creating a unique compile-time type, trimmed down from the other macro post.

#include <utility> template <char ... Chars> struct String {}; template <typename Str, std::size_t ... indices> decltype(auto) build_string(std::index_sequence<indices...>) { return String<Str().chars[indices]...>(); } #define make_string(str) []{\ struct Str { const char * chars = str; };\ return build_string<Str>(std::make_index_sequence<sizeof(str)>());\ }() auto HelloStrObject = make_string("hello"); 

It's really too bad that user-defined literals can't be used for this yet.

Nobody seems to like my other answer :-<. So here I show how to convert a str_const to a real type:

#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; } 

Compiles with clang++ -stdlib=libc++ -std=c++14 (clang 3.7)

I believe it should be possible to define a C preprocessor macro that takes a string and the size of the string as arguments, and returns a sequence consisting of the characters in the string (using BOOST_PP_FOR, stringification, array subscripts, and the like)

There is article: Using strings in C++ template metaprograms by Abel Sinkovics and Dave Abrahams.

It has some improvement over your idea of using macro + BOOST_PP_REPEAT - it doesn't require passing explicit size to macro. In short, it is based on fixed upper limit for string size and "string overrun protection":

template <int N> constexpr char at(char const(&s)[N], int i) { return i >= N ? '\0' : s[i]; } 

plus conditional boost::mpl::push_back.

I changed my accepted answer to Yankes' solution, since it solves this specific problem, and does so elegantly without the use of constexpr or complex preprocessor code.

If you accept trailing zeros, hand-written macro looping, 2x repetion of string in expanded macro, and don't have Boost - then I agree - it is better. Though, with Boost it would be just three lines:

LIVE DEMO

#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 

A colleague challenged me to concatenate strings in memory at compile-time. It includes instantiating individual strings at compile-time as well. The full code listing is here:

//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"; } 

Your approach #1 is the correct one.

However, the array would need to have external linkage, so to get approach 1 to work, we would have to write something like this: constexpr const char str[] = "Hello, world!";

No, not correct. This compiles with clang and gcc. I hope its standard c++11, but i am not a language laywer.

#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; } 

What I would really love for c++17 would be the following to be equivalent (to complete approach #1)

// for template <char...> <"Text"> == <'T','e','x','t'> 

Something very similar already exists in the standard for templated user defined literals,as void-pointer also mentions, but only for digits. Until then another little trick is to use the override editing mode + copy and paste of

string_t<' ',' ',' ',' ',' ',' ',' ',' ',' ',' ',' ',' '>; 

If you do not mind the macro, than this works(slighty modified from Yankes answer):

#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!)>(); 

kacey's solution for creating a unique compile-time type can, with minor modifications, also be used with C++11:

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(); \ }() 

Use:

template <typename String> void test(String) { // ... String = string_t<'H','e','l','l','o','\0'> } test(CSTR("Hello")); 

While playing with the boost hana map, I came across this thread. As non of the answers solved my problem, I found a different solution which I want to add here as it could be potentially helpful for others.

My problem was that when using the boost hana map with hana strings, the compiler still generated some runtime code (see below). The reason was obviously that to query the map at compile-time it must be constexpr. This isn't possible as the BOOST_HANA_STRING macro generates a lambda, which can't be used in constexpr context. On the other hand, the map needs strings with different content to be different types.

As the solutions in this thread are either using a lambda or not providing different types for different contents, I found the following approach helpful. Also it avoids the hacky str<'a', 'b', 'c'> syntax.

The basic idea is having a version of Scott Schurr's str_const templated on the hash of the characters. It is c++14, but c++11 should be possible with a recursive implementation of the crc32 function (see 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 ))> 

Usage:

#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; } 

Resulting assembler code with clang-cl 5.0 is:

012A1370 mov eax,2 012A1375 ret 

In C++17 with a helper macro function it's easy to create compile time strings:

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)>{} ) 

And this is a usage example:

auto n = CONSTEXPR_STRING( "ab" ); auto m = CONSTEXPR_STRING( "ab" ); static_assert(n == m); 

C++17 solution

I would recommend you to use a custom literal operator

template <char... chars> using text = std::integer_sequence<char, chars...>; template <typename T, T... chars> constexpr text<chars...> operator""_text() { return { }; } 

Which will allow you to get from "some_text"_text equivalent of the std::integer_sequence<'s','o','m','e','_','t','e','x','t'> And then create a template where you will check and use this type

template <typename> struct Field; template <char... chars> struct Field<text<chars...>> { static constexpr std::string_view Name() { constexpr const char data[] = {chars..., '\0'}; return data; } }; 

Finally you will be able to use it like:

Field<decltype("field_name"_text)> object; static_assert(object.Name() == std::string_view("field_name")); 

C++20 solution

You will need a custom class for you text, it will store you text in memory

template <size_t N> struct StringLiteral { constexpr StringLiteral(const char (&_str)[N]) : value_(std::to_array(_str)) {} constexpr std::string_view Name() const { return std::string_view(value_.data(), N - 1); } const std::array<char, N> value_; }; 

And in C++20 you can finally use custom classes as a template argument and you can write:

template <StringLiteral name__> struct Field { constexpr std::string_view Name() const { return name__.Name(); } }; 

The creation of such an object will be really simple:

Field<"field_name"> object; static_assert(object.Name() == std::string_view("field_name")); 

based on idea from Howard Hinnant you can create literal class that will add two literals together.

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" 

What you are looking for is N3599 Literal operator templates for strings. It was proposed for C++ in 2013 but there was no consensus on the details and it was never added to the standard.

However, GCC and Clang support it as an extension. It lets you split string literals to a template parameter pack of characters:

// some template type to represent a string template <char... chars> struct TemplateString { static constexpr char value[] = { chars... }; template <char... chars2> constexpr auto operator+(TemplateString<chars2...>) const { // compile-time concatenation, oh yeah! return TemplateString<chars..., chars2...>{}; } }; // a custom user-defined literal called by the compiler when you use your _suffix template <typename CharType, CharType... chars> constexpr auto operator""_tstr () { // since all the chars are constants here, you can do compile-time // processing with constexpr functions and/or template metaprogramming, // and then return whatever converted type you like return TemplateString<chars...>{}; } // auto = TemplateString<'H', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '!'> constexpr auto str = "Hello"_tstr + " world!"_tstr; cout << str.value << endl; 

As a fallback, the tricks using a macro get you to the same place (as shown in the answer by smilingthax, for example).

Please note that those are the only two ways to accept string literals and split them to constexpr chars: either you use the extension, or you use macro hackery at the call site.

I'd like to add two very small improvements to the answer of @user1115339. I mentioned them in the comments to the answer, but for convenience I'll put a copy paste solution here.

The only difference is the FIXED_CSTRING macro, which allows to use the strings within class templates and as arguments to the index operator (useful if you have e.g. a compiletime map).

Live example.

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")]; } 

Adapted from #QuarticCat's answer

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)) 

Full code here

Non lambda version, using std::min and sizeof.
Buy the length of string is limited to 256.
This can be used in unevaluated context, such as decltype or sizeof.
I used stamp macros to reduce the code size.

#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"))>>); } 

@smilingthax's solution can be shorter by using std::index_sequence:

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>{}); \ }() 

or even shorter:

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>{}) 

My own implementation is based on approach from the Boost.Hana string (template class with variadic characters), but utilizes only the C++11 standard and constexpr functions with strict check on compiletimeness (would be a compile time error if not a compile time expression). Can be constructed from the usual raw C string instead of fancy {'a', 'b', 'c' } (through a macro).

Implementation: https://github.com/andry81/tacklelib/tree/HEAD/include/tacklelib/tackle/tmpl_string.hpp

Tests: https://github.com/andry81/tacklelib/tree/HEAD/src/tests/unit/test_tmpl_string.cpp

Usage examples:

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. 

The details about a constexpr function compile time border: https://www.boost.org/doc/libs/1_65_0/libs/hana/doc/html/index.html#tutorial-appendix-constexpr

For other usage details see the tests.

The entire project currently is experimental.

Some improvements were made:

#pragma once // jaf 2023-6-17 // Compile-time string concatenation // Usage: // constexpr auto demo_1 = constr::Str("123"); // constexpr auto demo_2 = constr::Str({ '4', '5', '6', 0 }); // When you define a string this way, you append an extra zero at the end // constexpr auto demo_sum = demo_1 + demo_2; namespace constr { // Creating sequences of integers // Converting an integer to a sequence of integers, such as 10 to 0,1,2,3,4,5,6,7,8,9 // The usage is roughly as follows: // gen_seq<10>::type; // Implement the C++14 make_integer_sequence function for use with the C++11 standard template<class T> using Invoke = typename T::type; template<unsigned...> struct seq { using type = seq; }; template<class S1, class S2> struct concat; template<unsigned... I1, unsigned... I2> struct concat<seq<I1...>, seq<I2...>> : seq<I1..., (sizeof...(I1) + I2)...> {}; template<class S1, class S2> using Concat = Invoke<concat<S1, S2>>; template<unsigned N> struct gen_seq; template<unsigned N> using GenSeq = Invoke<gen_seq<N>>; template<unsigned N> struct gen_seq : Concat<GenSeq<N / 2>, GenSeq<N - N / 2>> {}; template<> struct gen_seq<0> : seq<> {}; template<> struct gen_seq<1> : seq<0> {}; } namespace constr { // Compile-time string template<int Size> struct String { char str[Size + 1]; // int size = Size; // The length of the string data, excluding the final 0 character // Define the character type.Let the types Char<0>, Char<1>, Char<2>, and Char<3> be defined as char template<char> using Char = char; // Constructor 1, in order to initialize str with the form "abc", a sequence of integers is used template <int... Index> constexpr String(const char* str, seq<Index...>) :str{ str[Index]..., 0 } {} // Constructor 2, which constructs the object by passing in a constant string constexpr String(const char* str) : String(str, gen_seq<Size>()) {} // Constructor 3, which concatenates strings by passing multiple characters template <int... Index> constexpr String(seq<Index...>, Char<Index>... str) : str{ str..., 0 } {} // Constructor 4, Copy constructor template <int... Index> constexpr String(const String<Size>& other) : str{ other.str[Index]..., 0 } {} // Constructor 5, initialized with a single character constexpr String(char c) : str{ c , 0} {} // Constructor 6, which initializes the empty compile time string constexpr String() : str{ 0 } {} }; // Auxiliary string addition struct Assist { template <int... AIndex,int... BIndex> constexpr static String<sizeof...(AIndex) + sizeof...(BIndex)> Add(const String<sizeof...(AIndex)>& str1, const String<sizeof...(BIndex)>& str2, seq<AIndex...>, seq<BIndex...>) { // Call constr::String constructor 3 to do the addition return String<sizeof...(AIndex) + sizeof...(BIndex)>(gen_seq<sizeof...(AIndex) + sizeof...(BIndex)>(), str1.str[AIndex]..., str2.str[BIndex]...); } template <int... Index> constexpr static String<sizeof...(Index) + 1> Add(const String<sizeof...(Index)>& str1, char c, seq<Index...>) { // Call constr::String constructor 3 to add a character return String<sizeof...(Index) + 1>(gen_seq<sizeof...(Index) + 1>(), str1.str[Index]..., c); } template <int... Index> constexpr static String<sizeof...(Index) + 1> Add(char c, const String<sizeof...(Index)>& str1, seq<Index...>) { // Call constr::String constructor 3 to add a character return String<sizeof...(Index) + 1>(gen_seq<sizeof...(Index) + 1>(), c, str1.str[Index]...); } }; // String plus string template<int ASize, int BSize> constexpr String<ASize + BSize> operator+(const String<ASize>& str1, const String<BSize>& str2) { return Assist::Add(str1, str2, gen_seq<ASize>(), gen_seq<BSize>()); } // Followed by a character template<int Size> constexpr String<Size + 1> operator+(const String<Size>& str, char c) { return Assist::Add(str, c, gen_seq<Size>()); } // Prefix it with a string template<int Size> constexpr String<Size + 1> operator+(char c, const String<Size>& str) { return Assist::Add(c, str, gen_seq<Size>()); } // Followed by a string ,example: constr::str("123") + "abc"; template<int Size1, int I> constexpr String<Size1 + I - 1> operator+(const String<Size1>& str1, const char(&str2)[I]) { return str1 + String<I - 1>(str2); } // Prefix it with a string ,example: "abc" + constr::str("123"); template<int Size1, int I> constexpr String<Size1 + I - 1> operator+(const char(&str1)[I], const String<Size1>& str2) { return String<I - 1>(str1) + str2; } template<int I> constexpr String<I - 1> Str(const char(&a)[I]) { return String<I - 1>(a); } constexpr String<0> Str() { return String<0>(); } constexpr String<1> Str(char c) { return String<1>(c); } } 

Test:

int main(int argc, char** argv) { constexpr auto demo_1 = constr::Str("123"); constexpr auto demo_2 = constr::Str({ '4', '5', '6', '\0'}); constexpr auto demo_3 = demo_1 + demo_2; constexpr auto demo_4 = demo_1 + 'a'; constexpr auto demo_5 = 'a' + demo_1; constexpr auto demo_6 = demo_1 + "abc"; constexpr auto demo_7 = "abc" + demo_1 ; constexpr auto demo_8 = constr::Str('a'); constexpr auto demo_9 = constr::Str(); std::cout << "demo_3.size = " << demo_3.size << std::endl; std::cout << "demo_3.str = " << demo_3.str << std::endl; std::cout << "demo_4.str = " << demo_4.str << std::endl; std::cout << "demo_5.str = " << demo_5.str << std::endl; std::cout << "demo_6.str = " << demo_6.str << std::endl; std::cout << "demo_7.str = " << demo_7.str << std::endl; std::cout << "demo_8.str = " << demo_8.str << std::endl; std::cout << "demo_9.str = " << demo_9.str << std::endl; return 0; } 

Test result:

demo_3.size = 6 demo_3.str = 123456 demo_4.str = 123a demo_5.str = a123 demo_6.str = 123abc demo_7.str = abc123 demo_8.str = a demo_9.str =