Conveniently Declaring Compile-Time Strings in C++
I haven't seen anything to match the elegance of Scott Schurr's str_const presented at C++ Now 2012. It does require constexpr though.
Here's how you can use it, and what it can do:
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!
}
It doesn't get much cooler than compile-time range checking!
Both the use, and the implementation, is free of macros. And there is no artificial limit on string size. I'd post the implementation here, but I'm respecting Scott's implicit copyright. The implementation is on a single slide of his presentation linked to above.
Update C++17
In the years since I posted this answer, std::string_view has become part of our tool chest. Here is how I would rewrite the above using string_view:
#include <string_view>
int
main()
{
constexpr std::string_view my_string = "Hello, world!";
static_assert(my_string.size() == 13);
static_assert(my_string[4] == 'o');
constexpr std::string_view my_other_string = my_string;
static_assert(my_string == my_other_string);
constexpr std::string_view world(my_string.substr(7, 5));
static_assert(world == "world");
// constexpr char x = world.at(5); // Does not compile because index is out of range!
}
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:
- lambdas variadic
- templates
- generalized constant expressions
- non-static data member initializers
- uniform initialization
(the latter two are not strictly required here)
-
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; }; } -
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]; } -
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.
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