boostorg
BOOST_METAPARSE_STRING_VALUE can be implemented without length limitation since C++17
We don't need to define BOOST_METAPARSE_LIMIT_STRING_SIZE macro anymore. Also I think compilation time will be reduced(haven't measured yet, but I believe).
#include <boost/metaparse/string.hpp>
template<typename CharT, size_t N> struct fixed_string {
const CharT* data;
constexpr fixed_string(const CharT (&s)[N]) noexcept:
data{s} {
}
constexpr auto operator[](size_t i) const noexcept { return data[i]; }
constexpr size_t size() const noexcept { return N; }
};
template<typename CharT, size_t N>
fixed_string(const CharT(&)[N]) -> fixed_string<CharT, N>;
template<const auto& S>
struct convert_string {
template<size_t... Is>
static constexpr auto apply(std::index_sequence<Is...>) noexcept {
return boost::metaparse::string<S[Is]...>{};
}
};
#define BOOST_METAPARSE_STRING_VALUE(S) ([]{ \
static constexpr fixed_string fs(S); \
return convert_string<fs>::apply(std::make_index_sequence<fs.size() - 1>{}); \
}())
The full snippet to play: https://godbolt.org/z/j117Karjq
Only one limitation - BOOST_METAPARSE_STRING_VALUE can't be used in decltype expressions:
decltype(BOOST_METAPARSE_STRING_VALUE("test")) a; // error: lambda-expression in unevaluated context only available with '-std=c++20' or '-std=gnu++20'
The workaround is to create temporary variable:
auto t = BOOST_METAPARSE_STRING_VALUE("test");
decltype(t) a; // OK
If it's interested, I can create PR.
This is a good idea, but the unevaluated context is the original purpose of BOOST_METAPARSE_STRING (Boost.Metaparse is for parsing with template metaprograms). ˋBOOST_METAPARSE_STRING_VALUEˋ is a nice to have feature on top of it. So this should only be enabled in standards where it works in an unevaluated context as well. If you take a look at the implementation of ˋBOOST_METAPARSE_STRINGˋ, it already has a framework for standard version specific implementations, so this can be extended to drop the need for the length limit where possible.
