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Use C++20 for ""_s if available #521

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Use C++20 for ""_s if available #521

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cd5c869
Use C++20 for ""_s if available
ecatmur Aug 8, 2023
110cbcb
formatting
ecatmur Aug 8, 2023
79b8f55
Fix detection of C++20 string UDL support
ecatmur Aug 8, 2023
2352f1a
use unsigned in ctor for consistency
ecatmur Aug 8, 2023
9502d00
formatting
ecatmur Aug 8, 2023
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3 changes: 2 additions & 1 deletion CMakeLists.txt
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Expand Up @@ -42,7 +42,8 @@ option(BOOST_HANA_ENABLE_DEBUG_MODE "Enable Hana's debug mode." OFF)

option(BOOST_HANA_ENABLE_STRING_UDL
"Enable the GNU extension allowing the special string literal operator\
template, which enables the _s suffix for creating compile-time strings." ON)
template, which enables the _s suffix for creating compile-time strings\
(unnecessary if you have a C++20 compiler)." ON)

option(BOOST_HANA_ENABLE_EXCEPTIONS
"Build with exceptions enabled. Note that Hana does not make use of exceptions,\
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2 changes: 1 addition & 1 deletion example/string/literal.cpp
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Expand Up @@ -13,7 +13,7 @@ namespace hana = boost::hana;
using namespace hana::literals;


// By default, this is disabled
// This requires C++20 or a compiler extension which is disabled by default
#ifdef BOOST_HANA_CONFIG_ENABLE_STRING_UDL

constexpr auto str = "Hello world!"_s;
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49 changes: 41 additions & 8 deletions include/boost/hana/config.hpp
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Expand Up @@ -80,6 +80,35 @@ Distributed under the Boost Software License, Version 1.0.
# endif
#endif

//////////////////////////////////////////////////////////////////////////////
// C++20 string literal UDL suport
//////////////////////////////////////////////////////////////////////////////
#if defined(_MSC_VER) && !defined(__clang__)
# if _MSVC_LANG > 201703 && _MSC_FULL_VER >= 192829921
# define BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
# endif
#elif defined(__clang__)
# if (__cplusplus > 201703 && \
BOOST_HANA_CONFIG_CLANG >= BOOST_HANA_CONFIG_VERSION(12, 0, 0))
# define BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
# if BOOST_HANA_CONFIG_CLANG < BOOST_HANA_CONFIG_VERSION(18, 0, 0)
# define BOOST_HANA_CONFIG_CXX20_STRING_UDL_CLANG_WORKAROUND
# endif
# endif
#elif defined(__GNUC__)
# if (__cplusplus > 201703 && \
BOOST_HANA_CONFIG_GCC >= BOOST_HANA_CONFIG_VERSION(9, 3, 0))
# define BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
# endif
#elif (__cpp_deduction_guides >= 201703 && \
__cpp_nontype_template_args >= 201911)
# define BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
#endif
#if defined(BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL) && \
!defined(BOOST_HANA_CONFIG_ENABLE_STRING_UDL)
# define BOOST_HANA_CONFIG_ENABLE_STRING_UDL
#endif

//////////////////////////////////////////////////////////////////////////////
// Caveats and other compiler-dependent options
//////////////////////////////////////////////////////////////////////////////
Expand Down Expand Up @@ -145,17 +174,21 @@ Distributed under the Boost Software License, Version 1.0.
# define BOOST_HANA_CONFIG_DISABLE_CONCEPT_CHECKS
#endif

#if defined(BOOST_HANA_DOXYGEN_INVOKED)
#if defined(BOOST_HANA_DOXYGEN_INVOKED) && \
!defined(BOOST_HANA_CONFIG_ENABLE_STRING_UDL)
//! @ingroup group-config
//! Enables usage of the "string literal operator template" GNU extension.
//! Enables usage of the "string literal operator template" GNU extension
//! before C++20.
//!
//! That operator is not part of the language yet, but it is supported by
//! both Clang and GCC. This operator allows Hana to provide the nice `_s`
//! user-defined literal for creating compile-time strings.
//! C++20 added functionality allowing Hana to provide the nice `_s`
//! user-defined literal for creating compile-time strings; prior to that,
//! both Clang and GCC supported an extension allowing the same
//! functionality but via a different mechanism.
//!
//! When this macro is not defined, the GNU extension will be not used
//! by Hana. Because this is a non-standard extension, the macro is not
//! defined by default.
//! When C++20 support is detected, this macro is defined by default and
//! the C++20 mechanism is used. Otherwise, if this macro is not defined,
//! the macro is not defined by default and the GNU extension will not be
//! used by Hana because it is a non-standard extension.
# define BOOST_HANA_CONFIG_ENABLE_STRING_UDL
#endif

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40 changes: 29 additions & 11 deletions include/boost/hana/fwd/string.hpp
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Expand Up @@ -233,13 +233,21 @@ namespace boost { namespace hana {
#endif

#ifdef BOOST_HANA_CONFIG_ENABLE_STRING_UDL
# ifdef BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
namespace string_detail {
template<unsigned N>
struct literal_helper;
}
# endif

namespace literals {
//! Creates a compile-time string from a string literal.
//! @relatesalso boost::hana::string
//!
//! The string literal is parsed at compile-time and the result is
//! returned as a `hana::string`. This feature is an extension that
//! is disabled by default; see below for details.
//! returned as a `hana::string`. This feature requires C++20 or a
//! compiler extension that is disabled by default; see below for
//! details.
//!
//! @note
//! Only narrow string literals are supported right now; support for
Expand All @@ -248,24 +256,34 @@ namespace boost { namespace hana {
//! [Hana.issue80] if you need this.
//!
//! @warning
//! This user-defined literal is an extension which requires a special
//! string literal operator that is not part of the standard yet.
//! That operator is supported by both Clang and GCC, and several
//! proposals were made for it to enter C++17. However, since it is
//! not standard, it is disabled by default and defining the
//! `BOOST_HANA_CONFIG_ENABLE_STRING_UDL` config macro is required
//! to get this operator. Hence, if you want to stay safe, just use
//! the `BOOST_HANA_STRING` macro instead. If you want to be fast and
//! furious (I do), define `BOOST_HANA_CONFIG_ENABLE_STRING_UDL`.
//! This user-defined literal requires a special string literal operator
//! that was added to the standard in C++20. Prior to that, an extension
//! supported by both Clang and GCC provides similar functionality, but
//! since it is not standard, it is disabled by default in pre=C++20
//! mode and defining the `BOOST_HANA_CONFIG_ENABLE_STRING_UDL` config
//! macro is required to get this operator. Hence, if you do not have
//! C++20 and want to stay safe, just use the `BOOST_HANA_STRING` macro
//! instead. If you want to be fast and furious (I do), define
//! `BOOST_HANA_CONFIG_ENABLE_STRING_UDL`.
//!
//!
//! Example
//! -------
//! @include example/string/literal.cpp
//!
//! [Hana.issue80]: https://github.com/boostorg/hana/issues/80
# if defined(BOOST_HANA_DOXYGEN_INVOKED)
template <implementation_defined>
constexpr auto operator"" _s();
# elif defined(BOOST_HANA_CONFIG_CXX20_STRING_UDL_CLANG_WORKAROUND)
// Older versions of clang get confused by a forward declaration.
# elif defined(BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL)
template <string_detail::literal_helper>
constexpr auto operator"" _s();
# else
template <typename CharT, CharT ...s>
constexpr auto operator"" _s();
# endif
}
#endif
}} // end namespace boost::hana
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25 changes: 25 additions & 0 deletions include/boost/hana/string.hpp
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Expand Up @@ -109,10 +109,34 @@ namespace boost { namespace hana {
/**/

#ifdef BOOST_HANA_CONFIG_ENABLE_STRING_UDL
# ifdef BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
namespace string_detail {
template<unsigned N>
struct literal_helper {
constexpr literal_helper(char const (&s)[N]) {
for (unsigned i = 0; i != N; ++i)
buf[i] = s[i];
}
static constexpr unsigned size() { return N - 1; }
char buf[N] = {};
};
template<unsigned N>
literal_helper(char const (&)[N]) -> literal_helper<N>;
}
# endif

//////////////////////////////////////////////////////////////////////////
// _s user-defined literal
//////////////////////////////////////////////////////////////////////////
namespace literals {
# ifdef BOOST_HANA_CONFIG_HAS_CXX20_STRING_UDL
template<string_detail::literal_helper s>
constexpr auto operator"" _s() {
return []<std::size_t... I>(std::index_sequence<I...>) {
return hana::string_c<s.buf[I]...>;
}(std::make_index_sequence<s.size()>());
}
# else
template <typename CharT, CharT ...s>
constexpr auto operator"" _s() {
static_assert(std::is_same<CharT, char>::value,
Expand All @@ -121,6 +145,7 @@ namespace boost { namespace hana {
"if you need support for fancier types of compile-time strings.");
return hana::string_c<s...>;
}
# endif
}
#endif

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