Make BOOST_METAPARSE_STRING_VALUE unlimited in C++17

doc/BOOST_METAPARSE_STRING.qbk

@@ -21,6 +21,10 @@ string literal. The macro requires C++11.
 The maximal length of the string is limited. This limit is defined by the
 `BOOST_METAPARSE_LIMIT_STRING_SIZE` macro.
 
+For C++17 and later, consider using
+[link BOOST_METAPARSE_STRING_VALUE `BOOST_METAPARSE_STRING_VALUE`]
+instead, as it doesn't have these limitations.
+
 On platforms where `BOOST_METAPARSE_STRING` is not supported, the `string.hpp`
 header defines the `BOOST_METAPARSE_V1_CONFIG_NO_BOOST_METAPARSE_STRING` macro.
 Defining this macro before including the header disables the

doc/BOOST_METAPARSE_STRING_VALUE.qbk

@@ -16,7 +16,15 @@ This is a macro.
 [h1 Description]
 
 This is a convenience macro for creating instances of the string types created
-using [link BOOST_METAPARSE_STRING `BOOST_METAPARSE_STRING`].
+using [link BOOST_METAPARSE_STRING `BOOST_METAPARSE_STRING`]. The macro
+requires C++11.
+
+[*C++11]: The maximum length of these strings is controlled by the
+`BOOST_METAPARSE_LIMIT_STRING_SIZE` macro.
+
+[*C++17]: The macro uses constexpr string literal.
+
+[caution It is [*not recommended] to use an expression with BOOST_METAPARSE_STRING_VALUE as the argument of `decltype`, for example `decltype(BOOST_METAPARSE_STRING_VALUE("foo"))`. Although such constructs may compile successfully in some environments, they tend to be [*non-portable]. ]
 
 [h1 Header]
 

include/boost/metaparse/config.hpp

@@ -40,8 +40,11 @@
       || (defined _MSC_VER && _MSC_VER >= 1900) \
     ) \
     && (!defined BOOST_GCC || BOOST_GCC >= 40700)
+#    if !defined BOOST_NO_CXX17_DEDUCTION_GUIDES
 
-#    if !defined BOOST_NO_CXX14_CONSTEXPR
+#      define BOOST_METAPARSE_STD 2017
+
+#    elif !defined BOOST_NO_CXX14_CONSTEXPR
 
 #      define BOOST_METAPARSE_STD 2014
 

include/boost/metaparse/v1/cpp17/impl/convert_string.hpp

@@ -0,0 +1,35 @@
+#ifndef BOOST_METAPARSE_V1_CPP17_IMPL_CONVERT_STRING_HPP
+#define BOOST_METAPARSE_V1_CPP17_IMPL_CONVERT_STRING_HPP
+
+// Copyright Denis Mikhailov 2025.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/metaparse/v1/string.hpp>
+
+#include <cstddef> // for std::size_t
+#include <utility> // for std::make_index_sequence
+
+namespace boost
+{
+  namespace metaparse
+  {
+    namespace v1
+    {
+      namespace impl
+      {
+        template<const auto& S>
+        struct convert_string {
+          template<std::size_t... Is>
+          static constexpr auto apply(std::index_sequence<Is...>) noexcept {
+            return ::boost::metaparse::v1::string<S[Is]...>{};
+          }
+        };
+      }
+    }
+  }
+}
+
+#endif
+

include/boost/metaparse/v1/cpp17/string_value.hpp

@@ -0,0 +1,25 @@
+#ifndef BOOST_METAPARSE_V1_CPP17_STRING_VALUE_HPP
+#define BOOST_METAPARSE_V1_CPP17_STRING_VALUE_HPP
+
+// Copyright Denis Mikhailov 2025.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#include <boost/metaparse/v1/impl/fixed_string.hpp>
+#include <boost/metaparse/v1/cpp17/impl/convert_string.hpp>
+
+#include <utility> // for std::make_index_sequence
+
+#ifdef BOOST_METAPARSE_V1_STRING_VALUE
+#error BOOST_METAPARSE_V1_STRING_VALUE already defined
+#endif
+
+#define BOOST_METAPARSE_V1_STRING_VALUE(s) ([]{                                                \
+    namespace impl = ::boost::metaparse::v1::impl;                                             \
+    static constexpr auto fs = impl::make_fixed_string(s);                                     \
+    return impl::convert_string<fs>::apply(std::make_index_sequence<fs.size() - 1>{});         \
+}())
+
+#endif
+

include/boost/metaparse/v1/impl/fixed_string.hpp

@@ -0,0 +1,39 @@
+#ifndef BOOST_METAPARSE_V1_IMPL_FIXED_STRING_HPP
+#define BOOST_METAPARSE_V1_IMPL_FIXED_STRING_HPP
+
+// Copyright Denis Mikhailov 2025.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#include <cstddef> // for std::size_t
+
+namespace boost
+{
+  namespace metaparse
+  {
+    namespace v1
+    {
+      namespace impl
+      {
+        template<typename CharT, std::size_t N>
+        struct fixed_string {
+            const CharT* data;
+            constexpr fixed_string(const CharT (&s)[N]) noexcept
+              : data{s}
+            {}
+            constexpr CharT operator[](std::size_t i) const noexcept { return data[i]; }
+            constexpr std::size_t size() const noexcept { return N; }
+        };
+
+        template<typename CharT, std::size_t N>
+        constexpr fixed_string<CharT, N> make_fixed_string(const CharT(&s)[N]) noexcept {
+            return {s};
+        }
+      }
+    }
+  }
+}
+
+#endif
+

include/boost/metaparse/v1/string_value.hpp

@@ -2,18 +2,27 @@
 #define BOOST_METAPARSE_V1_STRING_VALUE_HPP
 
 // Copyright Abel Sinkovics (abel@sinkovics.hu)  2021.
+// Copyright Denis Mikhailov 2025.
 // Distributed under the Boost Software License, Version 1.0.
 //    (See accompanying file LICENSE_1_0.txt or copy at
 //          http://www.boost.org/LICENSE_1_0.txt)
 
-#include <boost/metaparse/v1/string.hpp>
+#include <boost/metaparse/config.hpp>
 
-#ifdef BOOST_METAPARSE_V1_STRING_VALUE
-#error BOOST_METAPARSE_V1_STRING_VALUE already defined
-#endif
+#if BOOST_METAPARSE_STD >= 2017
+#  include <boost/metaparse/v1/cpp17/string_value.hpp>
+#else
+
+#  include <boost/metaparse/v1/string.hpp>
+
+#  ifdef BOOST_METAPARSE_V1_STRING_VALUE
+#  error BOOST_METAPARSE_V1_STRING_VALUE already defined
+#  endif
+
+#  ifdef BOOST_METAPARSE_V1_STRING
+#  define BOOST_METAPARSE_V1_STRING_VALUE(s) (BOOST_METAPARSE_V1_STRING(s){})
+#  endif
 
-#ifdef BOOST_METAPARSE_V1_STRING
-#define BOOST_METAPARSE_V1_STRING_VALUE(s) (BOOST_METAPARSE_V1_STRING(s){})
 #endif
 
 #endif

test/Jamfile.v2

@@ -1,4 +1,5 @@
 import testing ;
+import config : requires ;
 
 project :
   requirements
@@ -103,6 +104,7 @@ test-suite metaparse-unit-tests :
   [ compile string_iterator_tag.cpp ]
   [ compile string_tag.cpp ]
   [ compile string_value.cpp ]
+  [ compile string_value_unlimited_cxx17.cpp : [ requires cxx17_deduction_guides ] ]
   [ compile swap.cpp ]
   [ compile token.cpp ]
   [ compile-fail too_long_string.cpp ]

test/string_value_unlimited_cxx17.cpp

@@ -0,0 +1,24 @@
+// Copyright Denis Mikhailov 2025.
+// Distributed under the Boost Software License, Version 1.0.
+//    (See accompanying file LICENSE_1_0.txt or copy at
+//          http://www.boost.org/LICENSE_1_0.txt)
+
+#define BOOST_TEST_MODULE string_value_unlimited_cxx17
+
+#define BOOST_METAPARSE_LIMIT_STRING_SIZE 4
+#include <boost/metaparse/string.hpp>
+#include <boost/metaparse/string_value.hpp>
+
+#include <boost/test/unit_test.hpp>
+
+#include <type_traits>
+
+BOOST_AUTO_TEST_CASE(test_string_value_unlimited_cxx17)
+{
+  auto abcde = BOOST_METAPARSE_STRING_VALUE("abcde");
+
+  BOOST_MPL_ASSERT((
+    std::is_same<boost::metaparse::string<'a', 'b', 'c', 'd', 'e'>, decltype(abcde)>
+  ));
+}
+