boostorg · anarthal · Feb 27, 2026 · Feb 27, 2026 · Feb 27, 2026 · Mar 5, 2026
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -3,17 +3,40 @@
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
-cmake_minimum_required(VERSION 3.8...3.20)
+cmake_minimum_required(VERSION 3.8...3.31)
 
 project(boost_charconv VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
 
+# We can't make source files conditionally module units.
+# In module builds, use cc files that include their respective cpp, plus the module infrastructure
+if (BOOST_USE_MODULES)
+  set(SOURCE_SUFFIX "cc")
+else()
+  set(SOURCE_SUFFIX "cpp")
+endif()
+
 add_library(boost_charconv
-  src/from_chars.cpp
-  src/to_chars.cpp
+  src/from_chars.${SOURCE_SUFFIX}
+  src/to_chars.${SOURCE_SUFFIX}
 )
 
 add_library(Boost::charconv ALIAS boost_charconv)
 
+if (BOOST_USE_MODULES)
+  target_sources(boost_charconv
+    PUBLIC FILE_SET CXX_MODULES BASE_DIRS modules FILES
+      modules/boost_charconv.cppm
+      modules/boost_charconv_interface.cppm
+    PRIVATE FILE_SET helpers TYPE CXX_MODULES BASE_DIRS src FILES
+      src/helpers.cppm
+    )
+
+  # Enable and propagate C++23, import std, and the modules macro
+  target_compile_features(boost_charconv PUBLIC cxx_std_23)
+  set_target_properties(boost_charconv PROPERTIES CXX_MODULE_STD 1)
+  target_compile_definitions(boost_charconv PUBLIC BOOST_USE_MODULES)
+endif()
+
 target_include_directories(boost_charconv PUBLIC include)
 
 

diff --git a/include/boost/charconv/chars_format.hpp b/include/boost/charconv/chars_format.hpp
@@ -5,11 +5,24 @@
 #ifndef BOOST_CHARCONV_CHARS_FORMAT_HPP
 #define BOOST_CHARCONV_CHARS_FORMAT_HPP
 
+#if defined(BOOST_USE_MODULES) && !defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
+#ifndef BOOST_IN_MODULE_PURVIEW
+import boost.charconv;
+#ifndef BOOST_CHARCONV_CONSTEXPR
+#define BOOST_CHARCONV_CONSTEXPR constexpr
+#endif 
+#endif
+
+#else
+
+#include <boost/charconv/config.hpp>
+
 namespace boost { namespace charconv {
 
 // Floating-point format for primitive numerical conversion
 // chars_format is a bitmask type (16.3.3.3.3)
-enum class chars_format : unsigned
+BOOST_CHARCONV_MODULE_EXPORT enum class chars_format : unsigned
 {
     scientific = 1 << 0,
     fixed = 1 << 1,
@@ -19,4 +32,6 @@ enum class chars_format : unsigned
 
 }} // Namespaces
 
+#endif // defined(BOOST_USE_MODULES) && !defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_CHARS_FORMAT_HPP
diff --git a/include/boost/charconv/config.hpp b/include/boost/charconv/config.hpp
@@ -1,3 +1,8 @@
+// Make the header safe to include from libraries supporting modules
+#if defined(BOOST_IN_MODULE_PURVIEW) && !defined(BOOST_CHARCONV_CONFIG_HPP_INCLUDED)
+#  error "Please #include <boost/charconv/config.hpp> in your module global fragment"
+#endif
+
 #ifndef BOOST_CHARCONV_CONFIG_HPP_INCLUDED
 #define BOOST_CHARCONV_CONFIG_HPP_INCLUDED
 
@@ -7,7 +12,7 @@
 // https://www.boost.org/LICENSE_1_0.txt
 
 #include <boost/config.hpp>
-#include <climits>
+#include <boost/config/std/climits.hpp>
 
 // This header implements separate compilation features as described in
 // http://www.boost.org/more/separate_compilation.html
@@ -36,4 +41,11 @@
 
 #endif
 
+// Modules
+#ifdef BOOST_USE_MODULES
+#  define BOOST_CHARCONV_MODULE_EXPORT export
+#else
+#  define BOOST_CHARCONV_MODULE_EXPORT
+#endif
+
 #endif // BOOST_CHARCONV_CONFIG_HPP_INCLUDED
diff --git a/include/boost/charconv/detail/apply_sign.hpp b/include/boost/charconv/detail/apply_sign.hpp
@@ -5,10 +5,12 @@
 #ifndef BOOST_CHARCONV_DETAIL_APPLY_SIGN_HPP
 #define BOOST_CHARCONV_DETAIL_APPLY_SIGN_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #include <boost/config.hpp>
 #include <boost/charconv/detail/emulated128.hpp>
 #include <boost/charconv/detail/type_traits.hpp>
-#include <type_traits>
+#include <boost/config/std/type_traits.hpp>
 
 // We are purposefully converting values here
 #ifdef BOOST_MSVC
@@ -47,4 +49,6 @@ constexpr Unsigned_Integer apply_sign(Unsigned_Integer val) noexcept
 # pragma clang diagnostic pop
 #endif
 
+#endif // !defined(BOOST_USE_MODULES) || !defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_APPLY_SIGN_HPP
diff --git a/include/boost/charconv/detail/bit_layouts.hpp b/include/boost/charconv/detail/bit_layouts.hpp
@@ -5,10 +5,12 @@
 #ifndef BOOST_CHARCONV_DETAIL_BIT_LAYOUTS_HPP
 #define BOOST_CHARCONV_DETAIL_BIT_LAYOUTS_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #include <boost/charconv/detail/config.hpp>
 #include <boost/charconv/detail/emulated128.hpp>
-#include <cstdint>
-#include <cfloat>
+#include <boost/config/std/cstdint.hpp>
+#include <boost/config/std/cfloat.hpp>
 
 // Layouts of floating point types as specified by IEEE 754
 // See page 23 of IEEE 754-2008
@@ -56,7 +58,7 @@ struct ieee754_binary64
 };
 
 // 80 bit long double (e.g. x86-64)
-#if LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
+#if BOOST_CHARCONV_LDBL_BITS == 80
 
 struct IEEEl2bits
 {
@@ -83,10 +85,8 @@ struct ieee754_binary80
     static constexpr int decimal_digits = 18;
 };
 
-#define BOOST_CHARCONV_LDBL_BITS 80
-
 // 128 bit long double (e.g. s390x, ppcle64)
-#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+#elif BOOST_CHARCONV_LDBL_BITS == 128
 
 struct IEEEl2bits
 {
@@ -103,10 +103,8 @@ struct IEEEl2bits
 #endif
 };
 
-#define BOOST_CHARCONV_LDBL_BITS 128
-
 // 64 bit long double (double == long double on ARM)
-#elif LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+#elif BOOST_CHARCONV_LDBL_BITS == 64
 
 struct IEEEl2bits
 {
@@ -123,11 +121,6 @@ struct IEEEl2bits
 #endif
 };
 
-#define BOOST_CHARCONV_LDBL_BITS 64
-
-#else // Unsupported long double representation
-#  define BOOST_CHARCONV_UNSUPPORTED_LONG_DOUBLE
-#  define BOOST_CHARCONV_LDBL_BITS -1
 #endif
 
 struct IEEEbinary128
@@ -157,4 +150,6 @@ struct ieee754_binary128
 
 }}} // Namespaces
 
+#endif // !defined(BOOST_USE_MODULES) || !defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_BIT_LAYOUTS_HPP
diff --git a/include/boost/charconv/detail/buffer_sizing.hpp b/include/boost/charconv/detail/buffer_sizing.hpp
@@ -5,9 +5,11 @@
 #ifndef BOOST_CHARCONV_DETAIL_BUFFER_SIZING_HPP
 #define BOOST_CHARCONV_DETAIL_BUFFER_SIZING_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERNAL_PARTITION_UNIT)
+
 #include <boost/charconv/detail/config.hpp>
 #include <boost/charconv/detail/integer_search_trees.hpp>
-#include <type_traits>
+#include <boost/config/std/type_traits.hpp>
 
 namespace boost {
 namespace charconv {
@@ -69,4 +71,6 @@ inline int total_buffer_length(int real_precision, Int exp, bool signed_value)
 } //namespace charconv
 } //namespace boost
 
+#endif // !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif //BOOST_CHARCONV_DETAIL_BUFFER_SIZING_HPP
diff --git a/include/boost/charconv/detail/compute_float32.hpp b/include/boost/charconv/detail/compute_float32.hpp
@@ -5,10 +5,12 @@
 #ifndef BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT32_HPP
 #define BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT32_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERNAL_PARTITION_UNIT)
+
 #include <boost/charconv/detail/compute_float64.hpp>
-#include <limits>
-#include <cstdint>
-#include <cmath>
+#include <boost/config/std/limits.hpp>
+#include <boost/config/std/cstdint.hpp>
+#include <boost/config/std/cmath.hpp>
 
 namespace boost { namespace charconv { namespace detail {
 
@@ -52,4 +54,6 @@ inline float compute_float32(std::int64_t power, std::uint64_t i, bool negative,
 
 }}} // Namespaces
 
+#endif // !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT32_HPP
diff --git a/include/boost/charconv/detail/compute_float64.hpp b/include/boost/charconv/detail/compute_float64.hpp
@@ -6,18 +6,20 @@
 #ifndef BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT64_HPP
 #define BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT64_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERNAL_PARTITION_UNIT)
+
 #include <boost/charconv/detail/config.hpp>
 #include <boost/charconv/detail/significand_tables.hpp>
 #include <boost/charconv/detail/emulated128.hpp>
 #include <boost/core/bit.hpp>
-#include <cstdint>
-#include <cfloat>
-#include <cstring>
-#include <cmath>
+#include <boost/config/std/cstdint.hpp>
+#include <boost/config/std/cfloat.hpp>
+#include <boost/config/std/cstring.hpp>
+#include <boost/config/std/cmath.hpp>
 
 namespace boost { namespace charconv { namespace detail { 
 
-static constexpr double powers_of_ten[] = {
+BOOST_INLINE_CONSTEXPR double powers_of_ten[] = {
     1e0,  1e1,  1e2,  1e3,  1e4,  1e5,  1e6,  1e7,  1e8,  1e9,  1e10, 1e11,
     1e12, 1e13, 1e14, 1e15, 1e16, 1e17, 1e18, 1e19, 1e20, 1e21, 1e22
 };
@@ -198,4 +200,6 @@ inline double compute_float64(std::int64_t power, std::uint64_t i, bool negative
 
 }}} // Namespaces
 
+#endif // !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT64_HPP
diff --git a/include/boost/charconv/detail/compute_float80.hpp b/include/boost/charconv/detail/compute_float80.hpp
@@ -5,28 +5,30 @@
 #ifndef BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT80_HPP
 #define BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT80_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERNAL_PARTITION_UNIT)
+
 #include <boost/charconv/detail/config.hpp>
 #include <boost/charconv/detail/emulated128.hpp>
 #include <boost/charconv/detail/bit_layouts.hpp>
-#include <system_error>
-#include <type_traits>
-#include <limits>
-#include <cstdint>
-#include <cmath>
-#include <climits>
-#include <cfloat>
+#include <boost/config/std/system_error.hpp>
+#include <boost/config/std/type_traits.hpp>
+#include <boost/config/std/limits.hpp>
+#include <boost/config/std/cstdint.hpp>
+#include <boost/config/std/cmath.hpp>
+#include <boost/config/std/climits.hpp>
+#include <boost/config/std/cfloat.hpp>
 
 #ifdef BOOST_CHARCONV_DEBUG_FLOAT128
-#include <iostream>
-#include <iomanip>
+#include <boost/config/std/iostream.hpp>
+#include <boost/config/std/iomanip.hpp>
 #include <boost/charconv/detail/to_chars_integer_impl.hpp>
 #endif
 
 namespace boost { namespace charconv { namespace detail {
 
 #if BOOST_CHARCONV_LDBL_BITS > 64
 
-static constexpr long double powers_of_ten_ld[] = {
+BOOST_INLINE_CONSTEXPR long double powers_of_ten_ld[] = {
     1e0L,  1e1L,  1e2L,  1e3L,  1e4L,  1e5L,  1e6L,
     1e7L,  1e8L,  1e9L,  1e10L, 1e11L, 1e12L, 1e13L,
     1e14L, 1e15L, 1e16L, 1e17L, 1e18L, 1e19L, 1e20L,
@@ -111,4 +113,6 @@ inline ResultType compute_float80(std::int64_t q, Unsigned_Integer w, bool negat
 
 }}} // Namespaces
 
+#endif // !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_COMPUTE_FLOAT80_HPP
diff --git a/include/boost/charconv/detail/config.hpp b/include/boost/charconv/detail/config.hpp
@@ -2,13 +2,39 @@
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
+// Make this header safe to include in our purview
+#if defined(BOOST_IN_MODULE_PURVIEW) && !defined(BOOST_CHARCONV_DETAIL_CONFIG_HPP)
+#  error "Please #include <boost/charconv/detail/config.hpp> in your module global fragment"
+#endif
+
 #ifndef BOOST_CHARCONV_DETAIL_CONFIG_HPP
 #define BOOST_CHARCONV_DETAIL_CONFIG_HPP
 
 #include <boost/config.hpp>
-#include <type_traits>
+#include <boost/config/std/type_traits.hpp>
 #include <cfloat>
 
+// Long double characteristics
+
+// 80 bit long double (e.g. x86-64)
+#if LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
+#  define BOOST_CHARCONV_LDBL_BITS 80
+
+// 128 bit long double (e.g. s390x, ppcle64)
+#elif LDBL_MANT_DIG == 113 && LDBL_MAX_EXP == 16384
+#  define BOOST_CHARCONV_LDBL_BITS 128
+
+// 64 bit long double (double == long double on ARM)
+#elif LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
+#  define BOOST_CHARCONV_LDBL_BITS 64
+
+// Unsupported long double representation
+#else
+#  define BOOST_CHARCONV_UNSUPPORTED_LONG_DOUBLE
+#  define BOOST_CHARCONV_LDBL_BITS -1
+
+#endif // long double feature detection
+
 #include <boost/assert.hpp>
 #define BOOST_CHARCONV_ASSERT(expr) BOOST_ASSERT(expr)
 #define BOOST_CHARCONV_ASSERT_MSG(expr, msg) BOOST_ASSERT_MSG(expr, msg)
@@ -169,7 +195,7 @@ static_assert((BOOST_CHARCONV_ENDIAN_BIG_BYTE || BOOST_CHARCONV_ENDIAN_LITTLE_BY
 // All of these types are optional so check for each of them individually
 #if (defined(_MSVC_LANG) && _MSVC_LANG > 202002L) || __cplusplus > 202002L
 #  if __has_include(<stdfloat>)
-#    include <stdfloat>
+#    include <boost/config/std/stdfloat.hpp>
 #  endif
 #endif
 #ifdef __STDCPP_FLOAT16_T__

diff --git a/include/boost/charconv/detail/dragonbox/dragonbox.hpp b/include/boost/charconv/detail/dragonbox/dragonbox.hpp
@@ -22,6 +22,8 @@
 #ifndef BOOST_CHARCONV_DETAIL_DRAGONBOX_HPP
 #define BOOST_CHARCONV_DETAIL_DRAGONBOX_HPP
 
+#if !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERNAL_PARTITION_UNIT)
+
 #include <boost/charconv/detail/config.hpp>
 #include <boost/charconv/detail/dragonbox/dragonbox_common.hpp>
 #include <boost/charconv/detail/bit_layouts.hpp>
@@ -30,10 +32,10 @@
 #include <boost/charconv/detail/to_chars_result.hpp>
 #include <boost/charconv/chars_format.hpp>
 #include <boost/core/bit.hpp>
-#include <type_traits>
-#include <limits>
-#include <cstdint>
-#include <cstring>
+#include <boost/config/std/type_traits.hpp>
+#include <boost/config/std/limits.hpp>
+#include <boost/config/std/cstdint.hpp>
+#include <boost/config/std/cstring.hpp>
 
 #ifdef BOOST_MSVC
 # pragma warning(push)
@@ -2749,4 +2751,6 @@ to_chars_result dragonbox_to_chars(Float x, char* first, char* last, chars_forma
 # pragma warning(pop)
 #endif
 
+#endif // !defined(BOOST_USE_MODULES) || defined(BOOST_CHARCONV_INTERFACE_UNIT)
+
 #endif // BOOST_CHARCONV_DETAIL_DRAGONBOX_HPP