Initial support for C++20 modules (#177)

CMakeLists.txt

@@ -1,5 +1,6 @@
-# Generated by `boostdep --cmake pfr`
 # Copyright 2020 Peter Dimov
+# Copyright (c) 2016-2024 Antony Polukhin
+#
 # Distributed under the Boost Software License, Version 1.0.
 # https://www.boost.org/LICENSE_1_0.txt
 
@@ -12,9 +13,11 @@ add_library(Boost::pfr ALIAS boost_pfr)
 
 target_include_directories(boost_pfr INTERFACE include)
 
-if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
+if (CMAKE_VERSION VERSION_GREATER_EQUAL "3.28.0" AND BUILD_MODULE)
+    add_subdirectory(module)
+endif()
 
+if (BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
   add_subdirectory(test)
-
 endif()
 

doc/pfr.qbk

@@ -549,6 +549,40 @@ parameters provided to `BOOST_PFR_CORE_NAME_PARSING` macro [*and] the initial ou
 
 [endsect]
 
+[section PFR as a C++20 module]
+
+[caution C++20 PFR module support is on early stage, targets and flags may change in the future]
+
+If using CMake of version 3.28.0 or higher define CMake option `-DBUILD_MODULE=1`
+to make the `Boost::pfr_module` and `Boost::pfr_module_migration` libraries
+available. With `Boost::pfr_module` C++20 module Boost.PFR could be used:
+
+[import ../module/usage_sample.cpp]
+[pfr_module_example]
+
+The `Boost::pfr_module_migration` CMake target gives an ability to
+mix includes and imports of the PFR library in different translation units.
+
+If not using CMake, then the module could be build manually from the
+`module/pfr.cppm` file. If mixing of includes in imports is desired, additionally
+define `BOOST_PFR_ATTACH_TO_GLOBAL_MODULE` preprocessor macro to attach all the
+module entities to a global module and avoid ODR issues.
+
+For manual module build the following commands could be used for clang compiler:
+
+```
+cd pfr/module
+clang++ -I ../include -std=c++20 --precompile -x c++-module pfr.cppm
+```
+
+After that, the module could be used in the following way:
+
+```
+clang++ -std=c++20 -fmodule-file=pfr.pcm pfr.pcm usage_sample.cpp
+```
+
+[endsect]
+
 [section How it works]
 
 [h2 Fields count detection and getting references to members]

include/boost/pfr/config.hpp

@@ -145,4 +145,12 @@
 
 #undef BOOST_PFR_NOT_SUPPORTED
 
+#ifndef BOOST_PFR_BEGIN_MODULE_EXPORT
+#   define BOOST_PFR_BEGIN_MODULE_EXPORT
+#endif
+
+#ifndef BOOST_PFR_END_MODULE_EXPORT
+#   define BOOST_PFR_END_MODULE_EXPORT
+#endif
+
 #endif // BOOST_PFR_CONFIG_HPP

include/boost/pfr/core.hpp

@@ -29,6 +29,8 @@
 
 namespace boost { namespace pfr {
 
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
 /// \brief Returns reference or const reference to a field with index `I` in \aggregate `val`.
 /// Overload taking the type `U` returns reference or const reference to a field
 /// with provided type `U` in \aggregate `val` if there's only one field of such type in `val`.
@@ -260,6 +262,8 @@ constexpr detail::tie_from_structure_tuple<Elements...> tie_from_structure(Eleme
     return detail::tie_from_structure_tuple<Elements...>(args...);
 }
 
+BOOST_PFR_END_MODULE_EXPORT
+
 }} // namespace boost::pfr
 
 #endif // BOOST_PFR_CORE_HPP

include/boost/pfr/core_name.hpp

@@ -33,6 +33,8 @@
 
 namespace boost { namespace pfr {
 
+BOOST_PFR_BEGIN_MODULE_EXPORT
+
 /// \brief Returns name of a field with index `I` in \aggregate `T`.
 ///
 /// \b Example:
@@ -82,6 +84,8 @@ names_as_array() noexcept {
     );
 }
 
+BOOST_PFR_END_MODULE_EXPORT
+
 }} // namespace boost::pfr
 
 #endif // BOOST_PFR_CORE_NAME_HPP

include/boost/pfr/detail/core14_classic.hpp

@@ -9,8 +9,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <utility>      // metaprogramming stuff
+#endif
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 #include <boost/pfr/detail/offset_based_getter.hpp>

include/boost/pfr/detail/core14_loophole.hpp

@@ -24,8 +24,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <utility>
+#endif
 
 #include <boost/pfr/detail/offset_based_getter.hpp>
 #include <boost/pfr/detail/fields_count.hpp>

include/boost/pfr/detail/core17_generated.hpp

@@ -21,7 +21,12 @@
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 #include <boost/pfr/detail/size_t_.hpp>
+
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits> // for std::conditional_t, std::is_reference
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/core_name20_static.hpp

@@ -19,10 +19,15 @@
 #include <boost/pfr/detail/fields_count.hpp>
 #include <boost/pfr/detail/stdarray.hpp>
 #include <boost/pfr/detail/fake_object.hpp>
+
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <string_view>
 #include <array>
 #include <memory> // for std::addressof
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/detectors.hpp

@@ -9,8 +9,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <functional>
 #include <type_traits>
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 ///////////////////// `value` is true if Detector<Tleft, Tright> does not compile (SFINAE)

include/boost/pfr/detail/fields_count.hpp

@@ -12,9 +12,13 @@
 #include <boost/pfr/detail/size_t_.hpp>
 #include <boost/pfr/detail/unsafe_declval.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <climits>      // CHAR_BIT
 #include <type_traits>
 #include <utility>      // metaprogramming stuff
+#endif
 
 #ifdef __clang__
 #   pragma clang diagnostic push

include/boost/pfr/detail/for_each_field_impl.hpp

@@ -9,7 +9,11 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <utility>      // metaprogramming stuff
+#endif
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 #include <boost/pfr/detail/rvalue_t.hpp>

include/boost/pfr/detail/functional.hpp

@@ -9,8 +9,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <functional>
 #include <cstdint>
+#endif
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 

include/boost/pfr/detail/io.hpp

@@ -10,6 +10,10 @@
 #include <boost/pfr/detail/config.hpp>
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
+
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <iosfwd>       // stream operators
 #include <iomanip>
 
@@ -19,6 +23,8 @@
 #   endif
 #endif
 
+#endif
+
 namespace boost { namespace pfr { namespace detail {
 
 inline auto quoted_helper(const std::string& s) noexcept {

include/boost/pfr/detail/make_flat_tuple_of_references.hpp

@@ -9,7 +9,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <utility>      // metaprogramming stuff
+#endif
+
 #include <boost/pfr/detail/sequence_tuple.hpp>
 #include <boost/pfr/detail/rvalue_t.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>

include/boost/pfr/detail/make_integer_sequence.hpp

@@ -10,9 +10,13 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <utility>
 #include <cstddef>
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/offset_based_getter.hpp

@@ -10,9 +10,14 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <utility>
 #include <memory>  // std::addressof
+#endif
+
 #include <boost/pfr/detail/sequence_tuple.hpp>
 #include <boost/pfr/detail/rvalue_t.hpp>
 #include <boost/pfr/detail/size_t_.hpp>

include/boost/pfr/detail/possible_reflectable.hpp

@@ -10,7 +10,11 @@
 #include <boost/pfr/detail/config.hpp>
 #include <boost/pfr/traits_fwd.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits> // for std::is_aggregate
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/rvalue_t.hpp

@@ -7,8 +7,12 @@
 #define BOOST_PFR_DETAIL_RVALUE_T_HPP
 #pragma once
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
 #include <utility>      // std::enable_if_t
+#endif
 
 // This header provides aliases rvalue_t and lvalue_t.
 //

include/boost/pfr/detail/sequence_tuple.hpp

@@ -10,8 +10,12 @@
 #include <boost/pfr/detail/config.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <utility>      // metaprogramming stuff
 #include <cstddef>      // std::size_t
+#endif
 
 ///////////////////// Tuple that holds its values in the supplied order
 namespace boost { namespace pfr { namespace detail { namespace sequence_tuple {

include/boost/pfr/detail/size_array.hpp

@@ -9,7 +9,11 @@
 
 #include <boost/pfr/detail/config.hpp>
 
-#include <cstddef>      // metaprogramming stuff
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
+#include <cstddef>
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/stdarray.hpp

@@ -9,10 +9,14 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <utility> // metaprogramming stuff
 #include <array>
 #include <type_traits> // for std::common_type_t
 #include <cstddef>
+#endif
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 

include/boost/pfr/detail/stdtuple.hpp

@@ -9,8 +9,12 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <utility>      // metaprogramming stuff
 #include <tuple>
+#endif
 
 #include <boost/pfr/detail/sequence_tuple.hpp>
 

include/boost/pfr/detail/tie_from_structure_tuple.hpp

@@ -16,7 +16,11 @@
 #include <boost/pfr/tuple_size.hpp>
 #include <boost/pfr/detail/make_integer_sequence.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <tuple>
+#endif
 
 namespace boost { namespace pfr { namespace detail {
 

include/boost/pfr/detail/unsafe_declval.hpp

@@ -9,7 +9,11 @@
 
 #include <boost/pfr/detail/config.hpp>
 
+#ifdef BOOST_PFR_HAS_STD_MODULE
+import std;
+#else
 #include <type_traits>
+#endif
 
 namespace boost { namespace pfr { namespace detail {