Store enums wider than int at full width instead of truncating

include/boost/archive/detail/iserializer.hpp

@@ -18,6 +18,7 @@
 // iserializer.hpp: interface for serialization system.
 
 // (C) Copyright 2002 Robert Ramey - http://www.rrsd.com .
+// Copyright 2026 Gennaro Prota.
 // 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)
@@ -35,6 +36,7 @@ namespace std{
 } // namespace std
 #endif
 
+#include <boost/core/underlying_type.hpp>
 #include <boost/static_assert.hpp>
 
 #include <boost/mpl/eval_if.hpp>
@@ -46,10 +48,12 @@ namespace std{
 #ifndef BOOST_SERIALIZATION_DEFAULT_TYPE_INFO
     #include <boost/serialization/extended_type_info_typeid.hpp>
 #endif
+#include <boost/serialization/library_version_type.hpp>
 #include <boost/serialization/throw_exception.hpp>
 #include <boost/serialization/smart_cast.hpp>
 #include <boost/serialization/static_warning.hpp>
 
+#include <boost/type_traits/conditional.hpp>
 #include <boost/type_traits/is_pointer.hpp>
 #include <boost/type_traits/is_enum.hpp>
 #include <boost/type_traits/is_const.hpp>
@@ -560,10 +564,32 @@ template<class Archive>
 struct load_enum_type {
     template<class T>
     static void invoke(Archive &ar, T &t){
-        // convert integers to correct enum to load
-        int i;
-        ar >> boost::serialization::make_nvp(NULL, i);
-        t = static_cast< T >(i);
+        // Enumerators were always stored as int before archive library version
+        // 21; read that layout from any older archive so existing data keeps
+        // loading.  From version 21 on, an enumerator is stored as an int if it
+        // fits in an int; otherwise as the enum's underlying type (see
+        // save_enum_type).
+        if(ar.get_library_version()
+            < boost::serialization::library_version_type(21)
+        ){
+            int i;
+            ar >> boost::serialization::make_nvp(NULL, i);
+            t = static_cast< T >(i);
+            return;
+        }
+        #ifndef BOOST_NO_UNDERLYING_TYPE
+            typedef typename boost::underlying_type< T >::type underlying_type;
+            typedef typename boost::conditional<
+                sizeof(underlying_type) <= sizeof(int),
+                int,
+                underlying_type
+            >::type load_type;
+        #else
+            typedef int load_type;
+        #endif
+        load_type lt;
+        ar >> boost::serialization::make_nvp(NULL, lt);
+        t = static_cast< T >(lt);
     }
 };
 

include/boost/archive/detail/oserializer.hpp

@@ -18,6 +18,7 @@
 // oserializer.hpp: interface for serialization system.
 
 // (C) Copyright 2002 Robert Ramey - http://www.rrsd.com .
+// Copyright 2026 Gennaro Prota.
 // 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)
@@ -29,6 +30,8 @@
 
 #include <boost/config.hpp>
 
+#include <boost/core/underlying_type.hpp>
+
 #include <boost/static_assert.hpp>
 #include <boost/detail/workaround.hpp>
 
@@ -46,6 +49,7 @@
 #include <boost/serialization/assume_abstract.hpp>
 #include <boost/serialization/static_warning.hpp>
 
+#include <boost/type_traits/conditional.hpp>
 #include <boost/type_traits/is_pointer.hpp>
 #include <boost/type_traits/is_enum.hpp>
 #include <boost/type_traits/is_const.hpp>
@@ -488,9 +492,25 @@ struct save_enum_type
 {
     template<class T>
     static void invoke(Archive &ar, const T &t){
-        // convert enum to integers on save
-        const int i = static_cast<int>(t);
-        ar << boost::serialization::make_nvp(NULL, i);
+        // Save an enumerator as an integer wide enough to hold every value of
+        // the enum's underlying type.  An underlying type no wider than int
+        // keeps the historical layout (a plain int), so existing archives are
+        // unaffected; a wider underlying type is written at full width instead
+        // of being truncated to int.  Archives that need the wider layout carry
+        // library version 21 or greater (see load_enum_type).
+        #ifndef BOOST_NO_UNDERLYING_TYPE
+            typedef typename boost::underlying_type< T >::type underlying_type;
+            typedef typename boost::conditional<
+                sizeof(underlying_type) <= sizeof(int),
+                int,
+                underlying_type
+            >::type save_type;
+        #else
+            // no way to query the underlying type: keep the historical int
+            typedef int save_type;
+        #endif
+        const save_type st = static_cast< save_type >(t);
+        ar << boost::serialization::make_nvp(NULL, st);
     }
 };
 

src/basic_archive.cpp

@@ -85,9 +85,11 @@ BOOST_ARCHIVE_SIGNATURE(){
 //     was fully constructed.
 // 19- Boost 1.76 April 2021
 // 20- Boost 1.84 April 2021
+// 21- enumerators are stored as int (if they fit) or as the enum's underlying
+//     type, instead of always as int
 BOOST_SYMBOL_VISIBLE boost::serialization::library_version_type
 BOOST_ARCHIVE_VERSION(){
-    return boost::serialization::library_version_type(20);
+    return boost::serialization::library_version_type(21);
 }
 
 } // namespace archive

test/Jamfile.v2

@@ -75,6 +75,7 @@ test-suite "serialization" :
      [ test-bsl-run_files test_derived_class_ptr : A ]
      [ test-bsl-run_files test_diamond ]
      [ test-bsl-run_files test_diamond_complex ]
+     [ test-bsl-run_files test_enum ]
      [ test-bsl-run_files test_filesystem_path ]
      [ test-bsl-run_files test_forward_list : A  : :  [ requires cxx11_hdr_forward_list ] ] # BOOST_NO_CXX11_HDR_FORWARD_LIST
      [ test-bsl-run_files test_forward_list_ptrs : A  : :  [ requires cxx11_hdr_forward_list ] ] # BOOST_NO_CXX11_HDR_FORWARD_LIST

test/test_enum.cpp

@@ -0,0 +1,93 @@
+/////////1/////////2/////////3/////////4/////////5/////////6/////////7/////////8
+// test_enum.cpp
+
+// Copyright 2026 Gennaro Prota
+// Distributed under the Boost Software License, Version 1.0.
+// (See accompanying file LICENSE_1_0.txt or copy at
+// https://www.boost.org/LICENSE_1_0.txt)
+
+// Tests serialization of enumerators.  In particular it checks that a value of
+// a scoped enum whose underlying type is wider than int round trips at full
+// width, rather than being truncated to int on save.
+
+#include <cstddef>
+#include <cstdio>
+#include <fstream>
+
+#include <boost/config.hpp>
+
+#if defined(BOOST_NO_STDC_NAMESPACE)
+namespace std{
+    using ::remove;
+}
+#endif
+
+#include <boost/serialization/nvp.hpp>
+#include "test_tools.hpp"
+
+// underlying type is int, so the archived form is unchanged from earlier
+// releases of the library
+enum color { red, green = 3, blue = 100 };
+
+#ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+
+// the value 2^32 does not fit in a 32 bit int; a plain cast to int on save
+// used to truncate it
+enum class wide : long long {
+    one = 1,
+    big = 1LL << 32
+};
+
+// a narrow underlying type: still stored through int, so unchanged on the
+// wire, but it must round trip
+enum class narrow : signed char {
+    minus_one = -1,
+    two = 2,
+    hundred = 100
+};
+
+#endif // BOOST_NO_CXX11_SCOPED_ENUMS
+
+int test_main(int /* argc */, char * /* argv */[]){
+    const char * testfile = boost::archive::tmpnam(NULL);
+    BOOST_REQUIRE(NULL != testfile);
+
+    const color c_save = blue;
+    #ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+    const wide w_save = wide::big;
+    const narrow n_save = narrow::minus_one;
+    #endif
+    {
+        test_ostream os(testfile, TEST_STREAM_FLAGS);
+        test_oarchive oa(os, TEST_ARCHIVE_FLAGS);
+        oa << boost::serialization::make_nvp("c", c_save);
+        #ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+        oa << boost::serialization::make_nvp("w", w_save);
+        oa << boost::serialization::make_nvp("n", n_save);
+        #endif
+    }
+    color c_load = red;
+    #ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+    wide w_load = wide::one;
+    narrow n_load = narrow::two;
+    #endif
+    {
+        test_istream is(testfile, TEST_STREAM_FLAGS);
+        test_iarchive ia(is, TEST_ARCHIVE_FLAGS);
+        ia >> boost::serialization::make_nvp("c", c_load);
+        #ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+        ia >> boost::serialization::make_nvp("w", w_load);
+        ia >> boost::serialization::make_nvp("n", n_load);
+        #endif
+    }
+    BOOST_CHECK(c_save == c_load);
+    #ifndef BOOST_NO_CXX11_SCOPED_ENUMS
+    BOOST_CHECK(w_save == w_load);
+    // the point of the test: the value survived without being truncated
+    BOOST_CHECK(static_cast<long long>(w_load) == (1LL << 32));
+    BOOST_CHECK(n_save == n_load);
+    #endif
+
+    std::remove(testfile);
+    return EXIT_SUCCESS;
+}