Add locale support to <format> support

Author: mborland

include/boost/decimal/format.hpp

@@ -47,6 +47,14 @@ constexpr auto parse_impl(ParseContext &ctx)
     boost::decimal::chars_format fmt = boost::decimal::chars_format::general;
     bool is_upper = false;
     int padding_digits = 0;
+    bool use_locale = false;
+
+    // Check for the locale character
+    if (*it == 'L')
+    {
+        use_locale = true;
+        ++it;
+    }
 
     // Check for a sign character
     if (it != ctx.end())
@@ -148,7 +156,7 @@ constexpr auto parse_impl(ParseContext &ctx)
         BOOST_DECIMAL_THROW_EXCEPTION(std::format_error("Expected '}' in format string")); // LCOV_EXCL_LINE
     }
 
-    return std::make_tuple(ctx_precision, fmt, is_upper, padding_digits, sign_character, it);
+    return std::make_tuple(ctx_precision, fmt, is_upper, padding_digits, sign_character, use_locale, it);
 }
 
 } // Namespace boost::decimal::detail
@@ -163,12 +171,14 @@ struct formatter<T>
     int ctx_precision;
     int padding_digits;
     bool is_upper;
+    bool use_locale;
 
     constexpr formatter() : fmt(boost::decimal::chars_format::general),
                             sign(boost::decimal::detail::format_sign_option::minus),
                             ctx_precision(6),
                             padding_digits(0),
-                            is_upper(false)
+                            is_upper(false),
+                            use_locale(false)
     {}
 
     constexpr auto parse(format_parse_context &ctx)
@@ -180,8 +190,9 @@ struct formatter<T>
         is_upper = std::get<2>(res);
         padding_digits = std::get<3>(res);
         sign = std::get<4>(res);
+        use_locale = std::get<5>(res);
 
-        return std::get<5>(res);
+        return std::get<6>(res);
     }
 
     template <typename FormatContext>
@@ -254,6 +265,16 @@ struct formatter<T>
             s.insert(s.begin() + static_cast<std::size_t>(has_sign), static_cast<std::size_t>(padding_digits) - s.size(), '0');
         }
 
+        if (use_locale)
+        {
+            // We need approximately 1/3 more space in order to insert the thousands separators,
+            // but after we have done our processing we need to shrink the string back down
+            const auto initial_length {s.length()};
+            s.resize(s.length() * 4 / 3 + 1);
+            const auto offset {static_cast<std::size_t>(convert_pointer_pair_to_local_locale(const_cast<char*>(s.data()), s.data() + s.length()))};
+            s.resize(initial_length + offset);
+        }
+
         return std::format_to(ctx.out(), "{}", s);
     }
 };

test/test_format.cpp

@@ -246,6 +246,25 @@ void test_cohort_preservation()
     }
 }
 
+template <typename T>
+void test_locale_conversion(const char* locale, const std::string& result)
+{
+    try
+    {
+        const std::locale a(locale);
+        std::locale::global(a);
+
+        const T value {112289, -2};
+        BOOST_TEST_EQ(std::format("{:L.2f}", value), result);
+    }
+    // LCOV_EXCL_START
+    catch (...)
+    {
+        std::cerr << "Test not run" << std::endl;
+    }
+    // LCOV_EXCL_STOP
+}
+
 int main()
 {
     test_general<decimal32_t>();
@@ -287,6 +306,17 @@ int main()
     test_cohort_preservation<decimal64_t>();
     test_cohort_preservation<decimal128_t>();
 
+    #if !(defined(__GNUC__) && __GNUC__ >= 5 && defined(__APPLE__)) && !defined(BOOST_DECIMAL_QEMU_TEST) && !defined(BOOST_DECIMAL_DISABLE_EXCEPTIONS) && !defined(_MSC_VER)
+
+    test_locale_conversion<decimal32_t>("en_US.UTF-8", "1,122.89");
+    test_locale_conversion<decimal32_t>("de_DE.UTF-8", "1.122,89");
+
+    #if (defined(__clang__) && __clang_major__ > 9) || (defined(__GNUC__) && __GNUC__ > 9)
+    test_locale_conversion<decimal32_t>("fr_FR.UTF-8", "1 122,89");
+    #endif
+
+    #endif
+
     return boost::report_errors();
 }