Reduce locale dependency

include/fmt/format-inl.h

@@ -31,7 +31,7 @@
 #include "locale.h"
 
 FMT_BEGIN_NAMESPACE
-template <typename Char> std::locale::id num_format_facet<Char>::id;
+template <typename Locale> typename Locale::id num_format_facet<Locale>::id;
 
 namespace detail {
 
@@ -131,8 +131,9 @@ FMT_FUNC auto write_int(unsigned long long value, locale_ref loc)
   auto out = std::ostreambuf_iterator<Char>(&buf);
   // We cannot use the num_put<char> facet because it may produce output in
   // a wrong encoding.
-  using facet_t = conditional_t<std::is_same<Char, char>::value,
-                                num_format_facet<>, std::num_put<Char>>;
+  using facet_t =
+      conditional_t<std::is_same<Char, char>::value,
+                    num_format_facet<std::locale>, std::num_put<Char>>;
   if (std::has_facet<facet_t>(locale)) {
     std::use_facet<facet_t>(locale).put(out, ios, ' ', value);
     return buf.str();

include/fmt/locale.h

@@ -8,17 +8,28 @@
 #ifndef FMT_LOCALE_H_
 #define FMT_LOCALE_H_
 
-#include <locale>  // std::num_put
+#include <ios>       // std::ios_base
+#include <iterator>  // std::ostreambuf_iterator
 
 #include "core.h"
 
 FMT_BEGIN_NAMESPACE
 
 // A locale facet that formats numeric values in UTF-8.
-template <typename Char = char>
-class num_format_facet : public std::num_put<Char> {
+template <typename Locale> class num_format_facet : public Locale::facet {
  public:
-  static FMT_API std::locale::id id;
+  static FMT_API typename Locale::id id;
+
+  using iter_type = std::ostreambuf_iterator<char>;
+
+  auto put(iter_type out, std::ios_base& str, char fill,
+           unsigned long long val) const -> iter_type {
+    return do_put(out, str, fill, val);
+  }
+
+ protected:
+  virtual auto do_put(iter_type out, std::ios_base& str, char fill,
+                      unsigned long long val) const -> iter_type = 0;
 };
 
 FMT_END_NAMESPACE