Add support for arbitrary output iterators

include/fmt/core.h

@@ -832,25 +832,32 @@ inline Container &get_container(std::back_insert_iterator<Container> it) {
 }
 }  // namespace internal
 
-// A range where begin() returns back_insert_iterator.
-template <typename Container>
-class back_insert_range {
+template <typename OutputIt, typename T = typename OutputIt::value_type>
+class output_range {
  private:
-  Container &container_;
+  OutputIt it_;
+
+  // Unused yet.
+  using sentinel = void;
+  sentinel end() const;
 
  public:
-  using iterator = std::back_insert_iterator<Container>;
+  using value_type = T;
+
+  explicit output_range(OutputIt it): it_(it) {}
+  OutputIt begin() const { return it_; }
+};
+
+// A range where begin() returns back_insert_iterator.
+template <typename Container>
+class back_insert_range:
+    public output_range<std::back_insert_iterator<Container>> {
+  using base = output_range<std::back_insert_iterator<Container>>;
+ public:
   using value_type = typename Container::value_type;
 
-  struct sentinel {
-    friend bool operator!=(sentinel, iterator) { return false; }
-    friend bool operator!=(iterator, sentinel) { return false; }
-  };
-
-  back_insert_range(Container &c) : container_(c) {}
-
-  iterator begin() const { return std::back_inserter(container_); }
-  sentinel end() const { return sentinel(); }
+  using base::base;
+  back_insert_range(Container &c): base(std::back_inserter(c)) {}
 };
 
 // Formatting context.

include/fmt/format.h

@@ -2023,7 +2023,7 @@ class arg_formatter: public internal::arg_formatter_base<Range> {
     \endrst
    */
   arg_formatter(basic_context<Range> &ctx, format_specs &spec)
-  : base(Range(internal::get_container(ctx.begin())), spec), ctx_(ctx) {}
+  : base(Range(ctx.begin()), spec), ctx_(ctx) {}
 
   using base::operator();
 
@@ -3036,6 +3036,13 @@ inline void format_to(wmemory_buffer &buf, wstring_view format_str,
   vformat_to(buf, format_str, make_args<wcontext>(args...));
 }
 
+template <typename Container, typename Context>
+typename std::enable_if<!is_contiguous<Container>::value>::type
+    vformat_to(std::back_insert_iterator<Container> out,
+               string_view format_str,
+               basic_format_args<Context> args) {
+}
+
 template <typename Container, typename... Args>
 inline typename std::enable_if<is_contiguous<Container>::value>::type
     format_to(std::back_insert_iterator<Container> out,
@@ -3043,15 +3050,13 @@ inline typename std::enable_if<is_contiguous<Container>::value>::type
   vformat_to(out, format_str, make_args(args...));
 }
 
-template <typename Container, typename... Args>
-inline typename std::enable_if<!is_contiguous<Container>::value>::type
-    format_to(std::back_insert_iterator<Container> out,
-              string_view format_str, const Args & ... args) {
-  using range = back_insert_range<Container>;
+template <typename OutputIt, typename... Args>
+inline void format_to(OutputIt out, string_view format_str,
+                      const Args & ... args) {
+  using range = output_range<OutputIt, char>;
   auto store = make_args<basic_context<range>>(args...);
   do_vformat_to<arg_formatter<range>>(
-        range(internal::get_container(out)), format_str,
-        basic_format_args<basic_context<range>>(store));
+        range(out), format_str, basic_format_args<basic_context<range>>(store));
 }
 
 inline std::string vformat(string_view format_str, format_args args) {

test/format-test.cc

@@ -1910,8 +1910,11 @@ TEST(FormatTest, ToString) {
   EXPECT_EQ("42", fmt::to_string(42));
 }
 
-TEST(WriterTest, NoncontiguousIterator) {
+TEST(WriterTest, OutputIterators) {
   std::list<char> out;
   fmt::format_to(std::back_inserter(out), "{}", 42);
   EXPECT_EQ("42", std::string(out.begin(), out.end()));
+  std::stringstream s;
+  fmt::format_to(std::ostream_iterator<char>(s), "{}", 42);
+  EXPECT_EQ("42", s.str());
 }