Decouple appender from back_insert_iterator

include/fmt/core.h

@@ -11,7 +11,7 @@
 #include <cstddef>   // std::byte
 #include <cstdio>    // std::FILE
 #include <cstring>   // std::strlen
-#include <iterator>  // std::back_insert_iterator
+#include <iterator>  // DEPRECATED!
 #include <limits>    // std::numeric_limits
 #include <string>
 #include <type_traits>
@@ -1099,14 +1099,25 @@ using has_formatter =
 
 // An output iterator that appends to a buffer.
 // It is used to reduce symbol sizes for the common case.
-class appender : public std::back_insert_iterator<detail::buffer<char>> {
-  using base = std::back_insert_iterator<detail::buffer<char>>;
+class appender {
+ private:
+  detail::buffer<char>* buffer_;
+
+  friend auto get_container(appender app) -> detail::buffer<char>& {
+    return *app.buffer_;
+  }
 
  public:
-  using std::back_insert_iterator<detail::buffer<char>>::back_insert_iterator;
-  appender(base it) noexcept : base(it) {}
+  using difference_type = ptrdiff_t;
   FMT_UNCHECKED_ITERATOR(appender);
 
+  appender(detail::buffer<char>& buf) : buffer_(&buf) {}
+
+  appender& operator=(char c) {
+    buffer_->push_back(c);
+    return *this;
+  }
+  appender& operator*() {return *this;}
   auto operator++() noexcept -> appender& { return *this; }
   auto operator++(int) noexcept -> appender { return *this; }
 };
@@ -1545,6 +1556,8 @@ template <typename...> using void_t = void;
 template <typename It, typename T, typename Enable = void>
 struct is_output_iterator : std::false_type {};
 
+template <> struct is_output_iterator<appender, char> : std::true_type {};
+
 template <typename It, typename T>
 struct is_output_iterator<
     It, T,

include/fmt/format.h

@@ -262,6 +262,14 @@ inline auto ctzll(uint64_t x) -> int {
 FMT_END_NAMESPACE
 #endif
 
+namespace std {
+template <>
+struct iterator_traits<fmt::appender> {
+  using value_type = void;
+  using iterator_category	= std::output_iterator_tag;
+};
+}
+
 FMT_BEGIN_NAMESPACE
 namespace detail {
 

test/core-test.cc

@@ -11,7 +11,6 @@
 
 #include "fmt/core.h"
 
-#include <algorithm>    // std::copy_n
 #include <climits>      // INT_MAX
 #include <cstring>      // std::strlen
 #include <functional>   // std::equal_to
@@ -33,6 +32,11 @@ using testing::Return;
 #  error core-test includes format.h
 #endif
 
+fmt::appender copy(fmt::string_view s, fmt::appender out) {
+  for (char c : s) *out++ = c;
+  return out;
+}
+
 TEST(string_view_test, value_type) {
   static_assert(std::is_same<string_view::value_type, char>::value, "");
 }
@@ -102,16 +106,6 @@ TEST(core_test, is_output_iterator) {
 }
 
 TEST(core_test, buffer_appender) {
-  // back_insert_iterator is not default-constructible before C++20, so
-  // buffer_appender can only be default-constructible when back_insert_iterator
-  // is.
-  static_assert(
-      std::is_default_constructible<
-          std::back_insert_iterator<fmt::detail::buffer<char>>>::value ==
-          std::is_default_constructible<
-              fmt::detail::buffer_appender<char>>::value,
-      "");
-
 #ifdef __cpp_lib_ranges
   static_assert(std::output_iterator<fmt::detail::buffer_appender<char>, char>);
 #endif
@@ -297,8 +291,7 @@ template <typename Char> struct formatter<test_struct, Char> {
   }
 
   auto format(test_struct, format_context& ctx) const -> decltype(ctx.out()) {
-    auto test = string_view("test");
-    return std::copy_n(test.data(), test.size(), ctx.out());
+    return copy("test", ctx.out());
   }
 };
 FMT_END_NAMESPACE
@@ -619,8 +612,7 @@ template <> struct formatter<const_formattable> {
 
   auto format(const const_formattable&, format_context& ctx)
       -> decltype(ctx.out()) {
-    auto test = string_view("test");
-    return std::copy_n(test.data(), test.size(), ctx.out());
+    return copy("test", ctx.out());
   }
 };
 
@@ -631,8 +623,7 @@ template <> struct formatter<nonconst_formattable> {
 
   auto format(nonconst_formattable&, format_context& ctx)
       -> decltype(ctx.out()) {
-    auto test = string_view("test");
-    return std::copy_n(test.data(), test.size(), ctx.out());
+    return copy("test", ctx.out());
   }
 };
 FMT_END_NAMESPACE
@@ -653,8 +644,7 @@ template <> struct formatter<convertible_to_pointer_formattable> {
 
   auto format(convertible_to_pointer_formattable, format_context& ctx) const
       -> decltype(ctx.out()) {
-    auto test = string_view("test");
-    return std::copy_n(test.data(), test.size(), ctx.out());
+    return copy("test", ctx.out());
   }
 };
 FMT_END_NAMESPACE
@@ -732,7 +722,7 @@ template <> struct formatter<convertible_to_int> {
   }
   auto format(convertible_to_int, format_context& ctx) const
       -> decltype(ctx.out()) {
-    return std::copy_n("foo", 3, ctx.out());
+    return copy("foo", ctx.out());
   }
 };
 
@@ -742,7 +732,7 @@ template <> struct formatter<convertible_to_cstring> {
   }
   auto format(convertible_to_cstring, format_context& ctx) const
       -> decltype(ctx.out()) {
-    return std::copy_n("bar", 3, ctx.out());
+    return copy("bar", ctx.out());
   }
 };
 FMT_END_NAMESPACE
@@ -853,8 +843,7 @@ template <> struct formatter<its_a_trap> {
   }
 
   auto format(its_a_trap, format_context& ctx) const -> decltype(ctx.out()) {
-    auto s = string_view("42");
-    return std::copy(s.begin(), s.end(), ctx.out());
+    return copy("42", ctx.out());
   }
 };
 FMT_END_NAMESPACE