Speed up iterator_buffer (#4679)

include/fmt/base.h

@@ -1858,6 +1858,65 @@ class fixed_buffer_traits {
   }
 };
 
+template <typename OutputIt, typename InputIt, typename = void>
+struct has_back_insert_iterator_container_append : std::false_type {};
+
+template <typename OutputIt, typename InputIt>
+struct has_back_insert_iterator_container_append<
+    OutputIt, InputIt,
+    void_t<decltype(get_container(std::declval<OutputIt>())
+                        .append(std::declval<InputIt>(),
+                                std::declval<InputIt>()))>> : std::true_type {};
+
+template <typename OutputIt, typename InputIt, typename = void>
+struct has_back_insert_iterator_container_insert_at_end : std::false_type {};
+
+template <typename OutputIt, typename InputIt>
+struct has_back_insert_iterator_container_insert_at_end<
+    OutputIt, InputIt,
+    void_t<decltype(get_container(std::declval<OutputIt>())
+                        .insert(get_container(std::declval<OutputIt>()).end(),
+                                std::declval<InputIt>(),
+                                std::declval<InputIt>()))>> : std::true_type {};
+
+// An optimized version of std::copy with the output value type (T).
+template <typename T, typename InputIt, typename OutputIt,
+          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value&&
+                            has_back_insert_iterator_container_append<
+                                OutputIt, InputIt>::value)>
+FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
+  get_container(out).append(begin, end);
+  return out;
+}
+
+template <typename T, typename InputIt, typename OutputIt,
+          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value &&
+                        !has_back_insert_iterator_container_append<
+                            OutputIt, InputIt>::value &&
+                        has_back_insert_iterator_container_insert_at_end<
+                            OutputIt, InputIt>::value)>
+FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
+  auto& c = get_container(out);
+  c.insert(c.end(), begin, end);
+  return out;
+}
+
+template <typename T, typename InputIt, typename OutputIt,
+          FMT_ENABLE_IF(!(is_back_insert_iterator<OutputIt>::value &&
+                          (has_back_insert_iterator_container_append<
+                               OutputIt, InputIt>::value ||
+                           has_back_insert_iterator_container_insert_at_end<
+                               OutputIt, InputIt>::value)))>
+FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
+  while (begin != end) *out++ = static_cast<T>(*begin++);
+  return out;
+}
+
+template <typename T, typename V, typename OutputIt>
+FMT_CONSTEXPR auto copy(basic_string_view<V> s, OutputIt out) -> OutputIt {
+  return copy<T>(s.begin(), s.end(), out);
+}
+
 // A buffer that writes to an output iterator when flushed.
 template <typename OutputIt, typename T, typename Traits = buffer_traits>
 class iterator_buffer : public Traits, public buffer<T> {
@@ -1875,7 +1934,12 @@ class iterator_buffer : public Traits, public buffer<T> {
     this->clear();
     const T* begin = data_;
     const T* end = begin + this->limit(size);
-    while (begin != end) *out_++ = *begin++;
+#if defined(__cpp_if_constexpr)
+    if constexpr (std::is_move_assignable<OutputIt>::value)
+      out_ = copy<T>(begin, end, out_);
+    else
+#endif
+      while (begin != end) *out_++ = *begin++;
   }
 
  public:

include/fmt/format.h

@@ -548,65 +548,6 @@ FMT_CONSTEXPR20 auto fill_n(T* out, Size count, char value) -> T* {
   return out + count;
 }
 
-template <typename OutputIt, typename InputIt, typename = void>
-struct has_back_insert_iterator_container_append : std::false_type {};
-
-template <typename OutputIt, typename InputIt>
-struct has_back_insert_iterator_container_append<
-    OutputIt, InputIt,
-    void_t<decltype(get_container(std::declval<OutputIt>())
-                        .append(std::declval<InputIt>(),
-                                std::declval<InputIt>()))>> : std::true_type {};
-
-template <typename OutputIt, typename InputIt, typename = void>
-struct has_back_insert_iterator_container_insert_at_end : std::false_type {};
-
-template <typename OutputIt, typename InputIt>
-struct has_back_insert_iterator_container_insert_at_end<
-    OutputIt, InputIt,
-    void_t<decltype(get_container(std::declval<OutputIt>())
-                        .insert(get_container(std::declval<OutputIt>()).end(),
-                                std::declval<InputIt>(),
-                                std::declval<InputIt>()))>> : std::true_type {};
-
-// An optimized version of std::copy with the output value type (T).
-template <typename T, typename InputIt, typename OutputIt,
-          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value&&
-                            has_back_insert_iterator_container_append<
-                                OutputIt, InputIt>::value)>
-FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
-  get_container(out).append(begin, end);
-  return out;
-}
-
-template <typename T, typename InputIt, typename OutputIt,
-          FMT_ENABLE_IF(is_back_insert_iterator<OutputIt>::value &&
-                        !has_back_insert_iterator_container_append<
-                            OutputIt, InputIt>::value &&
-                        has_back_insert_iterator_container_insert_at_end<
-                            OutputIt, InputIt>::value)>
-FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
-  auto& c = get_container(out);
-  c.insert(c.end(), begin, end);
-  return out;
-}
-
-template <typename T, typename InputIt, typename OutputIt,
-          FMT_ENABLE_IF(!(is_back_insert_iterator<OutputIt>::value &&
-                          (has_back_insert_iterator_container_append<
-                               OutputIt, InputIt>::value ||
-                           has_back_insert_iterator_container_insert_at_end<
-                               OutputIt, InputIt>::value)))>
-FMT_CONSTEXPR auto copy(InputIt begin, InputIt end, OutputIt out) -> OutputIt {
-  while (begin != end) *out++ = static_cast<T>(*begin++);
-  return out;
-}
-
-template <typename T, typename V, typename OutputIt>
-FMT_CONSTEXPR auto copy(basic_string_view<V> s, OutputIt out) -> OutputIt {
-  return copy<T>(s.begin(), s.end(), out);
-}
-
 template <typename OutChar, typename InputIt, typename OutputIt>
 FMT_CONSTEXPR FMT_NOINLINE auto copy_noinline(InputIt begin, InputIt end,
                                               OutputIt out) -> OutputIt {