Simplify format string parsing

include/fmt/core.h

@@ -2190,7 +2190,8 @@ template <typename Char> struct arg_ref {
 // Format specifiers with width and precision resolved at formatting rather
 // than parsing time to allow reusing the same parsed specifiers with
 // different sets of arguments (precompilation of format strings).
-template <typename Char> struct dynamic_format_specs : format_specs<Char> {
+template <typename Char = char>
+struct dynamic_format_specs : format_specs<Char> {
   arg_ref<Char> width_ref;
   arg_ref<Char> precision_ref;
 };
@@ -2461,25 +2462,19 @@ FMT_CONSTEXPR inline void require_numeric_argument(type arg_type) {
     throw_format_error("format specifier requires numeric argument");
 }
 
-template <typename Char> struct parse_format_specs_result {
-  const Char* end;
-  dynamic_format_specs<Char> specs;
-};
-
 // Parses standard format specifiers.
 template <typename Char>
 FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
-    const Char* begin, const Char* end, basic_format_parse_context<Char>& ctx,
+    const Char* begin, const Char* end, dynamic_format_specs<Char>& specs,
-    type arg_type) -> parse_format_specs_result<Char> {
+    basic_format_parse_context<Char>& ctx, type arg_type) -> const Char* {
-  auto specs = dynamic_format_specs<Char>();
   if (1 < end - begin && begin[1] == '}' && is_ascii_letter(*begin) &&
       *begin != 'L') {
     specs.type = parse_presentation_type(*begin++);
     if (specs.type == presentation_type::none)
       throw_format_error("invalid type specifier");
-    return {begin, specs};
+    return begin;
   }
-  if (begin == end) return {begin, specs};
+  if (begin == end) return begin;
 
   auto align = parse_align(begin, end);
   if (align.align != align::none) {
@@ -2488,7 +2483,7 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
     if (fill_size > 0) specs.fill = {begin, to_unsigned(fill_size)};
   }
   begin = align.end;
-  if (begin == end) return {begin, specs};
+  if (begin == end) return begin;
 
   // Parse sign.
   switch (to_ascii(*begin)) {
@@ -2515,12 +2510,12 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
       throw_format_error("format specifier requires signed argument");
     }
   }
-  if (begin == end) return {begin, specs};
+  if (begin == end) return begin;
 
   if (*begin == '#') {
     require_numeric_argument(arg_type);
     specs.alt = true;
-    if (++begin == end) return {begin, specs};
+    if (++begin == end) return begin;
   }
 
   // Parse zero flag.
@@ -2531,11 +2526,11 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
       specs.align = align::numeric;
       specs.fill[0] = Char('0');
     }
-    if (++begin == end) return {begin, specs};
+    if (++begin == end) return begin;
   }
 
   begin = parse_dynamic_spec(begin, end, specs.width, specs.width_ref, ctx);
-  if (begin == end) return {begin, specs};
+  if (begin == end) return begin;
 
   // Parse precision.
   if (*begin == '.') {
@@ -2543,7 +2538,7 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
         parse_precision(begin, end, specs.precision, specs.precision_ref, ctx);
     if (is_integral_type(arg_type) || arg_type == type::pointer_type)
       throw_format_error("precision not allowed for this argument type");
-    if (begin == end) return {begin, specs};
+    if (begin == end) return begin;
   }
 
   if (*begin == 'L') {
@@ -2558,7 +2553,7 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
     if (specs.type == presentation_type::none)
       throw_format_error("invalid type specifier");
   }
-  return {begin, specs};
+  return begin;
 }
 
 template <typename Char, typename Handler>
@@ -2911,8 +2906,7 @@ struct formatter<T, Char,
     auto begin = ctx.begin(), end = ctx.end();
     if (begin == end) return begin;
     auto type = detail::type_constant<T, Char>::value;
-    auto result = detail::parse_format_specs(begin, end, ctx, type);
+    begin = detail::parse_format_specs(begin, end, specs_, ctx, type);
-    specs_ = result.specs;
     auto eh = detail::error_handler();
     switch (type) {
     case detail::type::none_type:
@@ -2937,19 +2931,19 @@ struct formatter<T, Char,
       break;
     case detail::type::float_type:
       if (detail::const_check(FMT_USE_FLOAT))
-        detail::parse_float_type_spec(result.specs, eh);
+        detail::parse_float_type_spec(specs_, eh);
       else
         FMT_ASSERT(false, "float support disabled");
       break;
     case detail::type::double_type:
       if (detail::const_check(FMT_USE_DOUBLE))
-        detail::parse_float_type_spec(result.specs, eh);
+        detail::parse_float_type_spec(specs_, eh);
       else
         FMT_ASSERT(false, "double support disabled");
       break;
     case detail::type::long_double_type:
       if (detail::const_check(FMT_USE_LONG_DOUBLE))
-        detail::parse_float_type_spec(result.specs, eh);
+        detail::parse_float_type_spec(specs_, eh);
       else
         FMT_ASSERT(false, "long double support disabled");
       break;
@@ -2967,7 +2961,7 @@ struct formatter<T, Char,
       // formatter specializations.
       break;
     }
-    return result.end;
+    return begin;
   }
 
   template <detail::type U = detail::type_constant<T, Char>::value,

include/fmt/format.h

@@ -3871,16 +3871,15 @@ class bytes {
 
 template <> struct formatter<bytes> {
  private:
-  detail::dynamic_format_specs<char> specs_;
+  detail::dynamic_format_specs<> specs_;
 
  public:
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
-    auto result = parse_format_specs(ctx.begin(), ctx.end(), ctx,
+    auto end = parse_format_specs(ctx.begin(), ctx.end(), specs_, ctx,
-                                     detail::type::string_type);
+                                  detail::type::string_type);
-    specs_ = result.specs;
     detail::check_string_type_spec(specs_.type, detail::error_handler());
-    return result.end;
+    return end;
   }
 
   template <typename FormatContext>
@@ -3913,16 +3912,15 @@ template <typename T> auto group_digits(T value) -> group_digits_view<T> {
 
 template <typename T> struct formatter<group_digits_view<T>> : formatter<T> {
  private:
-  detail::dynamic_format_specs<char> specs_;
+  detail::dynamic_format_specs<> specs_;
 
  public:
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
-    auto result =
+    auto end = parse_format_specs(ctx.begin(), ctx.end(), specs_, ctx,
-        parse_format_specs(ctx.begin(), ctx.end(), ctx, detail::type::int_type);
+                                  detail::type::int_type);
-    specs_ = result.specs;
     detail::check_string_type_spec(specs_.type, detail::error_handler());
-    return result.end;
+    return end;
   }
 
   template <typename FormatContext>
@@ -4139,16 +4137,15 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
         visit_format_arg(custom_formatter<Char>{parse_context, context}, arg);
         return parse_context.begin();
       }
-      auto result = parse_format_specs(begin, end, parse_context, arg.type());
+      auto specs = detail::dynamic_format_specs<Char>();
+      begin = parse_format_specs(begin, end, specs, parse_context, arg.type());
       detail::handle_dynamic_spec<detail::width_checker>(
-          result.specs.width, result.specs.width_ref, context);
+          specs.width, specs.width_ref, context);
       detail::handle_dynamic_spec<detail::precision_checker>(
-          result.specs.precision, result.specs.precision_ref, context);
+          specs.precision, specs.precision_ref, context);
-      begin = result.end;
       if (begin == end || *begin != '}')
         on_error("missing '}' in format string");
-      auto f =
+      auto f = arg_formatter<Char>{context.out(), specs, context.locale()};
-          arg_formatter<Char>{context.out(), result.specs, context.locale()};
       context.advance_to(visit_format_arg(f, arg));
       return begin;
     }

test/core-test.cc

@@ -523,9 +523,10 @@ TEST(format_test, constexpr_parse_arg_id) {
 template <size_t N> constexpr auto parse_test_specs(const char (&s)[N]) {
   auto ctx = fmt::detail::compile_parse_context<char>(fmt::string_view(s, N),
                                                       43, nullptr);
-  auto result = fmt::detail::parse_format_specs(s, s + N - 1, ctx,
+  auto specs = fmt::detail::dynamic_format_specs<>();
-                                                fmt::detail::type::float_type);
+  fmt::detail::parse_format_specs(s, s + N - 1, specs, ctx,
-  return result.specs;
+                                  fmt::detail::type::float_type);
+  return specs;
 }
 
 TEST(core_test, constexpr_parse_format_specs) {

test/xchar-test.cc

@@ -518,11 +518,10 @@ template <class charT> struct formatter<std::complex<double>, charT> {
  public:
   FMT_CONSTEXPR typename basic_format_parse_context<charT>::iterator parse(
       basic_format_parse_context<charT>& ctx) {
-    auto result = parse_format_specs(ctx.begin(), ctx.end(), ctx,
+    auto end = parse_format_specs(ctx.begin(), ctx.end(), specs_, ctx,
-                                     detail::type::string_type);
+                                  detail::type::string_type);
-    specs_ = result.specs;
     detail::parse_float_type_spec(specs_, detail::error_handler());
-    return result.end;
+    return end;
   }
 
   template <class FormatContext>