Separate parsing and formatting

fmt/format.h

@@ -451,6 +451,11 @@ class basic_string_view {
   /** Returns the string size. */
   std::size_t size() const { return size_; }
 
+  void remove_prefix(size_t n) {
+    data_ += n;
+    size_ -= n;
+  }
+
   // Lexicographically compare this string reference to other.
   int compare(basic_string_view other) const {
     std::size_t size = size_ < other.size_ ? size_ : other.size_;
@@ -483,6 +488,9 @@ class basic_string_view {
 typedef basic_string_view<char> string_view;
 typedef basic_string_view<wchar_t> wstring_view;
 
+template <typename Char>
+inline const Char *begin(basic_string_view<Char> s) { return s.data(); }
+
 /** A formatting error such as invalid format string. */
 class format_error : public std::runtime_error {
  public:
@@ -496,8 +504,8 @@ class format_error : public std::runtime_error {
 };
 
 // A formatter for objects of type T.
-template <typename Char, typename T>
-class formatter;
+template <typename T, typename Char = char, typename Enable = void>
+struct formatter;
 
 namespace internal {
 
@@ -847,6 +855,109 @@ struct conditional { typedef T type; };
 template<class T, class F>
 struct conditional<false, T, F> { typedef F type; };
 
+template <typename Char>
+class null_terminating_iterator;
+
+template <typename Char>
+const Char *pointer_from(null_terminating_iterator<Char> it);
+
+// An iterator that produces a null terminator on *end. This simplifies parsing
+// and allows comparing the performance of processing a null-terminated string
+// vs string_view.
+template <typename Char>
+class null_terminating_iterator {
+ public:
+  typedef Char value_type;
+  typedef std::ptrdiff_t difference_type;
+
+  null_terminating_iterator() : ptr_(0), end_(0) {}
+
+  null_terminating_iterator(const Char *ptr, const Char *end)
+    : ptr_(ptr), end_(end) {}
+
+  explicit null_terminating_iterator(basic_string_view<Char> s)
+    : ptr_(s.data()), end_(s.data() + s.size()) {}
+
+  null_terminating_iterator &operator=(const Char *ptr) {
+    assert(ptr <= end_);
+    ptr_ = ptr;
+    return *this;
+  }
+
+  Char operator*() const {
+    return ptr_ != end_ ? *ptr_ : 0;
+  }
+
+  null_terminating_iterator operator++() {
+    ++ptr_;
+    return *this;
+  }
+
+  null_terminating_iterator operator++(int) {
+    null_terminating_iterator result(*this);
+    ++ptr_;
+    return result;
+  }
+
+  null_terminating_iterator operator--() {
+    --ptr_;
+    return *this;
+  }
+
+  null_terminating_iterator operator+(difference_type n) {
+    return null_terminating_iterator(ptr_ + n, end_);
+  }
+
+  null_terminating_iterator operator+=(difference_type n) {
+    ptr_ += n;
+    return *this;
+  }
+
+  difference_type operator-(null_terminating_iterator other) const {
+    return ptr_ - other.ptr_;
+  }
+
+  bool operator!=(null_terminating_iterator other) const {
+    return ptr_ != other.ptr_;
+  }
+
+  bool operator>=(null_terminating_iterator other) const {
+    return ptr_ >= other.ptr_;
+  }
+
+  friend const Char *pointer_from<Char>(null_terminating_iterator it);
+
+ private:
+  const Char *ptr_;
+  const Char *end_;
+};
+
+template <
+  typename T,
+  typename Char,
+  typename std::enable_if<
+      std::is_same<T, null_terminating_iterator<Char>>::value, int>::type = 0>
+null_terminating_iterator<Char> to_iterator(basic_string_view<Char> v) {
+  const Char *s = v.data();
+  return null_terminating_iterator<Char>(s, s + v.size());
+}
+
+template <
+  typename T,
+  typename Char,
+  typename std::enable_if<std::is_same<T, const Char*>::value, int>::type = 0>
+const Char *to_iterator(basic_string_view<Char> v) {
+  return v.data();
+}
+
+template <typename T>
+const T *pointer_from(const T *p) { return p; }
+
+template <typename Char>
+const Char *pointer_from(null_terminating_iterator<Char> it) {
+  return it.ptr_;
+}
+
 // Returns true if value is negative, false otherwise.
 // Same as (value < 0) but doesn't produce warnings if T is an unsigned type.
 template <typename T>
@@ -1122,7 +1233,8 @@ struct string_value {
 template <typename Char>
 struct custom_value {
   typedef void (*FormatFunc)(
-      basic_buffer<Char> &buffer, const void *arg, void *ctx);
+      basic_buffer<Char> &buffer, const void *arg,
+      basic_string_view<Char>& format, void *ctx);
 
   const void *value;
   FormatFunc format;
@@ -1243,9 +1355,16 @@ class value {
   // Formats an argument of a custom type, such as a user-defined class.
   template <typename T>
   static void format_custom_arg(
-      basic_buffer<Char> &buffer, const void *arg, void *context) {
-    format_value(buffer, *static_cast<const T*>(arg),
-                 *static_cast<Context*>(context));
+      basic_buffer<Char> &buffer, const void *arg,
+      basic_string_view<Char> &format, void *context) {
+    Context &ctx = *static_cast<Context*>(context);
+    // Get the formatter type through the context to allow different contexts
+    // have different extension points, e.g. `formatter<T>` for `format` and
+    // `printf_formatter<T>` for `printf`.
+    typename Context::template formatter_type<T> f;
+    auto it = f.parse(format);
+    format.remove_prefix(it - begin(format));
+    f.format(buffer, *static_cast<const T*>(arg), ctx);
   }
 
  public:
@@ -1475,14 +1594,6 @@ basic_arg<Context> make_arg(const T &value) {
   typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
 #endif
 
-template <typename Formatter, typename T, typename Char>
-void format_value(basic_buffer<Char> &, const T &, Formatter &, const Char *) {
-  FMT_STATIC_ASSERT(sizeof(T) < 0,
-                    "Cannot format argument. To enable the use of ostream "
-                    "operator<< include fmt/ostream.h. Otherwise provide "
-                    "an overload of format_value.");
-}
-
 template <typename Context>
 struct named_arg : basic_arg<Context> {
   typedef typename Context::char_type Char;
@@ -1947,115 +2058,17 @@ class arg_formatter_base {
   }
 };
 
-template <typename Char>
-class null_terminating_iterator;
-
-template <typename Char>
-const Char *pointer_from(null_terminating_iterator<Char> it);
-
-// An iterator that produces a null terminator on *end. This simplifies parsing
-// and allows comparing the performance of processing a null-terminated string
-// vs string_view.
-template <typename Char>
-class null_terminating_iterator {
- public:
-  typedef Char value_type;
-  typedef std::ptrdiff_t difference_type;
-
-  null_terminating_iterator() : ptr_(0), end_(0) {}
-
-  null_terminating_iterator(const Char *ptr, const Char *end)
-    : ptr_(ptr), end_(end) {}
-
-  Char operator*() const {
-    return ptr_ != end_ ? *ptr_ : 0;
-  }
-
-  null_terminating_iterator operator++() {
-    ++ptr_;
-    return *this;
-  }
-
-  null_terminating_iterator operator++(int) {
-    null_terminating_iterator result(*this);
-    ++ptr_;
-    return result;
-  }
-
-  null_terminating_iterator operator--() {
-    --ptr_;
-    return *this;
-  }
-
-  null_terminating_iterator operator+(difference_type n) {
-    return null_terminating_iterator(ptr_ + n, end_);
-  }
-
-  null_terminating_iterator operator+=(difference_type n) {
-    ptr_ += n;
-    return *this;
-  }
-
-  difference_type operator-(null_terminating_iterator other) const {
-    return ptr_ - other.ptr_;
-  }
-
-  bool operator!=(null_terminating_iterator other) const {
-    return ptr_ != other.ptr_;
-  }
-
-  bool operator>=(null_terminating_iterator other) const {
-    return ptr_ >= other.ptr_;
-  }
-
-  friend const Char *pointer_from<Char>(null_terminating_iterator it);
-
- private:
-  const Char *ptr_;
-  const Char *end_;
-};
-
-template <
-  typename T,
-  typename Char,
-  typename std::enable_if<
-      std::is_same<T, null_terminating_iterator<Char>>::value, int>::type = 0>
-null_terminating_iterator<Char> to_iterator(basic_string_view<Char> v) {
-  const Char *s = v.data();
-  return null_terminating_iterator<Char>(s, s + v.size());
-}
-
-template <
-  typename T,
-  typename Char,
-  typename std::enable_if<std::is_same<T, const Char*>::value, int>::type = 0>
-const Char *to_iterator(const basic_string_view<Char> v) {
-  return v.data();
-}
-
-template <typename T>
-const T *pointer_from(const T *p) { return p; }
-
-template <typename Char>
-const Char *pointer_from(null_terminating_iterator<Char> it) {
-  return it.ptr_;
-}
-
 template <typename Char, typename Context>
 class context_base {
- public:
-  typedef null_terminating_iterator<Char> iterator;
-
  private:
-  iterator pos_;
   basic_args<Context> args_;
   int next_arg_index_;
 
  protected:
   typedef basic_arg<Context> format_arg;
 
-  context_base(basic_string_view<Char> format_str, basic_args<Context> args)
-  : pos_(to_iterator<iterator>(format_str)), args_(args), next_arg_index_(0) {}
+  explicit context_base(basic_args<Context> args)
+  : args_(args), next_arg_index_(0) {}
   ~context_base() {}
 
   basic_args<Context> args() const { return args_; }
@@ -2091,10 +2104,6 @@ class context_base {
     next_arg_index_ = -1;
     return true;
   }
-
- public:
-  // Returns an iterator to the current position in the format string.
-  iterator &pos() { return pos_; }
 };
 }  // namespace internal
 
@@ -2125,7 +2134,8 @@ class arg_formatter : public internal::arg_formatter_base<Char> {
 
   /** Formats an argument of a custom (user-defined) type. */
   void operator()(internal::custom_value<Char> c) {
-    c.format(this->writer().buffer(), c.value, &ctx_);
+    basic_string_view<Char> format_str;
+    c.format(this->writer().buffer(), c.value, format_str, &ctx_);
   }
 };
 
@@ -2134,7 +2144,10 @@ class basic_context :
   public internal::context_base<Char, basic_context<Char>> {
  public:
   /** The character type for the output. */
-  typedef Char char_type;
+  using char_type = Char;
+
+  template <typename T>
+  using formatter_type = formatter<T, Char>;
 
  private:
   internal::arg_map<basic_context<Char>> map_;
@@ -2153,9 +2166,7 @@ class basic_context :
    stored in the object so make sure they have appropriate lifetimes.
    \endrst
    */
-  basic_context(
-      basic_string_view<Char> format_str, basic_args<basic_context> args)
-  : Base(format_str, args) {}
+  basic_context(basic_args<basic_context> args): Base(args) {}
 
   format_arg next_arg() {
     const char *error = 0;
@@ -2177,8 +2188,6 @@ class basic_context :
   // Checks if manual indexing is used and returns the argument with
   // specified name.
   format_arg get_arg(basic_string_view<Char> name);
-
-  using Base::pos;
 };
 
 /**
@@ -3210,14 +3219,16 @@ template <typename Char, typename Context>
 class custom_formatter {
  private:
   basic_buffer<Char> &buffer_;
+  basic_string_view<Char> &format_;
   Context &ctx_;
 
  public:
-  custom_formatter(basic_buffer<Char> &buffer, Context &ctx)
-  : buffer_(buffer), ctx_(ctx) {}
+  custom_formatter(basic_buffer<Char> &buffer, basic_string_view<Char> &format,
+                   Context &ctx)
+  : buffer_(buffer), format_(format), ctx_(ctx) {}
 
   bool operator()(internal::custom_value<Char> custom) {
-    custom.format(buffer_, custom.value, &ctx_);
+    custom.format(buffer_, custom.value, format_, &ctx_);
     return true;
   }
 
@@ -3267,16 +3278,14 @@ struct precision_handler {
   }
 };
 
-template <typename Context>
-class specs_handler {
+template <typename Char>
+class specs_handler_base {
  public:
-  typedef typename Context::char_type char_type;
-
-  specs_handler(basic_format_specs<char_type> &specs, Context &ctx)
-    : specs_(specs), context_(ctx) {}
+  explicit specs_handler_base(basic_format_specs<Char> &specs)
+    : specs_(specs) {}
 
   void on_align(alignment align) { specs_.align_ = align; }
-  void on_fill(char_type fill) { specs_.fill_ = fill; }
+  void on_fill(Char fill) { specs_.fill_ = fill; }
   void on_plus() { specs_.flags_ |= SIGN_FLAG | PLUS_FLAG; }
   void on_minus() { specs_.flags_ |= MINUS_FLAG; }
   void on_space() { specs_.flags_ |= SIGN_FLAG; }
@@ -3288,46 +3297,107 @@ class specs_handler {
   }
 
   void on_width(unsigned width) { specs_.width_ = width; }
+  void on_precision(unsigned precision) { specs_.precision_ = precision; }
+  void on_type(char type) { specs_.type_ = type; }
+
+ protected:
+  ~specs_handler_base() {}
+
+  basic_format_specs<Char> &specs_;
+};
+
+template <typename Handler, typename T, typename Context>
+inline void set_dynamic_spec(T &value, basic_arg<Context> arg) {
+  ulong_long big_value = visit(Handler(), arg);
+  if (big_value > (std::numeric_limits<int>::max)())
+    FMT_THROW(format_error("number is too big"));
+  value = static_cast<int>(big_value);
+}
+
+template <typename Context>
+class specs_handler : public specs_handler_base<typename Context::char_type> {
+ public:
+  typedef typename Context::char_type char_type;
+
+  specs_handler(basic_format_specs<char_type> &specs, Context &ctx)
+    : specs_handler_base<char_type>(specs), context_(ctx) {}
 
   template <typename Id>
   void on_dynamic_width(Id arg_id) {
-    auto width_arg = get_arg(arg_id);
-    ulong_long width = visit(internal::width_handler(), width_arg);
-    if (width > (std::numeric_limits<int>::max)())
-      FMT_THROW(format_error("number is too big"));
-    specs_.width_ = static_cast<int>(width);
+    set_dynamic_spec<internal::width_handler>(
+          this->specs_.width_, get_arg(arg_id));
   }
 
-  void on_precision(unsigned precision) { specs_.precision_ = precision; }
-
   template <typename Id>
   void on_dynamic_precision(Id arg_id) {
-    auto precision_arg = get_arg(arg_id);
-    ulong_long precision = visit(internal::precision_handler(), precision_arg);
-    if (precision > (std::numeric_limits<int>::max)())
-      FMT_THROW(format_error("number is too big"));
-    specs_.precision_ = static_cast<int>(precision);
+    set_dynamic_spec<internal::precision_handler>(
+          this->specs_.precision_, get_arg(arg_id));
   }
 
-  void on_type(char type) { specs_.type_ = type; }
-
  private:
   basic_arg<Context> get_arg(monostate) {
     return context_.next_arg();
   }
 
-  basic_arg<Context> get_arg(unsigned index) {
-    return context_.get_arg(index);
+  template <typename Id>
+  basic_arg<Context> get_arg(Id arg_id) {
+    return context_.get_arg(arg_id);
   }
 
-  basic_arg<Context> get_arg(basic_string_view<char_type> name) {
-    return context_.get_arg(name);
-  }
-
-  basic_format_specs<char_type> &specs_;
   Context &context_;
 };
 
+// An argument reference.
+template <typename Char>
+struct arg_ref {
+  enum Kind { NONE, INDEX, NAME };
+
+  arg_ref() : kind(NONE) {}
+  explicit arg_ref(unsigned index) : kind(INDEX), index(index) {}
+  explicit arg_ref(basic_string_view<Char> name) : kind(NAME), name(name) {}
+
+  Kind kind;
+  union {
+    unsigned index;
+    basic_string_view<Char> name;
+  };
+};
+
+template <typename Char>
+struct dynamic_format_specs : basic_format_specs<Char> {
+  arg_ref<Char> width_ref;
+  arg_ref<Char> precision_ref;
+};
+
+template <typename Char>
+class dynamic_specs_handler : public specs_handler_base<Char> {
+ public:
+  explicit dynamic_specs_handler(dynamic_format_specs<Char> &specs)
+    : specs_handler_base<Char>(specs), specs_(specs) {}
+
+  template <typename Id>
+  void on_dynamic_width(Id arg_id) {
+    set(specs_.width_ref, arg_id);
+  }
+
+  template <typename Id>
+  void on_dynamic_precision(Id arg_id) {
+    set(specs_.precision_ref, arg_id);
+  }
+
+ private:
+  template <typename Id>
+  void set(arg_ref<Char> &ref, Id arg_id) {
+    ref = arg_ref<Char>(arg_id);
+  }
+
+  void set(arg_ref<Char> &ref, monostate) {
+    ref.kind = arg_ref<Char>::NONE;
+  }
+
+  dynamic_format_specs<Char> &specs_;
+};
+
 template <typename Iterator, typename Handler>
 Iterator parse_arg_id(Iterator it, Handler handler) {
   typedef typename Iterator::value_type char_type;
@@ -3338,15 +3408,13 @@ Iterator parse_arg_id(Iterator it, Handler handler) {
   }
   if (c >= '0' && c <= '9') {
     unsigned index = parse_nonnegative_int(it);
-    if (*it != '}' && *it != ':') {
+    if (*it != '}' && *it != ':')
       FMT_THROW(format_error("invalid format string"));
-    }
     handler(index);
     return it;
   }
-  if (!is_name_start(c)) {
+  if (!is_name_start(c))
     FMT_THROW(format_error("invalid format string"));
-  }
   auto start = it;
   do {
     c = *++it;
@@ -3357,6 +3425,9 @@ Iterator parse_arg_id(Iterator it, Handler handler) {
 
 // Parses standard format specifiers and sends notifications about parsed
 // components to handler.
+// it: an iterator pointing to the beginning of a null-terminated range of
+//     characters, possibly emulated via null_terminating_iterator, representing
+//     format specifiers.
 template <typename Iterator, typename Handler>
 Iterator parse_format_specs(Iterator it, Type arg_type, Handler &handler) {
   typedef typename Iterator::value_type char_type;
@@ -3482,13 +3553,16 @@ Iterator parse_format_specs(Iterator it, Type arg_type, Handler &handler) {
 
 // Formats a single argument.
 template <typename ArgFormatter, typename Char, typename Context>
-void do_format_arg(basic_buffer<Char> &buffer, const basic_arg<Context> &arg,
-                   Context &ctx) {
-  auto &it = ctx.pos();
+const Char *do_format_arg(basic_buffer<Char> &buffer,
+                          const basic_arg<Context> &arg,
+                          basic_string_view<Char> format,
+                          Context &ctx) {
+  auto it = null_terminating_iterator<Char>(format);
   basic_format_specs<Char> specs;
   if (*it == ':') {
-    if (visit(custom_formatter<Char, Context>(buffer, ctx), arg))
-      return;
+    format.remove_prefix(1);
+    if (visit(custom_formatter<Char, Context>(buffer, format, ctx), arg))
+      return begin(format);
     specs_handler<Context> handler(specs, ctx);
     it = parse_format_specs(it + 1, arg.type(), handler);
   }
@@ -3498,28 +3572,69 @@ void do_format_arg(basic_buffer<Char> &buffer, const basic_arg<Context> &arg,
 
   // Format argument.
   visit(ArgFormatter(buffer, ctx, specs), arg);
+  return pointer_from(it);
 }
+
+// Specifies whether to format enums.
+template <typename T, typename Enable = void>
+struct format_enum : std::integral_constant<bool, std::is_enum<T>::value> {};
 }  // namespace internal
 
-template <typename T, typename Char = char>
-class formatter {
- public:
-  explicit formatter(basic_context<Char> &ctx) {
-    auto &it = ctx.pos();
-    if (*it == ':') {
-      ++it;
-      internal::specs_handler<basic_context<Char>> handler(specs_, ctx);
-      it = parse_format_specs(it, internal::gettype<T>(), handler);
-    }
+// Formatter of objects of type T.
+template <typename T, typename Char>
+struct formatter<T, Char,
+    typename std::enable_if<internal::gettype<T>() != internal::CUSTOM>::type> {
+
+  // Parses format specifiers stopping either at the end of the range or at the
+  // terminating '}'.
+  template <typename Range>
+  auto parse(Range format) -> decltype(begin(format)) {
+    auto it = internal::null_terminating_iterator<Char>(format);
+    internal::dynamic_specs_handler<Char> handler(specs_);
+    it = parse_format_specs(it, internal::gettype<T>(), handler);
+    return pointer_from(it);
   }
 
   void format(basic_buffer<Char> &buf, const T &val, basic_context<Char> &ctx) {
+    handle_dynamic_spec<internal::width_handler>(
+      specs_.width_, specs_.width_ref, ctx);
+    handle_dynamic_spec<internal::precision_handler>(
+      specs_.precision_, specs_.precision_ref, ctx);
     visit(arg_formatter<Char>(buf, ctx, specs_),
           internal::make_arg<basic_context<Char>>(val));
   }
 
  private:
-  basic_format_specs<Char> specs_;
+  using arg_ref = internal::arg_ref<Char>;
+
+  template <typename Handler, typename Spec>
+  static void handle_dynamic_spec(
+      Spec &value, arg_ref ref, basic_context<Char> &ctx) {
+    switch (ref.kind) {
+    case arg_ref::NONE:
+      // Do nothing.
+      break;
+    case arg_ref::INDEX:
+      internal::set_dynamic_spec<Handler>(value, ctx.get_arg(ref.index));
+      break;
+    case arg_ref::NAME:
+      internal::set_dynamic_spec<Handler>(value, ctx.get_arg(ref.name));
+      break;
+    // TODO: handle automatic numbering
+    }
+  }
+
+  internal::dynamic_format_specs<Char> specs_;
+};
+
+template <typename T, typename Char>
+struct formatter<T, Char,
+    typename std::enable_if<internal::format_enum<T>::value>::type>
+    : public formatter<int, Char> {
+  template <typename Range>
+  auto parse(Range format) -> decltype(begin(format)) {
+    return begin(format);
+  }
 };
 
 template <typename Char>
@@ -3541,9 +3656,9 @@ inline typename basic_context<Char>::format_arg
 template <typename ArgFormatter, typename Char, typename Context>
 void vformat_to(basic_buffer<Char> &buffer, basic_string_view<Char> format_str,
                 basic_args<Context> args) {
-  basic_context<Char> ctx(format_str, args);
-  auto &it = ctx.pos();
-  auto start = it;
+  basic_context<Char> ctx(args);
+  auto start = internal::null_terminating_iterator<Char>(format_str);
+  auto it = start;
   using internal::pointer_from;
   while (*it) {
     Char c = *it++;
@@ -3573,7 +3688,8 @@ void vformat_to(basic_buffer<Char> &buffer, basic_string_view<Char> format_str,
     } handler(ctx, arg);
 
     it = parse_arg_id(it, handler);
-    internal::do_format_arg<ArgFormatter>(buffer, arg, ctx);
+    format_str.remove_prefix(pointer_from(it) - format_str.data());
+    it = internal::do_format_arg<ArgFormatter>(buffer, arg, format_str, ctx);
     if (*it != '}')
       FMT_THROW(format_error(fmt::format("unknown format specifier")));
     start = ++it;

fmt/ostream.h

@@ -77,18 +77,28 @@ void format_value(basic_buffer<Char> &buffer, const T &value) {
   output << value;
   buffer.resize(format_buf.size());
 }
+
+// Disable builtin formatting of enums and use operator<< instead.
+template <typename T>
+struct format_enum<T,
+    typename std::enable_if<std::is_enum<T>::value>::type> : std::false_type {};
 }  // namespace internal
 
-// Formats a value.
-template <typename Char, typename T>
-void format_value(basic_buffer<Char> &buf, const T &value,
-                  basic_context<Char> &ctx) {
-  basic_memory_buffer<Char> buffer;
-  internal::format_value(buffer, value);
-  basic_string_view<Char> str(buffer.data(), buffer.size());
-  internal::do_format_arg< arg_formatter<Char> >(
-        buf, internal::make_arg< basic_context<Char> >(str), ctx);
-}
+// Formats an object of type T that has an overloaded ostream operator<<.
+template <typename T, typename Char>
+struct formatter<T, Char,
+    typename std::enable_if<
+      internal::gettype<T>() == internal::CUSTOM>::type>
+    : formatter<basic_string_view<Char>, Char> {
+
+  void format(basic_buffer<Char> &buf, const T &value,
+              basic_context<Char> &ctx) {
+    basic_memory_buffer<Char> buffer;
+    internal::format_value(buffer, value);
+    basic_string_view<Char> str(buffer.data(), buffer.size());
+    formatter<basic_string_view<Char>, Char>::format(buf, str, ctx);
+  }
+};
 
 FMT_API void vprint(std::ostream &os, string_view format_str, args args);
 

fmt/printf.cc

@@ -14,7 +14,7 @@ FMT_FUNC int vfprintf(std::FILE *f, string_view format, printf_args args) {
 }
 
 #ifndef FMT_HEADER_ONLY
-template void printf_context<char>::format(buffer &);
-template void printf_context<wchar_t>::format(wbuffer &);
+template void printf_context<char>::format(string_view, buffer &);
+template void printf_context<wchar_t>::format(wstring_view, wbuffer &);
 #endif
 }

fmt/printf.h

@@ -286,28 +286,46 @@ class printf_arg_formatter : public internal::arg_formatter_base<Char> {
 
   /** Formats an argument of a custom (user-defined) type. */
   void operator()(internal::custom_value<Char> c) {
-    const Char format_str[] = {'}', '\0'};
+    const Char format_str_data[] = {'}', '\0'};
+    basic_string_view<Char> format_str = format_str_data;
     auto args = basic_args<basic_context<Char>>();
-    basic_context<Char> ctx(format_str, args);
-    c.format(this->writer().buffer(), c.value, &ctx);
+    basic_context<Char> ctx(args);
+    c.format(this->writer().buffer(), c.value, format_str, &ctx);
+  }
+};
+
+template <typename Char,
+          typename ArgFormatter = printf_arg_formatter<Char> >
+class printf_context;
+
+template <typename T, typename Char = char>
+struct printf_formatter {
+  const Char *parse(basic_string_view<Char> s) {
+    return s.data();
+  }
+
+  void format(basic_buffer<Char> &buf, const T &value, printf_context<Char> &) {
+    internal::format_value(buf, value);
   }
 };
 
 /** This template formats data and writes the output to a writer. */
-template <typename Char,
-          typename ArgFormatter = printf_arg_formatter<Char> >
+template <typename Char, typename ArgFormatter>
 class printf_context :
   private internal::context_base<
     Char, printf_context<Char, ArgFormatter>> {
  public:
   /** The character type for the output. */
-  typedef Char char_type;
+  using char_type = Char;
+
+  template <typename T>
+  using formatter_type = printf_formatter<T>;
 
  private:
   typedef internal::context_base<Char, printf_context> Base;
   typedef typename Base::format_arg format_arg;
   typedef basic_format_specs<Char> format_specs;
-  typedef typename Base::iterator iterator;
+  typedef internal::null_terminating_iterator<Char> iterator;
 
   void parse_flags(format_specs &spec, iterator &it);
 
@@ -328,12 +346,11 @@ class printf_context :
    appropriate lifetimes.
    \endrst
    */
-  explicit printf_context(basic_string_view<Char> format_str,
-                          basic_args<printf_context> args)
-    : Base(format_str, args) {}
+  explicit printf_context(basic_args<printf_context> args): Base(args) {}
 
   /** Formats stored arguments and writes the output to the buffer. */
-  FMT_API void format(basic_buffer<Char> &buffer);
+  FMT_API void format(
+      basic_string_view<Char> format_str, basic_buffer<Char> &buffer);
 };
 
 template <typename Char, typename AF>
@@ -415,8 +432,9 @@ unsigned printf_context<Char, AF>::parse_header(
 }
 
 template <typename Char, typename AF>
-void printf_context<Char, AF>::format(basic_buffer<Char> &buffer) {
-  auto start = this->pos();
+void printf_context<Char, AF>::format(
+    basic_string_view<Char> format_str, basic_buffer<Char> &buffer) {
+  auto start = iterator(format_str);
   auto it = start;
   using internal::pointer_from;
   while (*it) {
@@ -515,17 +533,10 @@ void printf_context<Char, AF>::format(basic_buffer<Char> &buffer) {
   buffer.append(pointer_from(start), pointer_from(it));
 }
 
-// Formats a value.
-template <typename Char, typename T>
-void format_value(basic_buffer<Char> &buf, const T &value,
-                  printf_context<Char>& ctx) {
-  internal::format_value(buf, value);
-}
-
 template <typename Char>
 void printf(basic_buffer<Char> &buf, basic_string_view<Char> format,
             basic_args<printf_context<Char>> args) {
-  printf_context<Char>(format, args).format(buf);
+  printf_context<Char>(args).format(format, buf);
 }
 
 typedef basic_args<printf_context<char>> printf_args;

fmt/time.h

@@ -15,40 +15,46 @@
 
 namespace fmt {
 
-void format_value(buffer &buf, const std::tm &tm, context &ctx) {
-  auto &it = ctx.pos();
-  if (*it == ':')
-    ++it;
-  auto end = it;
-  while (*end && *end != '}')
-    ++end;
-  if (*end != '}')
-    FMT_THROW(format_error("missing '}' in format string"));
-  memory_buffer format;
-  format.reserve(end - it + 1);
-  using internal::pointer_from;
-  format.append(pointer_from(it), pointer_from(end));
-  format.push_back('\0');
-  std::size_t start = buf.size();
-  for (;;) {
-    std::size_t size = buf.capacity() - start;
-    std::size_t count = std::strftime(&buf[start], size, &format[0], &tm);
-    if (count != 0) {
-      buf.resize(start + count);
-      break;
-    }
-    if (size >= format.size() * 256) {
-      // If the buffer is 256 times larger than the format string, assume
-      // that `strftime` gives an empty result. There doesn't seem to be a
-      // better way to distinguish the two cases:
-      // https://github.com/fmtlib/fmt/issues/367
-      break;
-    }
-    const std::size_t MIN_GROWTH = 10;
-    buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
+template <>
+struct formatter<std::tm> {
+  template <typename Range>
+  auto parse(Range format) -> decltype(begin(format)) {
+    auto it = internal::null_terminating_iterator<char>(format);
+    if (*it == ':')
+      ++it;
+    auto end = it;
+    while (*end && *end != '}')
+      ++end;
+    tm_format.reserve(end - it + 1);
+    using internal::pointer_from;
+    tm_format.append(pointer_from(it), pointer_from(end));
+    tm_format.push_back('\0');
+    return pointer_from(end);
   }
-  it = end;
-}
+
+  void format(buffer &buf, const std::tm &tm, context &ctx) {
+    std::size_t start = buf.size();
+    for (;;) {
+      std::size_t size = buf.capacity() - start;
+      std::size_t count = std::strftime(&buf[start], size, &tm_format[0], &tm);
+      if (count != 0) {
+        buf.resize(start + count);
+        break;
+      }
+      if (size >= tm_format.size() * 256) {
+        // If the buffer is 256 times larger than the format string, assume
+        // that `strftime` gives an empty result. There doesn't seem to be a
+        // better way to distinguish the two cases:
+        // https://github.com/fmtlib/fmt/issues/367
+        break;
+      }
+      const std::size_t MIN_GROWTH = 10;
+      buf.reserve(buf.capacity() + (size > MIN_GROWTH ? size : MIN_GROWTH));
+    }
+  }
+
+  memory_buffer tm_format;
+};
 }
 
 #endif  // FMT_TIME_H_

test/custom-formatter-test.cc

@@ -65,8 +65,8 @@ std::string custom_vsprintf(
     const char* format_str,
     fmt::basic_args<CustomPrintfFormatter> args) {
   fmt::memory_buffer buffer;
-  CustomPrintfFormatter formatter(format_str, args);
-  formatter.format(buffer);
+  CustomPrintfFormatter formatter(args);
+  formatter.format(format_str, buffer);
   return std::string(buffer.data(), buffer.size());
 }
 

test/format-test.cc

@@ -1227,8 +1227,18 @@ TEST(FormatterTest, FormatStringView) {
   EXPECT_EQ("test", format("{0}", string_view("test")));
 }
 
-void format_value(fmt::buffer &buf, const Date &d, fmt::context &) {
-  fmt::format_to(buf, "{}-{}-{}", d.year(), d.month(), d.day());
+namespace fmt {
+template <>
+struct formatter<Date> {
+  template <typename Range>
+  auto parse(Range format) -> decltype(begin(format)) {
+    return begin(format);
+  }
+
+  void format(buffer &buf, const Date &d, context &) {
+    format_to(buf, "{}-{}-{}", d.year(), d.month(), d.day());
+  }
+};
 }
 
 TEST(FormatterTest, FormatCustom) {
@@ -1239,9 +1249,13 @@ TEST(FormatterTest, FormatCustom) {
 
 class Answer {};
 
-void format_value(fmt::buffer &buf, Answer, fmt::context &ctx) {
-  fmt::formatter<int> f(ctx);
-  f.format(buf, 42, ctx);
+namespace fmt {
+template <>
+struct formatter<Answer> : formatter<int> {
+  void format(fmt::buffer &buf, Answer, fmt::context &ctx) {
+    formatter<int>::format(buf, 42, ctx);
+  }
+};
 }
 
 TEST(FormatterTest, CustomFormat) {

test/ostream-test.cc

@@ -65,7 +65,7 @@ struct TestArgFormatter : fmt::arg_formatter<char> {
 
 TEST(OStreamTest, CustomArg) {
   fmt::memory_buffer buffer;
-  fmt::context ctx("}", fmt::args());
+  fmt::context ctx((fmt::args()));
   fmt::format_specs spec;
   TestArgFormatter af(buffer, ctx, spec);
   visit(af, fmt::internal::make_arg<fmt::context>(TestEnum()));

test/util-test.cc

@@ -66,19 +66,27 @@ namespace {
 
 struct Test {};
 
-template <typename Char>
-void format_value(fmt::basic_buffer<Char> &b, Test,
-                  fmt::basic_context<Char> &) {
-  const Char *test = "test";
-  b.append(test, test + std::strlen(test));
-}
-
 template <typename Context, typename T>
 basic_arg<Context> make_arg(const T &value) {
   return fmt::internal::make_arg<Context>(value);
 }
 }  // namespace
 
+namespace fmt {
+template <typename Char>
+struct formatter<Test, Char> {
+  template <typename Range>
+  auto parse(Range format) -> decltype(begin(format)) {
+    return begin(format);
+  }
+
+  void format(basic_buffer<Char> &b, Test, basic_context<Char> &) {
+    const Char *test = "test";
+    b.append(test, test + std::strlen(test));
+  }
+};
+}
+
 void CheckForwarding(
     MockAllocator<int> &alloc, AllocatorRef< MockAllocator<int> > &ref) {
   int mem;
@@ -424,20 +432,33 @@ TEST(UtilTest, FormatArgs) {
 }
 
 struct CustomContext {
-  typedef char char_type;
-  bool called;
-};
+  using char_type = char;
 
-void format_value(fmt::buffer &, const Test &, CustomContext &ctx) {
-  ctx.called = true;
-}
+  template <typename T>
+  struct formatter_type {
+    template <typename Range>
+    auto parse(Range range) -> decltype(begin(range)) {
+      return begin(range);
+    }
+
+    void format(fmt::buffer &, const T &, CustomContext& ctx) {
+      ctx.called = true;
+    }
+  };
+
+  bool called;
+
+  fmt::string_view format() { return ""; }
+  void advance_to(const char *) {}
+};
 
 TEST(UtilTest, MakeValueWithCustomFormatter) {
   ::Test t;
   fmt::internal::value<CustomContext> arg(t);
   CustomContext ctx = {false};
   fmt::memory_buffer buffer;
-  arg.custom.format(buffer, &t, &ctx);
+  fmt::string_view format_str;
+  arg.custom.format(buffer, &t, format_str, &ctx);
   EXPECT_TRUE(ctx.called);
 }
 
@@ -581,8 +602,9 @@ TEST(UtilTest, CustomArg) {
         testing::Invoke([&](fmt::internal::custom_value<char> custom) {
     EXPECT_EQ(&test, custom.value);
     fmt::memory_buffer buffer;
-    fmt::context ctx("}", fmt::args());
-    custom.format(buffer, &test, &ctx);
+    fmt::context ctx((fmt::args()));
+    fmt::string_view format_str;
+    custom.format(buffer, &test, format_str, &ctx);
     EXPECT_EQ("test", std::string(buffer.data(), buffer.size()));
     return Visitor::Result();
   }));