Implement format_to_n

include/fmt/format.h

@@ -702,7 +702,7 @@ FMT_CONSTEXPR const Char *pointer_from(null_terminating_iterator<Char> it) {
 template <typename T>
 class counting_iterator {
  private:
-  std::size_t& count_;
+  std::size_t* count_;
   mutable T blackhole_;
 
  public:
@@ -712,16 +712,10 @@ class counting_iterator {
   typedef T* pointer;
   typedef T& reference;
 
-  explicit counting_iterator(std::size_t &count): count_(count) {}
-  counting_iterator(const counting_iterator &other): count_(other.count_) {}
-
-  counting_iterator& operator=(const counting_iterator &other) {
-    count_ = other.count_;
-    return *this;
-  }
+  explicit counting_iterator(std::size_t &count): count_(&count) {}
 
   counting_iterator& operator++() {
-    ++count_;
+    ++*count_;
     return *this;
   }
 
@@ -730,6 +724,41 @@ class counting_iterator {
   T &operator*() const { return blackhole_; }
 };
 
+// An output iterator that truncates the output and counts the number of objects
+// written to it.
+template <typename OutputIt>
+class truncating_iterator {
+ private:
+  typedef std::iterator_traits<OutputIt> traits;
+
+  OutputIt out_;
+  std::size_t limit_;
+  std::size_t *count_;
+  mutable typename traits::value_type blackhole_;
+
+ public:
+  typedef std::output_iterator_tag iterator_category;
+  typedef typename traits::value_type value_type;
+  typedef typename traits::difference_type difference_type;
+  typedef typename traits::pointer pointer;
+  typedef typename traits::reference reference;
+
+  truncating_iterator(OutputIt out, std::size_t limit, std::size_t &count)
+    : out_(out), limit_(limit), count_(&count) {}
+
+  OutputIt base() const { return out_; }
+
+  truncating_iterator& operator++() {
+    if ((*count_)++ < limit_)
+      ++out_;
+    return *this;
+  }
+
+  truncating_iterator operator++(int) { return ++*this; }
+
+  reference operator*() const { return *count_ < limit_ ? *out_ : blackhole_; }
+};
+
 // 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>
@@ -3374,6 +3403,22 @@ inline typename std::enable_if<
   return vformat_to(out, format_str, make_args(args...));
 }
 
+template <typename OutputIt>
+struct format_to_n_result {
+  OutputIt out;
+  std::size_t size;
+};
+
+template <typename OutputIt, typename... Args>
+inline format_to_n_result<OutputIt> format_to_n(
+    OutputIt out, std::size_t n, string_view format_str, const Args & ... args) {
+  typedef internal::truncating_iterator<OutputIt> It;
+  std::size_t count = 0;
+  auto it = vformat_to(It(out, n, count), format_str,
+      *make_args<typename context_t<It>::type>(args...));
+  return {it.base(), count};
+}
+
 inline std::string vformat(string_view format_str, format_args args) {
   memory_buffer buffer;
   vformat_to(buffer, format_str, args);

test/format-test.cc

@@ -1505,6 +1505,14 @@ TEST(FormatTest, OutputSize) {
   EXPECT_EQ(2u, fmt::count("{}", 42));
 }
 
+TEST(FormatTest, FormatToN) {
+  char buffer[4];
+  buffer[3] = 'x';
+  auto result = fmt::format_to_n(buffer, 3, "{}", 12345);
+  EXPECT_EQ(5u, result.size);
+  EXPECT_EQ("123x", fmt::string_view(buffer, 4));
+}
+
 #if FMT_USE_CONSTEXPR
 struct test_arg_id_handler {
   enum result { NONE, EMPTY, INDEX, NAME, ERROR };