Make truncating_iterator an output_iterator (#2158)

include/fmt/format.h

@@ -469,15 +469,17 @@ template <typename OutputIt> class truncating_iterator_base {
  protected:
   OutputIt out_;
   size_t limit_;
-  size_t count_;
+  size_t count_ = 0;
 
+  truncating_iterator_base() : out_(), limit_(0) {}
+  
   truncating_iterator_base(OutputIt out, size_t limit)
-      : out_(out), limit_(limit), count_(0) {}
+      : out_(out), limit_(limit) {}
 
  public:
   using iterator_category = std::output_iterator_tag;
   using value_type = typename std::iterator_traits<OutputIt>::value_type;
-  using difference_type = void;
+  using difference_type = std::ptrdiff_t;
   using pointer = void;
   using reference = void;
   using _Unchecked_type =
@@ -502,6 +504,8 @@ class truncating_iterator<OutputIt, std::false_type>
  public:
   using value_type = typename truncating_iterator_base<OutputIt>::value_type;
 
+  truncating_iterator() = default;
+  
   truncating_iterator(OutputIt out, size_t limit)
       : truncating_iterator_base<OutputIt>(out, limit) {}
 
@@ -525,6 +529,8 @@ template <typename OutputIt>
 class truncating_iterator<OutputIt, std::true_type>
     : public truncating_iterator_base<OutputIt> {
  public:
+  truncating_iterator() = default;
+
   truncating_iterator(OutputIt out, size_t limit)
       : truncating_iterator_base<OutputIt>(out, limit) {}
 

test/format-test.cc

@@ -12,9 +12,11 @@
 #include <climits>
 #include <cmath>
 #include <cstring>
+#include <iterator>
 #include <list>
 #include <memory>
 #include <string>
+#include <type_traits>
 
 // Check if fmt/format.h compiles with windows.h included before it.
 #ifdef _WIN32
@@ -157,6 +159,24 @@ TEST(IteratorTest, TruncatingIterator) {
   EXPECT_EQ(it.base(), p + 1);
 }
 
+
+TEST(IteratorTest, TruncatingIteratorDefaultConstruct) {
+  static_assert(
+      std::is_default_constructible<fmt::detail::truncating_iterator<char*>>::value,
+      "");
+  
+  fmt::detail::truncating_iterator<char*> it;
+  EXPECT_EQ(nullptr, it.base());
+  EXPECT_EQ(std::size_t{0}, it.count());
+}
+
+#ifdef __cpp_lib_ranges
+TEST(IteratorTest, TruncatingIteratorOutputIterator) {
+  static_assert(std::output_iterator<fmt::detail::truncating_iterator<char*>,
+      char>);
+}
+#endif
+
 TEST(IteratorTest, TruncatingBackInserter) {
   std::string buffer;
   auto bi = std::back_inserter(buffer);