Make classes derived from buffer<T> final to silence the virtual destructor warning. (#1937)

Co-authored-by: Bart Siwek <bsiwek@cisco.com>

include/fmt/core.h

@@ -761,7 +761,7 @@ class fixed_buffer_traits {
 
 // 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> {
+class iterator_buffer final : public Traits, public buffer<T> {
  private:
   OutputIt out_;
   enum { buffer_size = 256 };
@@ -787,7 +787,7 @@ class iterator_buffer : public Traits, public buffer<T> {
   size_t count() const { return Traits::count() + this->size(); }
 };
 
-template <typename T> class iterator_buffer<T*, T> : public buffer<T> {
+template <typename T> class iterator_buffer<T*, T> final : public buffer<T> {
  protected:
   void grow(size_t) final FMT_OVERRIDE {}
 
@@ -801,7 +801,7 @@ template <typename T> class iterator_buffer<T*, T> : public buffer<T> {
 template <typename Container>
 class iterator_buffer<std::back_insert_iterator<Container>,
                       enable_if_t<is_contiguous<Container>::value,
-                                  typename Container::value_type>>
+                                  typename Container::value_type>> final
     : public buffer<typename Container::value_type> {
  private:
   Container& container_;
@@ -823,7 +823,7 @@ class iterator_buffer<std::back_insert_iterator<Container>,
 };
 
 // A buffer that counts the number of code units written discarding the output.
-template <typename T = char> class counting_buffer : public buffer<T> {
+template <typename T = char> class counting_buffer final : public buffer<T> {
  private:
   enum { buffer_size = 256 };
   T data_[buffer_size];