common: add {,make_}ostream_joiner

it's handy to print arrays like containers.

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/common/backport14.h b/src/common/backport14.h
index c89b3310d218d6fb673ef708474ee2c03e9c5dbb..5d5c9ffcbed734194571991ff03c6a64d6ff8a64 100644 (file)
--- a/src/common/backport14.h
+++ b/src/common/backport14.h
@@ -133,6 +133,68 @@ template<typename T>
 constexpr in_place_type_t<T> in_place_type{};
 #endif // __cpp_variable_templates
 } // namespace _backport17
+
+namespace _backport_ts {
+template <class DelimT,
+          class CharT = char,
+          class Traits = std::char_traits<CharT>>
+class ostream_joiner {
+public:
+  using char_type = CharT;
+  using traits_type = Traits;
+  using ostream_type = std::basic_ostream<CharT, Traits>;
+  using iterator_category = std::output_iterator_tag;
+  using value_type = void;
+  using difference_type = void;
+  using pointer = void;
+  using reference = void;
+
+  ostream_joiner(ostream_type& s, const DelimT& delimiter)
+    : out_stream(std::addressof(out_stream)),
+      delim(delimiter),
+      first_element(true)
+  {}
+  ostream_joiner(ostream_type& s, DelimT&& delimiter)
+    : out_stream(std::addressof(s)),
+      delim(std::move(delimiter)),
+      first_element(true)
+  {}
+
+  template<typename T>
+  ostream_joiner& operator=(const T& value) {
+    if (!first_element)
+      *out_stream << delim;
+    first_element = false;
+    *out_stream << value;
+    return *this;
+  }
+
+  ostream_joiner& operator*() noexcept {
+    return *this;
+  }
+  ostream_joiner& operator++() noexcept {
+    return *this;
+  }
+  ostream_joiner& operator++(int) noexcept {
+    return this;
+  }
+
+private:
+  ostream_type* out_stream;
+  DelimT delim;
+  bool first_element;
+};
+
+template <class CharT, class Traits, class DelimT>
+ostream_joiner<decay_t<DelimT>, CharT, Traits>
+make_ostream_joiner(std::basic_ostream<CharT, Traits>& os,
+                    DelimT&& delimiter) {
+    return ostream_joiner<decay_t<DelimT>,
+                          CharT, Traits>(os, std::forward<DelimT>(delimiter));
+}
+
+} // namespace _backport_ts
+
 using _backport14::make_unique;
 using _backport17::size;
 using _backport14::max;
@@ -143,6 +205,8 @@ using _backport17::in_place_type_t;
 #ifdef __cpp_variable_templates
 using _backport17::in_place_type;
 #endif // __cpp_variable_templates
+using _backport_ts::ostream_joiner;
+using _backport_ts::make_ostream_joiner;
 } // namespace ceph
 
 #endif // CEPH_COMMON_BACKPORT14_H