mdbx++: add cursor::update_current() and cursor::reverse_current().

mdbx.h++

@@ -4487,6 +4487,11 @@ public:
   inline void upsert(const slice &key, const slice &value);
   inline slice upsert_reserve(const slice &key, size_t value_length);
 
+  /// \brief Updates value associated with a key at the current cursor position.
+  void update_current(const slice &value);
+  /// \brief Reserves and returns the space to storing a value associated with a key at the current cursor position.
+  slice reverse_current(size_t value_length);
+
   inline void update(const slice &key, const slice &value);
   inline bool try_update(const slice &key, const slice &value);
   inline slice update_reserve(const slice &key, size_t value_length);

src/mdbx.c++

@@ -1609,6 +1609,26 @@ __cold bool txn::rename_map(const ::std::string &old_name, const ::std::string &
 
 //------------------------------------------------------------------------------
 
+void cursor::update_current(const slice &value) {
+  default_buffer holder;
+  auto key = current().key;
+  if (error::boolean_or_throw(mdbx_is_dirty(handle_->txn, key.iov_base)))
+    key = holder.assign(key);
+
+  update(key, value);
+}
+
+slice cursor::reverse_current(size_t value_length) {
+  default_buffer holder;
+  auto key = current().key;
+  if (error::boolean_or_throw(mdbx_is_dirty(handle_->txn, key.iov_base)))
+    key = holder.assign(key);
+
+  return update_reserve(key, value_length);
+}
+
+//------------------------------------------------------------------------------
+
 __cold ::std::ostream &operator<<(::std::ostream &out, const slice &it) {
   out << "{";
   if (!it.is_valid())