HDFS-13534. libhdfs++: Fix GCC7 build. Contributed by James Clampffer.

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/include/hdfspp/ioservice.h

@@ -58,6 +58,7 @@
 #ifndef INCLUDE_HDFSPP_IOSERVICE_H_
 #define INCLUDE_HDFSPP_IOSERVICE_H_
 
+#include <functional>
 #include <memory>
 
 // forward decl

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/lib/reader/datatransfer.h

@@ -59,7 +59,7 @@ public:
   void Connect(std::function<void(Status status, std::shared_ptr<DataNodeConnection> dn)> handler) override
   {(void)handler;  /*TODO: Handshaking goes here*/};
 
-  void Cancel();
+  void Cancel() override;
 private:
   DataTransferSaslStream(const DataTransferSaslStream &) = delete;
   DataTransferSaslStream &operator=(const DataTransferSaslStream &) = delete;

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/tests/mock_connection.h

@@ -87,7 +87,7 @@ public:
   virtual void cancel() {}
   virtual void close() {}
 protected:
-  virtual ProducerResult Produce() = 0;
+  ProducerResult Produce() override = 0;
   ::asio::io_service *io_service_;
 
 private:

hadoop-hdfs-project/hadoop-hdfs-native-client/src/main/native/libhdfspp/tests/remote_block_reader_test.cc

@@ -82,7 +82,7 @@ public:
     this->MockConnectionBase::async_write_some(buf, handler);
   }
 
-  void Cancel() {
+  void Cancel() override {
     /* no-op, declared pure virtual */
   }
 };