Support flushing output streams

libs/io/include/psemek/io/file_stream.hpp

@@ -83,6 +83,8 @@ namespace psemek::io
 
 		std::size_t write(char const * p, std::size_t size) override;
 
+		void flush() override;
+
 		~file_ostream() override
 		{
 			reset();

libs/io/include/psemek/io/stream.hpp

@@ -18,6 +18,7 @@ namespace psemek::io
 	struct ostream
 	{
 		virtual std::size_t write(char const * p, std::size_t size) = 0;
+		virtual void flush() {}
 
 		virtual ~ostream() {}
 	};

libs/io/source/file_stream.cpp

@@ -63,4 +63,10 @@ namespace psemek::io
 		return std::fwrite(p, 1, size, file_);
 	}
 
+	void file_ostream::flush()
+	{
+		if (!file_) throw null_ostream{};
+		std::fflush(file_);
+	}
+
 }