Provide a Flush() API within Writer

This is helpful if you’re writing code that needs to control flush
behavior and you don’t want to pass around your buffer object to each
handler function alongside the writer. Seems like an easy convenience
to add.

include/rapidjson/prettywriter.h

@@ -152,7 +152,7 @@ public:
         (void)ret;
         RAPIDJSON_ASSERT(ret == true);
         if (Base::level_stack_.Empty()) // end of json text
-            Base::os_->Flush();
+            Base::Flush();
         return true;
     }
 
@@ -176,7 +176,7 @@ public:
         (void)ret;
         RAPIDJSON_ASSERT(ret == true);
         if (Base::level_stack_.Empty()) // end of json text
-            Base::os_->Flush();
+            Base::Flush();
         return true;
     }
 

include/rapidjson/writer.h

@@ -267,6 +267,14 @@ public:
         return EndValue(WriteRawValue(json, length));
     }
 
+    //! Flush the output stream.
+    /*!
+        Allows the user to flush the output stream immediately.
+     */
+    void Flush() {
+        os_->Flush();
+    }
+
 protected:
     //! Information for each nested level
     struct Level {
@@ -473,7 +481,7 @@ protected:
     // Flush the value if it is the top level one.
     bool EndValue(bool ret) {
         if (RAPIDJSON_UNLIKELY(level_stack_.Empty()))   // end of json text
-            os_->Flush();
+            Flush();
         return ret;
     }