Update md

Author: yyin-dev

ch16/16_1_6.md

@@ -17,6 +17,15 @@ The deleter type is part of the type of `unique_ptr` that must be specified at d
 2. more flexibility
 3. overhead 
 
+Difference:
 https://stackoverflow.com/a/27742765/9057530
 https://stackoverflow.com/a/21355146/9057530
 
+Type erasure:
+http://davekilian.com/cpp-type-erasure.html
+
+std::function:  
+https://stackoverflow.com/questions/14936539/how-stdfunction-works  
+https://shaharmike.com/cpp/naive-std-function/
+
+