refactor section 15.8

ch15/README.md

@@ -283,3 +283,85 @@ The reason is that a constructor taking 4 parameters has been defined, which pre
 > Redefine your `Bulk_quote` class to inherit its constructors.
 
 [code](ex15_27_Bulk_quote.h)
+
+## Exercise 15.28:
+> Define a `vector` to hold `Quote` objects but put `Bulk_quote` objects into that `vector`. Compute the total `net_price` of all the elements in the `vector`.
+
+```cpp
+#include <vector>
+#include <numeric>
+#include "ex15_27_Bulk_quote.h"
+
+int main()
+{
+    std::vector<Quote> vecQuote;
+
+    Bulk_quote bulk0("0-201-82470-1", 50, 5, 0.2);
+    Bulk_quote bulk1("0-201-82470-1", 50, 3, 0.5);
+
+    // total price should be:
+    std::cout << "bulk_quote's total: " << bulk0.net_price(5) + bulk1.net_price(5) << std::endl;
+
+    vecQuote.push_back(bulk0);
+    vecQuote.push_back(bulk1);
+
+    double total = std::accumulate(vecQuote.cbegin(), vecQuote.cend(),0.0, [](double ret, const Quote &obj){
+        return ret += obj.net_price(5);
+    });
+
+    // total price in the vector.
+    std::cout << "total in the vector: " << total << std::endl;
+}
+```
+
+**Output:**
+
+```
+bulk_quote's total: 325
+total in the vector: 500
+```
+
+## Exercise 15.29:
+> Repeat your program, but this time store `shared_ptrs` to objects of type `Quote`. Explain any discrepancy in the sum generated by the this version and the previous program. If there is no discrepancy, explain why there isn’t one.
+
+```cpp
+#include <vector>
+#include <numeric>
+#include <memory>
+
+#include "ex15_27_Bulk_quote.h"
+
+int main()
+{
+    std::vector<std::shared_ptr<Quote>> vecQuote;
+
+    std::shared_ptr<Bulk_quote> spBulk0 = std::make_shared<Bulk_quote>("0-201-82470-1", 50, 5, 0.2);
+    std::shared_ptr<Bulk_quote> spBulk1 = std::make_shared<Bulk_quote>("0-201-82470-1", 50, 3, 0.5);
+
+    // total price should be:
+    std::cout << "bulk_quote's total: " << spBulk0->net_price(5) + spBulk1->net_price(5) << std::endl;
+
+    vecQuote.push_back(spBulk0);
+    vecQuote.push_back(spBulk1);
+
+    double total = std::accumulate(vecQuote.cbegin(), vecQuote.cend(),0.0, [](double ret, std::shared_ptr<Quote> sp){
+        return ret += sp->net_price(5);
+    });
+
+    // total price in the vector.
+    std::cout << "total in the vector: " << total << std::endl;
+}
+```
+
+**Output:**
+
+```
+bulk_quote's total: 325
+total in the vector: 325
+```
+
+Because derived objects are "sliced down" when assigned to a base-type object. Thus, when put `Bulk_quote` object into the `vector` of `Quote`, all the object "slice down" to `Quote`.
+
+If we want to holds objects related by inheritance, we should define the `vector` to hold pointers(preferable smart pointers) to the base class. So, the `vector` contains are dynamic type of the objects.
+
+Different from previous program, it can be found that 20% and 50% discount has been applied to the total price calculation.