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diff --git a/CM2005 Object Oriented Programming/Topic 2/2.3.9/main.cpp b/CM2005 Object Oriented Programming/Topic 2/2.3.9/main.cpp
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+#include <iostream>
+#include <string>
+#include <vector>
+
+enum class OrderBookType
+{
+  bid,
+  ask
+};
+
+// We add a constructor with an initialization list :
+// This is mor effecient because it doesn't copy but assigns the data right away.
+class OrderBookEntry
+{
+  public:
+    OrderBookEntry
+    (
+      double price,
+      double amount,
+      std::string timestamp,
+      std::string product,
+      OrderBookType orderType
+    )
+    : price{price},
+      amount(amount),
+      timestamp(timestamp),
+      product(product),
+      orderType(orderType)
+      // can use parenthesis or curly braces
+    {
+      
+    }
+    double price;
+    double amount;
+    std::string timestamp;
+    std::string product;
+    OrderBookType orderType;
+};
+
+int main()
+{
+
+  std::vector<OrderBookEntry> orders;
+
+
+  OrderBookEntry order1{  10000,
+                          0.002,
+                          "2020/03/17 17:01:24.884492",
+                          "BTC/USDT",
+                          OrderBookType::bid};
+  orders.push_back(order1);
+  orders.push_back(order1);
+  std::cout << orders[0].price << std::endl;
+  std::cout << orders[1].price << std::endl;
+}
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