Update implemented toString function

CM2005 Object Oriented Programming/Topic 5/5.1.9/Wallet.cpp

@@ -36,5 +36,12 @@ bool Wallet::containsCurrency(std::string type, double amount)
 
 std::string Wallet::toString()
 {
-  return "Oink!";
+  std::string s;
+  for(std::pair<std::string, double> pair : currencies)
+  {
+    std::string currency = pair.first;
+    double amount = pair.second;
+    s += currency + " : " + std::to_string(amount) + "\n";
+  }
+  return s;
 }