Classy Cars<\/h2>\n\n\n\n
Start with a base class for an automobile with a color property.<\/p>\n\n\n\n
class Automobile;\n string color;\nendclass<\/pre>\n\n\n\nNow extend this to make a Pickup class with a bed in the back to carry big things.<\/p>\n\n\n\n
class Pickup extends Automobile;\n int bed_size;\nendclass<\/pre>\n\n\n\nA place for everything<\/h2>\n\n\n\n
As you create Automobile and Pickup objects, you need a place to hold them. How about a parking lot? This class has an array of Automobile class variables. This can hold handles to base Automobiles and derived Pickups, just like a real parking lot. Include methods to park and retrieve vehicles.<\/p>\n\n\n\n
class ParkingLot;\n Automobile spaces[4];\n\n function void park(Automobile a, int spot);\n spaces[spot] = a;\n endfunction\n\n function Automobile retrieve(int spot);\n return spaces[spot];\n endfunction\nendclass<\/pre>\n\n\n\nLet\u2019s try this out! Declare some class variables, construct the parking lot, then an Automobile object, park it, then retrieve it.<\/p>\n\n\n\n
Automobile a1, a2;\nParkingLot lot;\n\ninitial begin\n lot = new(); \/\/ Construct the parking lot\n a1 = new(); \/\/ Construct a blue Automobile\n a1.color = \"blue\";\n lot.park(a1, 0); \/\/ Park it in spot 0\n\n \/\/ Run some errands, then come back to get your car\n\na2 = lot.retrieve(0);<\/code>\/\/ Retrieve automobile with different var<\/code>iable\n$display(\"Found automobile %p\", a2.color);<\/code>\nend<\/pre>\n\n\n\nTry this code on your favorite simulator. You should see that the blue car was parked in space 0, and was successfully retrieved.<\/p>\n\n\n\n
Parking lot with an Automobile<\/figcaption><\/figure>\n\n\n\n Dude, where is my car?<\/h2>\n\n\n\n
Construct a red pickup and park it in spot 3.<\/p>\n\n\n\n
Pickup p1, p2;\ninitial begin\n p1 = new();\n p1.color = \"red\";\n lot.park(p1, 3);\nend<\/pre>\n\n\n\nParking lot with an Automobile and a Pickup<\/figcaption><\/figure>\n\n\n\n Wait, is that legal? The park() method has the formal argument a that is an Automobile class variable. You just tried to assign it a Pickup handle. The previous post showed that a base class variable can hold a handle to base and derived objects.<\/p>\n\n\n\n
Base and derived classes and their handles<\/figcaption><\/figure>\n\n\n\n OK, you\u2019ve now parked the Pickup object in space 3. Can you get it back? Use a different class variable to make sure the assignment worked.<\/p>\n\n\n\n
p2 = lot.retreive(3); \/\/ Won\u2019t compile<\/pre>\n\n\n\nWhen you compile this code, you get an error that you can\u2019t assign to a Pickup variable from an Automobile variable. This is because retrieve() returns an Automobile variable. The compiler does not know if that variable refers to an Automobile or Pickup object.<\/p>\n\n\n\n
Think about this in the real world. Spaces in a parking lot can hold cars, pickups, motorcycles, or might be empty. If you parked your pickup in space 3, you expect to find your vehicle there \u2013 the space better have the same type! But you won’t know this until you retrieve it at run time.<\/p>\n\n\n\n
![\"Parking](\"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2021\/08\/5_3_lot_auto.png\")
![\"Parking](\"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2021\/08\/5_4_lot_pickup.png\")
![\"Base](\"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2021\/07\/4_1_venn-1024x428.png\")
![\"\"](\"https:\/\/blogs.sw.siemens.com\/wp-content\/uploads\/sites\/54\/2021\/08\/5_2_cast.png\")
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