Card Objects

By now we have seen several examples of composition (the ability to combine language features in a variety of arrangements). One of the first examples we saw was using a function invocation as part of an expression. Another example is the nested structure of statements: you can put an if statement within a while loop, or within another if statement, etc.

Having seen this pattern, and having learned about vectors and objects, you should not be surprised to learn that you can have vectors of objects. In fact, you can also have objects that contain vectors (as instance variables); you can have vectors that contain vectors; you can have objects that contain objects, and so on.

In the next two chapters we will look at some examples of these combinations, using Card objects as a case study.

If you are not familiar with common playing cards, now would be a good time to get a deck, or else this chapter might not make much sense. There are 52 cards in a deck, each of which belongs to one of four suits and one of 13 ranks. The suits are Spades, Hearts, Diamonds and Clubs (in descending order in Bridge). The ranks are Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen and King. Depending on what game you are playing, the rank of the Ace may be higher than King or lower than 2.

If we want to define a new object to represent a playing card, it is pretty obvious what the instance variables should be: rank and suit. It is not as obvious what type the instance variables should be. One possibility is pstrings, containing things like "Spade" for suits and "Queen" for ranks. One problem with this implementation is that it would not be easy to compare cards to see which had higher rank or suit.

An alternative is to use integers to encode the ranks and suits. By "encode," I do not mean what some people think, which is to encrypt, or translate into a secret code. What a computer scientist means by "encode" is something like "define a mapping between a sequence of numbers and the things I want to represent." For example,

The symbol -> is mathematical notation for "maps to." The obvious feature of this mapping is that the suits map to integers in order, so we can compare suits by comparing integers. The mapping for ranks is fairly obvious; each of the numerical ranks maps to the corresponding integer, and for face cards:

The reason I am using mathematical notation for these mappings is that they are not part of the C++ program. They are part of the program design, but they never appear explicitly in the code. The class definition for the Card type looks like this:

There are two constructors for Cards. You can tell that they are constructors because they have no return type and their name is the same as the name of the structure. The first constructor takes no arguments and initializes the instance variables to a useless value (the zero of clubs).

The second constructor is more useful. It takes two parameters, the suit and rank of the card.

The following code creates an object named threeOfClubs that represents the 3 of Clubs:

The first argument, 0 represents the suit Clubs, the second, naturally, represents the rank 3.