Slot Assignment

Assignment to Inherited Slots

If you cannot write to an inherited slot with an assignment, what does happen? Pretty much what you would expect--a new slot is created. The new slot is assigned the value and is placed in the inheriting frame. Thus, the easy example of copyCat and original and this assignment:

copyCat.sharedSlot4 := 69;

creates a new slot in copyCat with the value of 69 (see FIGURE 4.5).

FIGURE 4.5 : Assignment to an inherited slot creates a new slot.

Now, let us examine a more interesting example. Here we have a frame account, and two others frames, savingsAccount and checkingAccount, that proto from it:

account:= {
   total: 0,
   Deposit: func(amount) 
            begin
               total := total + amount;
            end,
   Withdraw:func(amount)
            begin
               total := total - amount;
            end,
   Balance: func() 
               return total,
};

savingsAccount := {
   _proto: account,
};

checkingAccount := {
   _proto: account,
};

We will send some messages to savingsAccount and checkingAccount and see what happens to the slots in our three frames:

savingsAccount:Deposit(2000);
checkingAccount:Deposit(500);
savingsAccount:Withdraw(200);

Before sending the messages, savingsAccount and checkingAccount only have one slot each, the _proto slot.

When the Deposit message is sent to savingsAccount:

savingsAccount:Deposit(2000);

the runtime system looks for the Deposit method within the savingsAccount frame. Since it isn't there, it follows the proto pointer to account. It finds the Deposit method there and executes it.

While Deposit is executing, the value of self is savingsAccount (self is the frame that receives the message). When Deposit attempts to access the current value of total, lookup begins. total is looked for first as a local variable, then using inheritance rules. Inheritance lookup starts with self and then follows the proto chain. total is found in account, with a value of 0.

When the Deposit method:

Deposit:func(amount)

begin

  total := total + amount;
end;

tries to assign to total, the lookup takes place again--starting first in self and then following the proto chain. total is found in account.

Notice that total in account is not changed; instead, a new total slot with a new value is created in self (savingsAccount). This overrides the total slot in account.

When the Deposit method finishes, savingsAccount contains two slots, a _proto and a total slot. After you execute the next message to checking- Account, the same process occurs all over again. When you have finished, checkingAccount will also have two slots, a _proto slot and a total slot. When you execute the third message:

savingsAccount:Withdraw(200);

savingsAccount will have no new slots, but the value of its total slot will now be 1800.

An online version of Programming for the Newton using Macintosh, 2nd ed. Š1996, 1994, Julie McKeehan and Neil Rhodes.

Last modified: 1 DEC 1996