7.3 Parameter transfer and return value

For a class instantiation, an object is created and for a method invocation, a method object is created.

Then the actual parameters of the invocation/instantiation are evaluated and transferred to the corresponding data-items of the method object/object. When the method has been executed, a possible value may be returned.

Parameter transfer

The transfer of parameters corresponds to assignment of the actual parameters to the formal parameters. Consider the following method invocation:

 transfer(account_1010,account_1022, 218)

The invocation consists of the following steps:

A transfer-object is generated – assume that aTransfer refers to this object. The situation is then as shown in the snapshot where the reference variables from and to have the initial value none and amount has the initial value 0 (zero).

2. The actual parameters are then assigned to the corresponding data-items in aTransfer:

aTransfer.from := account_1010aTransfer.to := account_1022aTransfer.amount := 218

The situation is then as shown in the snapshot where the from refers to account_1010, to refers to account_1022, and amount has the value 218.

3. The statements of aTransfer are executed:

from.withdraw(amount)
to.deposit(amount)

The situation is then as shown in the snapshot.

In the example, we have two types of parameters:

Amount is a a value parameter of type integer. A value type is copied to the data-item of the method object.

From and to are reference parameters. For a reference parameter the reference is assigned to the data-item in the method object.

For an elaboration of the difference between value and object and thus value assign and reference assign, see section .

There is a third kind of parameter being virtual methods and/or virtual classes. In section . we saw an example of a virtual class as a parameter:class ElmType as a virtual class parameter of class Set. In section , we show examples of virtual method parameters.

Parameter transfer for a class instantiation, takes place in a similar way.

Return value

As mentioned, a method invocation may return a value being computed by the method. This is e.g. the case for the withdraw method of Account:

class Account:
   ...
   withdraw(amount: var float) -> newB: var float:
      balance := balance - amount
      newB := balance

The value to be returned is defined by the clause -> newB: var float. It defines a data-item newB, which holds the value to be returned. In withdraw, the statement newB := balance assigns a value to newB.

The return value of withdraw may e.g. be used in as shown here:

anAmount := anAccount.withdraw(300)