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

1. A transfer-object is generated – assume that aTransfer refers to this object.
2. The actual parameters are then assigned to the corresponding data-items in aTransfer.
   • aTransfer.from := account_1010
   • aTransfer.to := account_1022
   • aTransfer.amount := 218
3. The statements of aTransfer are executed

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.

Perhaps an OSD here?

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. +++ do we need to spell this out in an example?

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)