Software Engineering for Students A Programming Approach

14.9 Parameter-passing mechanisms 189
Java provides the following scheme. All primitive data terms are passed by value, which
is most commendable, but all proper objects are passed by reference. No distinction is
made between procedures and functions. Thus a method of either type (procedure or
function) can modify any non-primitive parameter and it is left to the programmer to
enforce a discipline over changing parameters. A small concession is that the pointer to
an object cannot be changed, for example to point to another object.
Fortran employs only a single parameter passing mode: call by reference. Thus,
undesirably, all actual parameters in Fortran may potentially be changed by any subroutine
or function. The programmer is responsible for ensuring the safe implementation
of input and output parameters. Using call by reference, the location of the
actual parameter is bound to the formal parameter. The formal and actual parameter
names are thus aliases; modification of the formal parameter automatically modifies
the actual parameter. This is what you might expect of a language where arrays are
often used, and the performance hit of copying arrays is unacceptable. Fortran also,
unfortunately, restricts the type of result that may be returned from functions to
scalar types only (i.e. not arrays etc.).
Visual Basic.Net provides the choice of value or reference parameters, described by
the key words ByVal (the default) and ByRef in the method header. But when objects
are passed, they are always passed by reference.
In C#, by default, primitive data items are passed by value, objects are passed by reference.
But you can pass a primitive data item by reference if the parameter is preceded
by the key word ref in both the method header and the method call. You can also precede
an object name by ref, in which case you are passing a pointer to a pointer. This
means that the method can return an entirely different object.
Call by value-result is often used as an alternative to call by reference for input-output
parameters. It avoids the use of aliases at the expense of copying. Parameters passed by
value-result are initially treated as in call by value; a copy of the value of the actual parameter
is passed to the formal parameter, which again acts as a local variable. Manipulation
of the formal parameter does not immediately affect the actual parameter. On exit from
the procedure, the final value of the formal parameter is assigned into the actual parameter.
Call by result may be used as an alternative to call by reference for output parameters.
Parameters passed by value are treated exactly as those passed by value-result except that
no initial value is assigned to the local formal parameter.
Ada identifies three types of parameter:
■ input parameters to allow a method read-only access to an actual parameter. The
actual parameter is purely an input parameter; the method should not be able to
modify the value of the actual parameter
■ output parameters to allow a procedure write-only access to an actual parameter.
The actual parameter is purely an output parameter; the procedure should not be
able to read the value of the actual parameter
■ input-output parameters to allow a procedure read-and-write access to an actual
parameter. The value of the actual parameter may be modified by the procedure.
Ada only allows input variables to functions. The parameter-passing mechanisms used
in Ada (described as in, out and in out) would therefore seem to be ideal. However,