What can we use to specify a byref parameter? Because a byref parameter is an alias of a column, it makes sense that only objects that represent columns (like variables and byval parameters) can be used to specify a byref parameter.
Let us consider the following example:
This produces the following trace.
A | B | C | D | E | F | |
| 1 | line# | |||||
| 2 | 10 | retline# | i | |||
| 3 | post | ? | ||||
| 4 | 7 | retline# | x | |||
| 5 | 8 | aka col C | ||||
|
“i” is a normal argument to parameter “x”. “i” is local
variable name that specifies column D. As a result, it
makes sense that parameter “x” becomes an alias of
column D.
| ||||||
| 6 | 3 | 0 | ||||
| 7 | 4 | retline# | x | |||
| 8 | 8 | expr | retline# | x | ||
|
Things get a little interesting here. The expression 20 ×
(i + 2) ↔ x does not specify a column. Instead, the
expression specifies a value that can only be determined
when line 8 executes. In theory, the value of an expression
does not have a column, and therefore it cannot be
an argument to a byref parameter. However, most
programming languages creates a column so that the
expression can be passed as an argument to a byref
parameter. Note that column D does not have a name
that the pseudocode in subroutine main can refer to (it
is anonymous).
| ||||||
| 9 | 40 | 9 | aka col D | |||
|
In this case, column D is allocated to store the result
of the expression 20 × (i + 2). This column is allocated
on-the-fly as the invoke statement executes. As a result,
parameter “x” can now be an alias to column D.
| ||||||
| 10 | 3 | 0 | ||||
|
When line 3 executes, whatever column parameter “x”
refers to is reset to zero. This time, it is column D.
This does not mean that 20 × (i + 2) = 0. It simply
overwrites the column that was used to store the value
of the expression to zero.
| ||||||
| 11 | 4 | expr | retline# | x | ||
|
When we return from “reset”, all three columns are
deallocated, including the one that was allocated to store
the result of 20×(i+2). This means that column D, now
reset to zero, is never accessible to subroutine “main”.
| ||||||
| 12 | 9 | retline# | i | |||
| 13 | post | |||||