Typecasting is pretending that a variable of one type is actually of another
type (this isn't a formal definition). For example, a boolean
variable could be typecast to a byte, which would allow the data
in the variable to be treated as a number. You might want to do this if you
wanted to add 1 to a number if a boolean variable is true, in which case you
can just add the byte version of the variable.
To typecast a variable in Pascal, use the type you want to typecast to as if
it were a function e.g. byte(a_boolean_var) returns the data in
the variable a_boolean_var as if it were a byte.
Turbo Pascal won't let you just typecast anything to anything; it has some
rules, one of which is that the data type and the data must have the data
size. It will also sometimes do some automatic conversion, for example if
you typecast an integer to a longint, it will
actually convert the integer into a longint.
Typecasting is only useful if you know something about the way different
types are stored internally. For example, a boolean is a byte
that contains 1 for true or 0 for false, or a char is a byte
containing the ASCII code of the character. Usually you use typecasting to
access the data in a variable in a different way.
A good example is typecasting an integer to a set. A set with the range 0 to
15 (note that it must start at 0 because of the way Turbo Pascal deals with
sets) is a 2 byte integer with each bit representing one element's presence
or absence. Thus if an integer is put in a loop from 0 to 65535 (use a
word, because 65535 is too big for an integer) and
typecast to a set of this type inside the loop, then the set will loop
through all subsets of the set 0..15!
Sometimes Turbo Pascal doesn't allow you to do a typecast that it thinks
doesn't make sense. If you know what you are doing, you can work around it
with the absolute keyword. When declaring a variable followed by
this keyword and the name of another variable, the compiler will make the
two variables point to the same block of memory if it can. For example:
var aset : set of 0..15; aword : word absolute aset;
Here the two variables use the same two bytes of memory.
Last updated Sun Nov 28 22:04:38.0000000000 2004. Copyright Bruce Merry (bmerry '@' gmail dot. com).