Compile time range checking/asserts
Borden Rhodes
jrvp at bordenrhodes.com
Tue Sep 14 22:42:07 PDT 2010
Good morning, list,
I know that D has support for ranges in for-each statements and in array
bounds checking, but I'm curious if it also has a facility for compile-time
range checking or assertions on individual variables.
For example, using the Java Tutorial's venerable Bicycle class, say I
had a property <code>int gear;</code> which indicated the bicycle's current
gear. Let's say all bicycles of this class have 6 gears. Typically, I would
write a changeGear method as:
<code>
void changeGear(int newGear) {
if ( 1 <= newGear && newGear <= 6)
gear = newGear;
else
throw SomeKindOfException();
}
</code>
which would include the if statement to make sure that newGear would be passed
a number in the correct range and a throw statement to deal with cheaters.
Does D allow - or would there be logic in creating - an ability to define the
acceptable ranges when the variable is initialised? So, say I wanted gear
only to accept values between 1 and 6 inclusive. Could I declare something to
the effect of:
<code>int gear {1...6} = 1;</code>
Which could read, "create a new int, called gear, which only accepts values
between 1 and 6, inclusive, and initialise it to 1." Then, if I, or some
coder who forgot how many gears my Bicycle has, tried to call:
</code>bicycle.changeGear( 12 );</code>
the compiler would catch that <code>gear = newGear;</code> is assigning a
value out of range and throw a compile-time error. Can D do this? Should it
do this?
With thanks,
Borden
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