The class called bool3 implements a three-valued boolean: the
possible values represent the notions false, uncertain and
true. A variable of type bool3 has a default initial value of
uncertain. To avoid problems with reserved words the three truth
values are actually called:
true3 |
false3 |
uncertain3 |
bool3(true) -- is the same as true3
bool3(false) -- is the same as false3
bool3() -- is the same as uncertain3
To convert a normal bool to a bool3 value, you must call
the ctor explicitly.
Nevertheless, a variable of type bool3 may be assigned a C++
bool value (e.g. bool3 b3 = true;) in which case true
maps to true3 and false to false3.
There are three functions for testing the value of a bool3 expression:
(note that these functions return a C++ bool value)
IsTrue3(bool3expr) -- true iff expr is true3
IsFalse3(bool3expr) -- true iff expr is false3
IsUncertain3(bool3expr) -- true iff expr is uncertain3
These functions are the only way of converting a bool3 to a
standard C++ bool value -- there is no automatic conversion from a
bool3 value to a standard C++ bool.
There are no arithmetic operations on bool3 values.
bool3 values may be printed in the usual way. The printed forms are:
true3 false3 uncertain3.
Note that bool3 is not the same as BOOST's tribool, though the
two are fairly similar. The principal differences are that bool3
does not have automatic conversion to bool, and there are no
logical operations on bool3 whereas tribool does have some.
The implementation is very simple. The only point to watch is that the
order of the constants in the enum Bool3TruthValues was chosen to allow a
simple implementation of the function cmp (which is currently removed
from bool3.H, see Bugs and Shortcomings below). If you change the
order, you will have to change the definition of cmp.
All functions/operations are implemented inline except for I/O. I have avoided const-ref arguments since it is surely cheaper simply to copy the enum value.
I made the bool3 ctor from bool explicit; if conversion from
bool to bool3 is automatic then machine integer value match
bool3 as well as they match MachineInt -- be careful.
I do feel quite uneasy about disagreeing with BOOST's tribool design, but
their example of a three-way if statement looks to me to be a recipe for
programmer grief -- one has to suppress the law of the excluded middle to
read their code without finding it odd and surprising.
Boolean arithmetic operations are not defined since we have not needed them so far. It would be a simple matter, but I prefer to wait until there is a real need for such operations.
Is the cmp function ever going to be useful???
There was also a function cmp for comparing two bool3 values:
cmp(b1, b2) returns an int <0, =0 or >0 according as b1 <,=,> b2
(assuming this ordering: false3 < uncertain3 < true3)
> friend int cmp(bool3 lhs, bool3 rhs); // must be friend function
> inline int cmp(bool3 lhs, bool3 rhs)
> {
> return lhs.myTruthValue - rhs.myTruthValue;
> }