bool3

User documentation for bool3

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

Examples

ex-bool3.C

Constructors

bool3() -- is the same as uncertain3
bool3(true) -- is the same as true3
bool3(false) -- is the same as false3

To convert a normal bool to a bool3 value, you must call the ctor explicitly.

For convenience, 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.

Queries

There are three functions for testing the value of a bool3 expression: (note that these functions return a C++ bool value)

IsTrue3(expr) -- true iff expr is true3
IsFalse3(expr) -- true iff expr is false3
IsUncertain3(expr) -- 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.

Operations on bool3

There are no arithmetic operations on bool3 values. There is also no equality test.

bool3 values may be printed in the usual way. The printed forms are: true3 and false3 and uncertain3.

Maintainer documentation for bool3

The implementation is very simple. The only point to watch is that the order of the constants in the enum TruthValueSet was chosen to allow a simple implementation of the function cmp (which is currently removed; 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.

Bugs, Shortcomings and other ideas

I made the bool3 ctor from bool explicit; if conversion from bool to bool3 is automatic then machine integer values match bool3 as well as they match MachineInt -- be careful.

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.

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;
>   }

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.