GNU Free Documentation License, Version 1.2

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()`

-- 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`

.

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`

.

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`

.

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.

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.