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cbooleanstate

Three-state Boolean

What is a good way to create a three-state Boolean in a C-based language?

Solution

  • While others have offered answers, I'd like to offer a justification for the best one.

    Use integer values -1/0/1 (or really, any negative/zero/positive).

    With this scheme, there is an extremely efficient check for any subset of possible values:

    (x<0)  /*   {-1} */
(!x)   /*    {0} */
(x>0)  /*    {1} */
(x<=0) /* {-1,0} */
(x)    /* {-1,1} */
(x>=0) /*  {0,1} */

    On x86, all of these will compile to a single test opcode followed by a conditional jump opcode with the appropriate conditions.

    If you want to hide the implementation, you can use predicate macros for testing each of the 6 subsets.