Hello Christopher,

CE> I thought the undefined behaviour happens only if the path
CE> is actually taken.

Undefined  behaviour  term  applies  to  compilation  as  well  as  to
execution  of  the  program; refusal to compile is one of the possible
options of handling it.

CE>  So for the function f above, the compiler
CE> would have to proof that this function is *ever* called with
CE> a value of x other than 1 or 2, to legally refuse compiling.

Compilation  of the function doesn't depend upon the use of it (BTW it
can  be  called  by the code that is not even written by the moment of
the compilation, so such proof is impossible).

CE> Assume f is really intended to take a boolean value,
CE> and only takes int to accomodate an interface that
CE> cannot be changed. Then f would never be called with
CE> anything other than 0 and 1, and the behaviour would
CE> be well defined.

The compiler can't deduce this from the function definition.

Best regards,
 Roman                            mailto:roman@nstl.nnov.ru




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