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(cpp-300.info)Assertions

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Assertions
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   "Assertions" are a deprecated alternative to macros in writing
conditionals to test what sort of computer or system the compiled
program will run on.  Assertions are usually predefined, but you can
define them with preprocessing directives or command-line options.

   Assertions were intended to provide a more systematic way to describe
the compiler's target system.  However, in practice they are just as
unpredictable as the system-specific predefined macros.  In addition,
they are not part of any standard, and only a few compilers support
them.  Therefore, the use of assertions is *less* portable than the use
of system-specific predefined macros.  We recommend you do not use them
at all.

   An assertion looks like this:

     #PREDICATE (ANSWER)

PREDICATE must be a single identifier.  ANSWER can be any sequence of
tokens; all characters are significant except for leading and trailing
whitespace, and differences in internal whitespace sequences are
ignored.  (This is similar to the rules governing macro redefinition.)
Thus, `(x + y)' is different from `(x+y)' but equivalent to
`( x + y )'.  Parentheses do not nest inside an answer.

   To test an assertion, you write it in an `#if'.  For example, this
conditional succeeds if either `vax' or `ns16000' has been asserted as
an answer for `machine'.

     #if #machine (vax) || #machine (ns16000)

You can test whether _any_ answer is asserted for a predicate by
omitting the answer in the conditional:

     #if #machine

   Assertions are made with the `#assert' directive.  Its sole argument
is the assertion to make, without the leading `#' that identifies
assertions in conditionals.

     #assert PREDICATE (ANSWER)

You may make several assertions with the same predicate and different
answers.  Subsequent assertions do not override previous ones for the
same predicate.  All the answers for any given predicate are
simultaneously true.

   Assertions can be cancelled with the `#unassert' directive.  It has
the same syntax as `#assert'.  In that form it cancels only the answer
which was specified on the `#unassert' line; other answers for that
predicate remain true.  You can cancel an entire predicate by leaving
out the answer:

     #unassert PREDICATE

In either form, if no such assertion has been made, `#unassert' has no
effect.

   You can also make or cancel assertions using command line options.
Note: Invocation.