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| <h3 class="section">11.3 Obsolete Features</h3> |
| |
| <p>CPP has some features which are present mainly for compatibility with |
| older programs. We discourage their use in new code. In some cases, |
| we plan to remove the feature in a future version of GCC. |
| |
| <h4 class="subsection">11.3.1 Assertions</h4> |
| |
| <p><a name="index-assertions-111"></a> |
| <dfn>Assertions</dfn> 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. |
| |
| <p>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 <strong>less</strong> portable than the use |
| of system-specific predefined macros. We recommend you do not use them at |
| all. |
| |
| <p><a name="index-predicates-112"></a>An assertion looks like this: |
| |
| <pre class="smallexample"> #<var>predicate</var> (<var>answer</var>) |
| </pre> |
| <p class="noindent"><var>predicate</var> must be a single identifier. <var>answer</var> 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, <code>(x + y)</code> is different from <code>(x+y)</code> but |
| equivalent to <code>( x + y )<!-- /@w --></code>. Parentheses do not nest inside an |
| answer. |
| |
| <p><a name="index-testing-predicates-113"></a>To test an assertion, you write it in an ‘<samp><span class="samp">#if</span></samp>’. For example, this |
| conditional succeeds if either <code>vax</code> or <code>ns16000</code> has been |
| asserted as an answer for <code>machine</code>. |
| |
| <pre class="smallexample"> #if #machine (vax) || #machine (ns16000) |
| </pre> |
| <p class="noindent">You can test whether <em>any</em> answer is asserted for a predicate by |
| omitting the answer in the conditional: |
| |
| <pre class="smallexample"> #if #machine |
| </pre> |
| <p><a name="index-g_t_0023assert-114"></a>Assertions are made with the ‘<samp><span class="samp">#assert</span></samp>’ directive. Its sole |
| argument is the assertion to make, without the leading ‘<samp><span class="samp">#</span></samp>’ that |
| identifies assertions in conditionals. |
| |
| <pre class="smallexample"> #assert <var>predicate</var> (<var>answer</var>) |
| </pre> |
| <p class="noindent">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. |
| |
| <p><a name="index-assertions_002c-canceling-115"></a><a name="index-g_t_0023unassert-116"></a>Assertions can be canceled with the ‘<samp><span class="samp">#unassert</span></samp>’ directive. It |
| has the same syntax as ‘<samp><span class="samp">#assert</span></samp>’. In that form it cancels only the |
| answer which was specified on the ‘<samp><span class="samp">#unassert</span></samp>’ line; other answers |
| for that predicate remain true. You can cancel an entire predicate by |
| leaving out the answer: |
| |
| <pre class="smallexample"> #unassert <var>predicate</var> |
| </pre> |
| <p class="noindent">In either form, if no such assertion has been made, ‘<samp><span class="samp">#unassert</span></samp>’ has |
| no effect. |
| |
| <p>You can also make or cancel assertions using command line options. |
| See <a href="Invocation.html#Invocation">Invocation</a>. |
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