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| <title>The C Preprocessor: Obsolete Features</title> |
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| Next: <a href="Differences-from-previous-versions.html#Differences-from-previous-versions" accesskey="n" rel="next">Differences from previous versions</a>, Previous: <a href="Implementation-limits.html#Implementation-limits" accesskey="p" rel="prev">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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| <hr> |
| <a name="Obsolete-Features-1"></a> |
| <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. |
| </p> |
| <a name="Assertions"></a> |
| <h4 class="subsection">11.3.1 Assertions</h4> |
| <a name="index-assertions"></a> |
| |
| <p><em>Assertions</em> 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> |
| <p>Assertions were intended to provide a more systematic way to describe |
| the compiler’s target system and we added them for compatibility with |
| existing compilers. 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"></a> |
| <p>An assertion looks like this: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#<var>predicate</var> (<var>answer</var>) |
| </pre></div> |
| |
| <p><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"></a> |
| <p>To test an assertion, you write it in an ‘<samp>#if</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>. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#if #machine (vax) || #machine (ns16000) |
| </pre></div> |
| |
| <p>You can test whether <em>any</em> answer is asserted for a predicate by |
| omitting the answer in the conditional: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#if #machine |
| </pre></div> |
| |
| <a name="index-_0023assert"></a> |
| <p>Assertions are made with the ‘<samp>#assert</samp>’ directive. Its sole |
| argument is the assertion to make, without the leading ‘<samp>#</samp>’ that |
| identifies assertions in conditionals. |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#assert <var>predicate</var> (<var>answer</var>) |
| </pre></div> |
| |
| <p>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"></a> |
| <a name="index-_0023unassert"></a> |
| <p>Assertions can be canceled with the ‘<samp>#unassert</samp>’ directive. It |
| has the same syntax as ‘<samp>#assert</samp>’. In that form it cancels only the |
| answer which was specified on the ‘<samp>#unassert</samp>’ line; other answers |
| for that predicate remain true. You can cancel an entire predicate by |
| leaving out the answer: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#unassert <var>predicate</var> |
| </pre></div> |
| |
| <p>In either form, if no such assertion has been made, ‘<samp>#unassert</samp>’ has |
| no effect. |
| </p> |
| <p>You can also make or cancel assertions using command line options. |
| See <a href="Invocation.html#Invocation">Invocation</a>. |
| </p> |
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| Next: <a href="Differences-from-previous-versions.html#Differences-from-previous-versions" accesskey="n" rel="next">Differences from previous versions</a>, Previous: <a href="Implementation-limits.html#Implementation-limits" accesskey="p" rel="prev">Implementation limits</a>, Up: <a href="Implementation-Details.html#Implementation-Details" accesskey="u" rel="up">Implementation Details</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
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