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<title>Issue 294: User defined macros and standard headers</title>
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<p><em>This page is a snapshot from the LWG issues list, see the <a href="lwg-active.html">Library Active Issues List</a> for more information and the meaning of <a href="lwg-active.html#CD1">CD1</a> status.</em></p>
<h3 id="294"><a href="lwg-defects.html#294">294</a>. User defined macros and standard headers</h3>
<p><b>Section:</b> 16.4.5.3.3 <a href="https://wg21.link/macro.names">[macro.names]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> James Kanze <b>Opened:</b> 2001-01-11 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View other</b> <a href="lwg-index-open.html#macro.names">active issues</a> in [macro.names].</p>
<p><b>View all other</b> <a href="lwg-index.html#macro.names">issues</a> in [macro.names].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>Paragraph 2 of 16.4.5.3.3 <a href="https://wg21.link/macro.names">[macro.names]</a> reads: &quot;A
translation unit that includes a header shall not contain any macros
that define names declared in that header.&quot; As I read this, it
would mean that the following program is legal:</p>

<pre>
  #define npos 3.14
  #include &lt;sstream&gt;
</pre>

<p>since npos is not defined in &lt;sstream&gt;.  It is, however, defined
in &lt;string&gt;, and it is hard to imagine an implementation in
which &lt;sstream&gt; didn't include &lt;string&gt;.</p>

<p>I think that this phrase was probably formulated before it was
decided that a standard header may freely include other standard
headers.  The phrase would be perfectly appropriate for C, for
example.  In light of 16.4.6.2 <a href="https://wg21.link/res.on.headers">[res.on.headers]</a> paragraph 1, however,
it isn't stringent enough.</p>


<p id="res-294"><b>Proposed resolution:</b></p>
<p>For 16.4.5.3.3 <a href="https://wg21.link/macro.names">[macro.names]</a>, replace the current wording, which reads:</p>
<blockquote>
     <p>Each name defined as a macro in a header is reserved to the
     implementation for any use if the translation unit includes
     the header.168)</p>

     <p>A translation unit that includes a header shall not contain any
     macros that define names declared or defined in that header. Nor shall
     such a translation unit define macros for names lexically
     identical to keywords.</p>

     <p>168) It is not permissible to remove a library macro definition by
     using the #undef directive.</p>
</blockquote>

<p>with the wording:</p>

<blockquote>
     <p>A translation unit that includes a standard library header shall not
     #define or #undef names declared in any standard library header.</p>

     <p>A translation unit shall not #define or #undef names lexically
     identical to keywords.</p>
</blockquote>

<p><i>[Lillehammer: Beman provided new wording]</i></p>






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