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<title>Issue 2428: "External declaration" used without being defined</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#C++17">C++17</a> status.</em></p>
<h3 id="2428"><a href="lwg-defects.html#2428">2428</a>. "External declaration" used without being defined</h3>
<p><b>Section:</b> 16.4.3.2 <a href="https://wg21.link/using.headers">[using.headers]</a> <b>Status:</b> <a href="lwg-active.html#C++17">C++17</a>
 <b>Submitter:</b> Tim Song <b>Opened:</b> 2014-09-03 <b>Last modified:</b> 2017-07-30</p>
<p><b>Priority: </b>0
</p>
<p><b>View all other</b> <a href="lwg-index.html#using.headers">issues</a> in [using.headers].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#C++17">C++17</a> status.</p>
<p><b>Discussion:</b></p>
<p>
16.4.3.2 <a href="https://wg21.link/using.headers">[using.headers]</a>/3 says
</p>
<blockquote><p>
A translation unit shall include a header only outside of any external declaration or definition [&hellip;]
</p></blockquote>
<p>
This wording appears to be borrowed from the C standard. However, the term "external declaration" is not defined 
in the C++ standard, and in fact is only used here as far as I can tell, so it is unclear what it means. The C 
standard does define external declarations as (WG14 N1570 6.9 External definitions/4-5):
</p>
<blockquote><p>
As discussed in 5.1.1.1, the unit of program text after preprocessing is a translation unit, which consists of a 
sequence of external declarations. These are described as "external" because they appear outside any function 
(and hence have file scope). [...] An external definition is an external declaration that is also a definition of 
a function (other than an inline definition) or an object.
</p></blockquote>
<p>
The corresponding description of a translation unit in C++ is "A translation unit consists of a sequence of declarations." 
(6.7 <a href="https://wg21.link/basic.link">[basic.link]</a>/3).
<p/>
So it appears that the C++ counterpart of "external declaration" in C is simply a "declaration" at file scope. There 
is no need to specifically limit the statement in 16.4.3.2 <a href="https://wg21.link/using.headers">[using.headers]</a>/3 to file-scope declarations, however, 
since every non-file-scope declaration is necessarily inside a file-scope declaration, so banning including a header 
inside file-scope declarations necessarily bans including one inside non-file-scope declarations as well.
</p>

<p><i>[Urbana 2014-11-07: Move to Ready]</i></p>




<p id="res-2428"><b>Proposed resolution:</b></p>
<p>
This wording is relative to N3936.
</p>
<ol>
<li><p>Edit 16.4.3.2 <a href="https://wg21.link/using.headers">[using.headers]</a> as indicated:</p>

<blockquote>
<p>
A translation unit shall include a header only outside of any <del>external</del> declaration or definition, and shall
include the header lexically before the first reference in that translation unit to any of the entities declared
in that header. No diagnostic is required.
</p>
</blockquote>
</li>
</ol>





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