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<title>Issue 263: Severe restriction on basic_string reference counting</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="263"><a href="lwg-defects.html#263">263</a>. Severe restriction on <code>basic_string</code> reference counting</h3>
<p><b>Section:</b> 27.4.3 <a href="https://wg21.link/basic.string">[basic.string]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Kevlin Henney <b>Opened:</b> 2000-09-04 <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#basic.string">active issues</a> in [basic.string].</p>
<p><b>View all other</b> <a href="lwg-index.html#basic.string">issues</a> in [basic.string].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
The note in paragraph 6 suggests that the invalidation rules for
references, pointers, and iterators in paragraph 5 permit a reference-
counted implementation (actually, according to paragraph 6, they permit
a &quot;reference counted implementation&quot;, but this is a minor editorial fix).
</p>

<p>
However, the last sub-bullet is so worded as to make a reference-counted
implementation unviable. In the following example none of the
conditions for iterator invalidation are satisfied:
</p>

<pre>
    // first example: &quot;*******************&quot; should be printed twice
    string original = &quot;some arbitrary text&quot;, copy = original;
    const string &amp; alias = original;

    string::const_iterator i = alias.begin(), e = alias.end();
    for(string::iterator j = original.begin(); j != original.end(); ++j)
        *j = '*';
    while(i != e)
        cout &lt;&lt; *i++;
    cout &lt;&lt; endl;
    cout &lt;&lt; original &lt;&lt; endl;
</pre>

<p>
Similarly, in the following example:
</p>

<pre>
    // second example: &quot;some arbitrary text&quot; should be printed out
    string original = &quot;some arbitrary text&quot;, copy = original;
    const string &amp; alias = original;

    string::const_iterator i = alias.begin();
    original.begin();
    while(i != alias.end())
        cout &lt;&lt; *i++;
</pre>

<p>
I have tested this on three string implementations, two of which were
reference counted. The reference-counted implementations gave
&quot;surprising behavior&quot; because they invalidated iterators on
the first call to non-const begin since construction. The current
wording does not permit such invalidation because it does not take
into account the first call since construction, only the first call
since various member and non-member function calls.
</p>


<p id="res-263"><b>Proposed resolution:</b></p>
<p>
Change the following sentence in 21.3 paragraph 5 from
</p>

<blockquote><p>
    Subsequent to any of the above uses except the forms of insert() and
    erase() which return iterators, the first call to non-const member
    functions operator[](), at(), begin(), rbegin(), end(), or rend().
</p></blockquote>

<p>to</p>

<blockquote><p>
    Following construction or any of the above uses, except the forms of
    insert() and erase() that return iterators, the first call to non-
    const member functions operator[](), at(), begin(), rbegin(), end(),
    or rend().
</p></blockquote>




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