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<title>Issue 381: detection of invalid mbstate_t in codecvt</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="381"><a href="lwg-defects.html#381">381</a>. detection of invalid mbstate_t in codecvt</h3>
<p><b>Section:</b> 28.3.4.2.6 <a href="https://wg21.link/locale.codecvt.byname">[locale.codecvt.byname]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Martin Sebor <b>Opened:</b> 2002-09-06 <b>Last modified:</b> 2016-01-28</p>
<p><b>Priority: </b>Not Prioritized
</p>
<p><b>View all other</b> <a href="lwg-index.html#locale.codecvt.byname">issues</a> in [locale.codecvt.byname].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
All but one codecvt member functions that take a state_type argument
list as one of their preconditions that the state_type argument have
a valid value. However, according to 22.2.1.5.2, p6,
codecvt::do_unshift() is the only codecvt member that is supposed to
return error if the state_type object is invalid.
</p>

<p>
It seems to me that the treatment of state_type by all codecvt member
functions should be the same and the current requirements should be
changed. Since the detection of invalid state_type values may be
difficult in general or computationally expensive in some specific
cases, I propose the following:
</p>


<p id="res-381"><b>Proposed resolution:</b></p>
<p>
Add a new paragraph before 22.2.1.5.2, p5, and after the function
declaration below
</p>
<pre>
    result do_unshift(stateT&amp; state,
    externT* to, externT* to_limit, externT*&amp; to_next) const;
</pre>
<p>
as follows:
</p>
<pre>
    Requires: (to &lt;= to_end) well defined and true; state initialized,
    if at the beginning of a sequence, or else equal to the result of
    converting the preceding characters in the sequence.
</pre>
<p>
and change the text in Table 54, row 4, the <b>error</b> row, under
the heading Meaning, from
</p>
<pre>
    state has invalid value
</pre>
<p>
to
</p>
<pre>
    an unspecified error has occurred
</pre>


<p><b>Rationale:</b></p>
<p>The intent is that implementations should not be required to detect
invalid state values; such a requirement appears nowhere else.  An
invalid state value is a precondition violation, <i>i.e.</i> undefined
behavior.  Implementations that do choose to detect invalid state
values, or that choose to detect any other kind of error, may return
<b>error</b> as an indication.</p>





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