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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="369"><a href="lwg-defects.html#369">369</a>. io stream objects and static ctors</h3>
<p><b>Section:</b> 31.4 <a href="https://wg21.link/iostream.objects">[iostream.objects]</a> <b>Status:</b> <a href="lwg-active.html#CD1">CD1</a>
 <b>Submitter:</b> Ruslan Abdikeev <b>Opened:</b> 2002-07-08 <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#iostream.objects">issues</a> in [iostream.objects].</p>
<p><b>View all issues with</b> <a href="lwg-status.html#CD1">CD1</a> status.</p>
<p><b>Discussion:</b></p>
<p>
Is it safe to use standard iostream objects from constructors of
static objects?  Are standard iostream objects constructed and are
their associations established at that time?
</p>

<p>Surpisingly enough, Standard does NOT require that.</p>

<p>
27.3/2 [lib.iostream.objects] guarantees that standard iostream
objects are constructed and their associations are established before
the body of main() begins execution.  It also refers to ios_base::Init
class as the panacea for constructors of static objects.
</p>

<p>
However, there's nothing in 27.3 [lib.iostream.objects],
in 27.4.2 [lib.ios.base], and in 27.4.2.1.6 [lib.ios::Init],
that would require implementations to allow access to standard
iostream objects from constructors of static objects.
</p>

<p>Details:</p>

<p>Core text refers to some magic object ios_base::Init, which will
be discussed below:</p>

<blockquote><p>
    "The [standard iostream] objects are constructed, and their
    associations are established at some time prior to or during
    first time an object of class basic_ios&lt;charT,traits&gt;::Init
    is constructed, and in any case before the body of main
    begins execution." (27.3/2 [lib.iostream.objects])
</p></blockquote>

<p>
The first <i>non-normative</i> footnote encourages implementations
to initialize standard iostream objects earlier than required.
</p>

<p>However, the second <i>non-normative</i> footnote makes an explicit
and unsupported claim:</p>

<blockquote><p>
  "Constructors and destructors for static objects can access these
  [standard iostream] objects to read input from stdin or write output
  to stdout or stderr." (27.3/2 footnote 265 [lib.iostream.objects])
</p></blockquote>

<p>
The only bit of magic is related to that ios_base::Init class.  AFAIK,
the rationale behind ios_base::Init was to bring an instance of this
class to each translation unit which #included &lt;iostream&gt; or
related header.  Such an inclusion would support the claim of footnote
quoted above, because in order to use some standard iostream object it
is necessary to #include &lt;iostream&gt;.
</p>

<p>
However, while Standard explicitly describes ios_base::Init as
an appropriate class for doing the trick, I failed to found a
mention of an _instance_ of ios_base::Init in Standard.
</p>


<p id="res-369"><b>Proposed resolution:</b></p>

<p>Add to 31.4 <a href="https://wg21.link/iostream.objects">[iostream.objects]</a>, p2, immediately before the last sentence
of the paragraph, the following two sentences:</p>

<blockquote><p>
If a translation unit includes &lt;iostream&gt;, or explicitly
constructs an ios_base::Init object, these stream objects shall
be constructed before dynamic initialization of non-local
objects defined later in that translation unit, and these stream
objects shall be destroyed after the destruction of dynamically
initialized non-local objects defined later in that translation unit.
</p></blockquote>

<p><i>[Lillehammer: Matt provided wording.]</i></p>

<p><i>[Mont Tremblant: Matt provided revised wording.]</i></p>



<p><b>Rationale:</b></p>
<p>
The original proposed resolution unconditionally required
implementations to define an ios_base::Init object of some
implementation-defined name in the header &lt;iostream>. That's an
overspecification. First, defining the object may be unnecessary
and even detrimental to performance if an implementation can
guarantee that the 8 standard iostream objects will be initialized
before any other user-defined object in a program. Second, there
is no need to require implementations to document the name of the
object.</p>

<p>
The new proposed resolution gives users guidance on what they need to
do to ensure that stream objects are constructed during startup.</p>





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