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Document that a derived archive must call init() itself #357

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24 changes: 22 additions & 2 deletions doc/derivation.html
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Expand Up @@ -70,8 +70,28 @@ <h3>Log Archive</h3>
<li><i>Note the</i> <code style="white-space: normal">log_archive</code> <i>between the</i> &lt;&gt;
This is required so that base classes can downcast their <code style="white-space: normal">this</code> pointer
to the most derived class. This is referred to as <b>C</b>uriously <b>R</b>ecurring
<b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>.
<b>T</b>emplate <b>P</b>attern (<b>CRTP</b>) <a href="bibliography.html#11">[11]</a>.
It is used to implement static polymorphism.
<li><i>The most derived class is responsible for calling</i> <code style="white-space: normal">init()</code>.
The archive header (signature and version) is written by <code style="white-space: normal">init()</code>.
Before Boost 1.73, the <b>CRTP</b> base called <code style="white-space: normal">init()</code> from its own
constructor, but that downcast <code style="white-space: normal">this</code> to the most derived class
while that class was still being constructed, which is undefined behavior. As of 1.73, the base
classes no longer do this, so a derived archive that wants the standard header must call
<code style="white-space: normal">init()</code> itself, in its own constructor body, after the base
sub-objects have been constructed, honoring the <code style="white-space: normal">no_header</code> flag:
<pre><code>
log_archive(std::ostream &amp; os, unsigned int flags = 0) :
xml_oarchive_impl&lt;log_archive&gt;(os, flags)
{
if(0 == (flags &amp; boost::archive::no_header))
init();
}
</code></pre>
The <code style="white-space: normal">log_archive</code> shown in this example passes
<code style="white-space: normal">no_header</code>, so it has no header to write and its constructor
body is empty; see <code style="white-space: normal">xml_oarchive</code>, or the
<code style="white-space: normal">portable_binary_oarchive</code> example, for archives that do write one.
<li><i>Base classes need to be explicitly given access to the derived class.</i>
This can be done by making members public or by including friend declarations for
the base classes.
Expand Down Expand Up @@ -135,7 +155,7 @@ <h3>Log Archive</h3>
{
public:
log_archive(std::ostream &amp; os, unsigned int flags = 0) :
log_archive_impl&lt;xml_oarchive&gt;(os, flags)
log_archive_impl&lt;log_archive&gt;(os, flags)
{}
};
</code></pre>
Expand Down