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PODoes the GotW #101 "solution" actually solve anything?
 

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  1. PODoes the GotW #101 "solution" actually solve anything?
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    <p>First read Herb's Sutters GotW posts concerning pimpl in C++11:</p> <ul> <li><p><a href="http://herbsutter.com/gotw/_100/" rel="noreferrer">GotW #100: Compilation Firewalls (Difficulty: 6/10)</a></p></li> <li><p><a href="http://herbsutter.com/gotw/_101/" rel="noreferrer">GotW #101: Compilation Firewalls, Part 2 (Difficulty: 8/10)</a></p></li> </ul> <p>I'm having some trouble understanding the solution proposed in GotW #101. As far as I can understand, all the problems laboriously solved in GotW #100 are back with a vengeance:</p> <ul> <li><p>The <code>pimpl</code> members are out-of-line templates, and the definitions are not visible at the point of use (in <code>class widget</code>'s class definition and implicitly generated special member functions of <code>widget</code>). There aren't any explicit instantiations either. This will cause unresolved external errors during linking.</p></li> <li><p><code>widget::impl</code> is still incomplete at the point where <code>pimpl&lt;widget::impl&gt;::~pimpl()</code> is <strike>instantiated</strike> defined (I don't think it actually IS instantiated at all, just referenced). So <code>std::unique_ptr&lt;widget::impl&gt;::~unique_ptr()</code> calls <code>delete</code> on a pointer to incomplete type, which produces undefined behavior if <code>widget::impl</code> has a non-trivial destructor.</p></li> </ul> <p>Please explain what forces the compiler to generate the special members in a context where <code>widget::impl</code> is complete. Because I can't see how this works.</p> <hr> <p>If GotW #101 still requires explicit definition of <code>widget::~widget()</code> in the implementation file, where <code>widget::impl</code> is complete, then please explain the "More Robust" comment (which @sehe quoted in his answer).</p> <p>I'm looking at the core claim of GotW #101 that the wrapper "eliminates some pieces of boilerplate", which seems to me (based on the remainder of the paragraph) to mean the <code>widget::~widget()</code> declaration and definition. So please don't rely on that in your answer, in GotW #101, that's gone!</p> <hr> <p>Herb, if you stop by, please let me know if it would be ok to cut+paste the solution code here for reference.</p>
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    1. CO`Paging` Dr. @HerbSutter
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    2. CO@Ben Voigt: I did search for and tag several other questions with gotw. Did you find some that I missed?
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    3. CO@Ben Voigt: Must be a propagation delay in Stack Exchange then. I added the tag to several other questions immediately after creating it for this one. I've found one more question since then and tagged it as well.
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