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POHow do I use unique_ptr for pimpl?
 

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  1. POHow do I use unique_ptr for pimpl?
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    <p>Here is a simplification of what I'm seeing when I try to use unique_ptr for pimpl. I chose unique_ptr because I really want the class to own the pointer - I want the lifetimes of the pimpl pointer and the class to be the same.</p> <p>Anyway, here is the header:</p> <pre><code>#ifndef HELP #define HELP 1 #include &lt;memory&gt; class Help { public: Help(int ii); ~Help() = default; private: class Impl; std::unique_ptr&lt;Impl&gt; _M_impl; }; #endif // HELP </code></pre> <p>Here is the source:</p> <pre><code>#include "Help.h" class Help::Impl { public: Impl(int ii) : _M_i{ii} { } private: int _M_i; }; Help::Help(int ii) : _M_impl{new Help::Impl{ii}} { } </code></pre> <p>I could compile these into a library just fine. But when I try to use it in a test program I get</p> <pre><code>ed@bad-horse:~/ext_distribution$ ../bin/bin/g++ -std=c++0x -o test_help test_help.cpp Help.cpp In file included from /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/memory:86:0, from Help.h:4, from test_help.cpp:3: /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h: In instantiation of 'void std::default_delete&lt;_Tp&gt;::operator()(_Tp*) const [with _Tp = Help::Impl]': /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:245:4: required from 'void std::unique_ptr&lt;_Tp, _Dp&gt;::reset(std::unique_ptr&lt;_Tp, _Dp&gt;::pointer) [with _Tp = Help::Impl; _Dp = std::default_delete&lt;Help::Impl&gt;; std::unique_ptr&lt;_Tp, _Dp&gt;::pointer = Help::Impl*]' /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:169:32: required from 'std::unique_ptr&lt;_Tp, _Dp&gt;::~unique_ptr() [with _Tp = Help::Impl; _Dp = std::default_delete&lt;Help::Impl&gt;]' Help.h:6:7: required from here /home/ed/bin/lib/gcc/x86_64-unknown-linux-gnu/4.7.0/../../../../include/c++/4.7.0/bits/unique_ptr.h:63:14: error: invalid application of 'sizeof' to incomplete type 'Help::Impl' </code></pre> <p>This is a well known <a href="https://stackoverflow.com/questions/6012157/does-stdunique-ptrt-requires-to-know-the-full-t-definition">safety feature</a>. I've tried to follow.</p> <p>My problem is that if I put Help::Impl declaration in a header it would seem to obviate any advantage of pimpl. The class layout is visible to users. The definition is hidden but I could have done that with the Help class and private members. Also, including the declaration of Impl brings in new headers that I would have liked to keep separate.</p> <p>What am I missing? What do folks put in an Impl declaration and where? Am I doing the Help dtor wrong? Argh!</p>
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    1. COSee also [GotW #101: Compilation Firewalls, Part 2](http://herbsutter.com/gotw/_101/) and [this related question](http://stackoverflow.com/q/8595471/636019).
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