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POAvoid leaking out external types in a C++ class
 

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  1. POAvoid leaking out external types in a C++ class
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    <p>I have a class defined in a header like so (abbreviated):</p> <pre><code>class CairoRenderer { public: CairoRenderer(); ~CairoRenderer(); ... protected: cairo_t* m_context; cairo_surface_t* m_surface; }; </code></pre> <p>cairo_t and cairo_surface_t are types defined by the <a href="http://cairographics.org/" rel="nofollow">Cairo</a> graphics library.</p> <p>The problem I have is that if I want to use this class or its derived classes from another library or application, I need to include the cairo headers as well because I am "leaking" the cairo types out through the CairoRenderer header. I want this class (or any subclass of it) in the same library to be usable externally without needing to include the cairo header or link against the cairo libraries.</p> <p>So the next thing I tried was to use the pimpl technique as per the <a href="http://en.wikipedia.org/wiki/Opaque_pointer" rel="nofollow">Wikipedia</a> example because it looked like it could do what I wanted to achieve:</p> <p>CairoRenderer.h (abbreviated)</p> <pre><code>class CairoRenderer { ... protected: struct CairoRendererImpl; CairoRendererImpl* m_pimpl; }; </code></pre> <p>CairoRenderer.cpp (abbreviated)</p> <pre><code>#include "CairoRenderer.h" #include "cairo.h" .... struct CairoRenderer::CairoRendererImpl { public: CairoRendererImpl() : m_surface(NULL), m_context(NULL) { } ~CairoRendererImpl() { cairo_surface_destroy(m_surface); cairo_destroy(m_context); } void Initialize(cairo_t* context, cairo_surface_t* surface) { m_context = context; m_surface = surface; } cairo_surface_t* surface() { return m_surface; } cairo_t* context() { return m_context; } private: cairo_surface_t* m_surface; cairo_t* m_context; }; CairoRenderer::CairoRenderer() : m_pimpl(new CairoRendererImpl()) { ... } CairoRenderer::~CairoRenderer() { delete m_pimpl; } </code></pre> <p>The problem I have is when I try to access the m_pimpl member from a derived class, I get the compiler error:</p> <pre><code>error C2027: use of undefined type 'CairoRenderer::CairoRendererImpl' </code></pre> <p>Am I doing pimpl wrong? Or is what I want to do even possible?</p>
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    1. COWhy do you want to access the pimpl from a derived class? The whole raison d'etre of the pimpl is to hide implementation details of a class. If at all then derived classes should only use the base class' interface.
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    2. COMain objective is to hide cairo_t and cairo_surface_t from external code. Pimpl looked like the solution, but obviously wasn't
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