The use of incomplete class declarations (also known as "forward" "forward" declarations) is common. While it is possible to declare pointers and references to incomplete classes, because the class definition is not available it is not possible to access a member of the class, determine the size of the class object, and so on. However, it is possible to cast and delete a pointer to an incomplete class, but this is never a good idea.
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class Handle {
public:
Handle();
~Handle() {} // correct.
// ...
private:
std::tr1::shared_ptr&lt;Body&gt;ptr<Body> impl_;
};
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Note that we used a shared_ptr to refer to the Body. Other common smart pointers, including std::auto_ptr, will still produce undefined behavior.
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class B {
// ...
};
B *getMeSomeSortOfB();
// ...
class D; // incomplete declaration
// ...
B *bp = getMeSomeSortOfB();
D *dp = (D *)bp; // old-stlye cast: legal, but inadvisable
dp = reinterpret_cast&lt;Dcast<D *&gt;>(bp); // new-style cast: legal, but inadvisable
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