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Pointer downcasting to a pointer of incomplete class type has similar caveats. Pointer upcasting (casting from a more derived type to a less derived type) is a standard implicit conversion operation. C++ allows static_cast to perform the inverse operation, pointer downcasting, via [expr.static.cast], paragraph 7. However, when the pointed-to type is incomplete, the compiler is unable to make any class offset adjustments that may be required in the presence of multiple inheritancesinheritance, resulting in a pointer that cannot be validly dereferenced.
The reinterpret_cast of a pointer type is defined by [expr.reinterpret.cast], paragraph 7, as being static_cast<cv T *>(static_cast<cv void *>(PtrValue)), meaning that reinterpret_cast is simply a sequence of static_cast operations. C-style casts of a pointer to incomplete object type are defined as using either static_cast or reinterpret_cast (which is picked is unspecified) in [expr.cast], paragraph 5.
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Implementation Details
When compiled with ClangBB. Definitions#clang3.8 and the function f() is executed, the noncompliant code example prints the following.
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