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Referring to objects of incomplete class type, also known as forward declarations, is a common practice. One such common usage is with the "pimpl idiom" [Sutter 00] whereby an opaque pointer is used to hide implementation details from a public-facing API. However, attempting to delete a pointer to an object of incomplete class type can lead to undefined behavior. The C++ Standard, [expr.delete], paragraph 5 [ISO/IEC 14882-2014], states the following:

If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.

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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 an incomplete object type are defined as using either static_cast or reinterpret_cast (it is unspecified which is picked is unspecified) in [expr.cast], paragraph 5.

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In this noncompliant code example, a class attempts to implement the pimpl idiom but deletes a pointer to an incomplete class type, resulting in undefined behavior if Body has a nontrivial destructor:.

class Handle {
  class Body *impl;  // Declaration of a pointer to an incomplete class
public:
  ~Handle() { delete impl; } // Deletion of pointer to an incomplete class
  // ...
};

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Casting pointers or references to incomplete classes can result in bad addresses. Deleting a pointer to an incomplete class results in undefined behavior if the class has a nontrivial destructor. Doing so can cause program termination, a runtime signal, or resource leaks.

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Bibliography

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