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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] where 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, states [ISO/IEC 14882-2014]:

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.

Do not attempt to delete a pointer to an object of incomplete type. While it is well-formed if the class has no nontrivial destructor and no associated deallocation function, it would become undefined behavior were a nontrivial destructor or deallocation function added later. It would be possible to check for a nontrivial destructor at compile time using a static_assert and the std::is_trivially_destructible type trait, not such type trait exists to test for the presence of a deallocation function.

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 inheritance, resulting in a pointer that cannot be validly dereferenced.

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.

Do not attempt to cast through a pointer to an object of incomplete type. The cast operation itself is well-formed, but dereferencing the resulting pointer may result in undefined behavior if the downcast is unable to adjust for multiple inheritance.

Noncompliant Code Example

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.
  // ...
};

Compliant Solution (delete)

In this compliant solution, the deletion of Impl is moved to a part of the code where Body is defined:

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

// Elsewhere.
class Body { /* ... */ };
 
Handle::~Handle() {
  delete Impl;
}

Compliant Solution (std::shared_ptr)

In this compliant solution, a std::shared_ptr is used to own the memory to Impl. Note that a std::shared_ptr is capable of referring to an incomplete type, but a std::unique_ptr is not.

#include <memory>
 
class Handle {
  std::shared_ptr<class Body> Impl;
  public:
    Handle();
    ~Handle() {}
    // ...
};

Noncompliant Code Example

Pointer downcasting (casting a pointer to a base class into a pointer to a derived class) may require adjusting the address of the pointer by a fixed amount that can only be determined when the layout of the class inheritance structure is known. In this noncompliant code example, f() retrieves a polymorphic pointer of complete type B from getD(). That pointer is then cast to a pointer of incomplete type, D, before being passed to g(). Casting to a pointer to the derived class may fail to properly adjust the resulting pointer, resulting in undefined behavior when the pointer is dereferenced by calling d->doSomething().

// File1.h
class B {
protected:
  double d;
public:
  B() : d(1.0) {}
};
 
// File2.h
void g(class D *);
class B *getD(); // Returns a D object

// File1.cpp
#include "File1.h"
#include "File2.h"

void f() {
  B *v = getD();
  g(reinterpret_cast<class D *>(v));
}
 
// File2.cpp
#include "File2.h"
#include "File1.h"
#include <iostream>

class Hah {
protected:
  short s;
public:
  Hah() : s(12) {}
};

class D : public Hah, public B {
  float f;
public:
  D() : Hah(), B(), f(1.2f) {}
  void doSomething() { std::cout << "f: " << f << ", d: " << d << ", s: " << s << std::endl; }
};

void g(D *d) {
  d->doSomething();
}

B *getD() {
  return new D;
}

Implementation Details

When compiled with Clang 3.5, the noncompliant code example prints:

f: 1.89367e-40, d: 5.27183e-315, s: 0

Similarly unexpected values are printed when the example is run in Microsoft Visual Studio 2013 and GCC 4.9.0.

Compliant Solution

This compliant solution assumes that the intent is to hide implementation details by using incomplete class types. Instead of requiring a D * to be passed to g(), it expects a B * type instead:

// File1.h
class B {
protected:
  double d;
public:
  B() : d(1.0) {}
};
 
// File2.h
void g(class B *); // Accepts a B object, expects a D object
class B *getD(); // Returns a D object

// File1.cpp
#include "File1.h"
#include "File2.h"

void f() {
  B *v = getD();
  g(v);
}
 
// File2.cpp
#include "File2.h"
#include "File1.h"
#include <iostream>

class Hah {
protected:
  short s;
public:
  Hah() : s(12) {}
};

class D : public Hah, public B {
  float f;
public:
  D() : Hah(), B(), f(1.2f) {}
  void doSomething() { std::cout << "f: " << f << ", d: " << d << ", s: " << s << std::endl; }
};

void g(B *d) {
  D *t = static_cast<D *>(d);
  if (t) {
    t->doSomething();
  } else {
    // Handle error.
  }
}

B *getD() {
  return new D;
}

Risk Assessment

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 non-trivial destructor. This can result in program termination, a runtime signal, or resource leaks.

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Bibliography