The use of incomplete class declarations (also known as "forward" declarations) is common. While it is possible to declare pointers and references to incomplete classes, because the class definition is not available it's 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.
Non-Compliant Code Example (deleting)
Consider a common application of a handle/body idiom that implements an abstract data type by a handle that contains a pointer to an implementation class.
class Body; // incomplete class declaration
class Handle {
public:
Handle();
~Handle() { delete impl_; } // deletion of pointer to incomplete class
...
private:
Body *impl_;
};
Because impl_ is a pointer to an undefined class, its deletion in Handle's destructor results in undefined behavior if Body has a non-trivial destructor. Even in the case where Body does have a non-trivial destructor, this practice should be avoided. During maintenance a non-trivial destructor could be added to Body, resulting in undefined behavior in the destructor for Handle.
Compliant Solution 1
The deletion of impl_ should be moved to a part of the code where Body is defined.
class Body {
...
};
Handle::~Handle() { delete impl_; } // correct.
Compliant Solution 2
Alternatively, an appropriate smart pointer may be used in place of a raw pointer.
class Handle {
public:
Handle();
~Handle() {} // correct.
...
private:
std::tr1::shared_ptr<Body> impl_;
};
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.
Non-Compliant Code Example (casting)
Similarly, while it is possible to cast a pointer or reference to an incomplete class, it is never a good idea to do so. Casting a class address often involves an adjustment of the address by a fixed amount that can only be determined after the layout and inheritance structure of the class is known, and this information is not available in the case of an incomplete class.
class B {
...
};
B *getMeSomeSortOfB();
...
class D; // incomplete declaration
...
B *bp = getMeSomeSortOfB();
D *dp = (D *)bp; // old-stlye cast: legal, but inadvisable
dp = reinterpret_cast<D *>(bp); // new-style cast: legal, but inadvisable
Both an old-style cast and a reinterpret_cast may be used to cast a pointer to an incomplete class. However, the cast may result in a bad address.
class D : public SomeClass, public B {
...
};
B *getMeSomeSortOfB() { return new D; }
In the case shown above, it is likely that a correct cast of a B * to a D * would have to adjust the address by a fixed amount. However, at the point the cast is translated by the compiler the required information is not available and the address adjustment will not take place.
In the case of an old-style cast, the address adjustment will, however, take place if the cast is performed at a point where the structure of the class D is known. This different, context-dependent behavior of the old-style cast can result in very challenging bugs.
Compliant Solution 3
Avoid casting to or from pointers to incomplete classes. If such a cast is, for some reason, absolutely necessary, prefer reinterpret_cast to an old-style cast because it will exhibit the same behavior whether or not the class is incomplete.
Risk Assessment
Casting pointers or references to incomplete classes can result in bad addresses.
Rule |
Severity |
Likelihood |
Remediation Cost |
Priority |
Level |
|---|---|---|---|---|---|
ERR01-A |
2 (medium) |
1 (unlikely) |
2 (medium) |
P4 |
L3 |
References
[[Dewhurst 03]] Gotcha 39: Casting Incomplete Types