错误的演员表 - 是演员表或使用未定义的行为

Question

If I do a cast from a Base to a Derived type, but the Base type isn't an instance of derived type, but only use the result if it is, do I get undefined behaviour?

Hard to understand what I'm asking? take a look at this example:

struct Animal { int GetType(){...} };
struct Dog : Animal { bool HasLoudBark(){...}};
struct Cat : Animal { bool HasEvilStare(){...} };

Animal * a = ...;
Dog* d = static_cast<Dog*>(a);

if(a->GetType() == DogType && d->HasLoudBark())
    ....

In this case a may or not be a Dog . We always do the static_cast of a to Dog * d but we never use d unless we're sure its a Dog .

Assuming that a is not a Dog , is this undefined behaviour at the point of the cast? Or is it defined as we don't actually use d unless it is really is a Dog ?

References to the relevant parts of the standard are appreciated.

(Yes I know I can use dynamic_cast, and RTTI, and probably this isn't great code, but I'm more interested in whether this is valid)

Answer 1

The cast itself has undefined behaviour. Quoting C++17 (n4659) [expr.static.cast] 8.2.10/11:

A prvalue of type “pointer to cv1 B ”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D ”, where D is a class derived (Clause 13) from B , if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1 . ... If the prvalue of type “pointer to cv1 B ” points to a B that is actually a subobject of an object of type D , the resulting pointer points to the enclosing object of type D . Otherwise, the behavior is undefined.

Answer 2

This is undefined behaviour , but (funny enough) if you would have used reinterpret_cast instead of static_cast , you would cast that demon away.

[expr.reinterpret.cast]/7

An object pointer can be explicitly converted to an object pointer of a different type. When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T ”, the result is static_cast<cv T*>(static_cast<cv void*>(v)) .

As noted by user Angew, this "requires a particular internal representation which ensures that static_cast<void*>(d) == static_cast<void*>(a) when a == d ".

This is expressed by [class.mem]/22 to 26 :

[class.mem]/26

If a standard-layout class object has any non-static data members, its address is the same as the address of its first non-static data member if that member is not a bit-field. Its address is also the same as the address of each of its base class subobjects.

So if GetType() of Animal returns the value of a non-static data member from the common initial sequence of Animal and Dog , the behavior is defined.

Those requirements are met when dealing with simple inheritance and default-aligned objects.