How Can `static_cast` Seemingly Allow Access to a Derived Class\'s Members When Downcasting a Base Pointer to a Distinct Base Object?

Downcasting Using the static_cast Operator: Demystifying Undefined Behavior

Consider the following concern:

class base {
    base();
    virtual void func();
};

class derived : public base {
    derived();
    void func();
    void func_d();
    int a;
};

int main() {
    base *b = new base();
    sizeof(*b); // Gives 4.
    derived *d = static_cast<derived>(b);
    sizeof(*d); // Gives 8 - means whole derived obj size...why?
    d->func_d();
}</derived>

In this scenario, casting the base pointer to a derived pointer using static_cast has seemingly allowed access to the full derived object's size and function. However, this raises the question: how is this possible if the base pointer originally pointed to a distinct base object?

Understanding the Undefined Behavior

The answer lies in the nature of static_cast and its impact on dynamic objects. Downcasting using static_cast to a type that the object does not actually have is classified as undefined behavior. The consequences of undefined behavior can vary dramatically, including allowing unexpected access to the derived class member function func_d() in this case.

The Rule of Downcasting

According to the C standard (section 5.2.9), downcasting using static_cast follows a specific rule:

• If the base pointer points to a base object that is a subobject of a derived object, then the resulting derived pointer will point to the enclosing derived object.
• Otherwise, the result of the cast is undefined.

In our example:

• The base pointer points to a base object that is distinct from any derived object.
• Therefore, the downcast to derived* results in undefined behavior.

The unexpected success of calling d->func_d() is a consequence of this undefined behavior. Do not rely on the ability to access derived class members after an unsafe downcast.

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