Strict Aliasing Rule and Type Aliasing in C++

Question

I am trying to get a grasp of undefined-behavior when violating the strict aliasing rule. I have read many articles on SO in order to understand it. However, one question remains: I do not really understand when two types illegaly alias. cpp-reference states:

Type aliasing

Whenever an attempt is made to read or modify the stored value of an object of type DynamicType through a glvalue of type AliasedType, the behavior is undefined unless one of the following is true:

• AliasedType and DynamicType are similar.
• AliasedType is the (possibly cv-qualified) signed or unsigned variant of DynamicType.
• AliasedType is std::byte, (since C++17)char, or unsigned char: this permits examination of the object representation of any object as an array of bytes.

I also found a nice example on SO where I clearly see the issue:

int foo( float *f, int *i ) { 
    *i = 1;               
    *f = 0.f;            

   return *i;
}

int main() {
    int x = 0;

    std::cout << x << "\n";   // Expect 0
    x = foo(reinterpret_cast<float*>(&x), &x);
    std::cout << x << "\n";   // Expect 0?
}

int and float are non-similar types and this program possibly wreaks havoc. What I fail to see and understand is the following modification:

struct A
{
    int a;
};

struct B
{
    int b;
};

A foo( A *a, B *b ) { 
    a->a = 1;               
    b->b = 0;            

    return *a;
}

int main() {
    A a;
    a.a = 0;


    std::cout << a.a << "\n";   // Expect 0
    a = foo(&a, reinterpret_cast<B*>(&a));
    std::cout << a.a << "\n";   // Expect 0?
}

Are A and B similar types and everything is fine, or are they illegaly aliasing and I have undefined-behavior. And if it is legal, is this because A and B are aggregates (if yes, what would I have to change to make it undefined-behavior)?

Any heads-up and help would be very appreciated.

EDIT On the issue of being duplicate

I am aware of this post, but I do not see where they clarify what types are similar. At least not to an extend that I would understand it. Therefore it would be kind if you would not close this question.

Answer 1

No, it's not legal and you have Undefined Behavior:

8.2.1 Value category [basic.lval]

11 If a program attempts to access the stored value of an object through a glvalue of other than one of the following types the behavior is undefined: ⁶³

(11.1) — the dynamic type of the object,

(11.2) — a cv-qualified version of the dynamic type of the object,

(11.3) — a type similar (as defined in 7.5) to the dynamic type of the object,

(11.4) — a type that is the signed or unsigned type corresponding to the dynamic type of the object,

(11.5) — a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,

(11.6) — an aggregate or union type that includes one of the aforementioned types among its elements or nonstatic data members (including, recursively, an element or non-static data member of a subaggregate or contained union),

(11.7) — a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,

(11.8) — a char, unsigned char, or std::byte type

---

63) The intent of this list is to specify those circumstances in which an object may or may not be aliased.