c++ - C++ 中的严格别名规则和类型别名

Question

我试图在违反严格的别名规则时了解未定义的行为。为了理解它，我阅读了很多关于 SO 的文章。然而，一个问题仍然存在:我真的不明白两种类型的别名是非法的。 cpp-reference状态:

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.

我还找到了一个 nice example on SO我清楚地看到问题的地方:

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 和 float 是不相似的类型，这个程序可能会造成严重破坏。我没有看到和理解的是以下修改:

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?
}

A 和 B 是相似的类型并且一切都很好，或者它们是非法别名并且我有未定义的行为。如果它是合法的，这是因为 A 和 B 是聚合(如果是的话，我需要更改什么才能使其成为未定义的行为)？

任何提示和帮助将不胜感激。

编辑 关于重复的问题

我知道 this发布，但我看不到他们在哪里澄清哪些类型是相似的。至少没有达到我能理解的程度。因此，如果您不关闭此问题，我们将不胜感激。

Answer 1

不，这是不合法的，你有未定义的行为:

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.