c++ - 通过指向其派生类的指针访问类对象是否会违反严格的别名规则？

Question

void foobar(Base* base)
{
    Derived* derived = dynamic_cast<Derived*>(base); // or static_cast
    derived->blabla = 0xC0FFEE;
    if (base->blabla == 0xC0FFEE)
        ...
}

在具有严格别名 的编译器上，“derived”是“base”的别名吗？

Answer 1

只要有可能通过它们访问同一个对象，两个指针就被别名化了。该标准的第 3.10/15 段指定了对对象的访问何时有效。

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

• the dynamic type of the object,
• a cv-qualified version of the dynamic type of the object,
• a type that is the signed or unsigned type corresponding to the dynamic type of the object,
• a type that is the signed or unsigned type corresponding to a cv-qualified version of the dynamic type of the object,
• an aggregate or union type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union),
• a type that is a (possibly cv-qualified) base class type of the dynamic type of the object,
• a char or unsigned char type.

在您的情况下，*derived 要么是对象动态类型的左值，要么是对象动态类型的基类类型。 *base 是对象动态类型的基类类型。

因此，您可以通过derived 和base 访问对象，使这两个指针成为别名。