为什么从char***到char*const**的转换无效？

Question

根据N4606,4.5 [conv.qual]第3段读到

如果满足以下条件,则T1可以将类型的prvalue表达式转换为类型T2,其中cv _i ^j表示T _j的cv限定签名中的cv限定符:

• ...
• 如果cv ¹ _i和cv ² _i不同,那么对于0 <k <i ,const在每个cv ² _k中.

上面的最后一个子弹表明以下转换失败.

T1 : pointer to / pointer to /       pointer to / T
T2 : pointer to / pointer to / const pointer to / T

为了成功,T2必须pointer to / const pointer to / const pointer to / T.是不是T2足够的只是更加CV-资格比T1？为什么转换成功所需的更低维度的cv-qualifiers更多？

Answer 1

请考虑以下代码:

char str[] = "p1 should always point here.";
char* const p1 = str;
char** p2 = nullptr;
char*** p3 = &p2;

char str2[] = "Can we make p1 point to here?"
// p1 = str2; // does not work: const violation

// But:
char*const** p4=p3; // if it were allowed
*p4 = &p1; // no const violation, as *p4 is of type char*const*
**p3 = str2; // oops, changed p1!

因此,如果允许转换,您将更改常量变量(p1)而不会发生任何正式的const违规.