为什么我不能使用static_cast <const char**> （str）而不是（const char **）str？

Question

I have an issue, it doesn't want to cast using static_cast<> . What can it be?

void myCompare(const void *str)
{
    const char *ca = *(static_cast<const char**>(str)); //error
    const char *a = *(const char **)str; //ok
}

Answer 1

You're casting away const on the second level, which static_cast is not allowed to do (in fact, no "C++" cast apart from const_cast ):

          void    const*
   char const*         *
// ^^^^^^^^^^^    ^^^^^
// pointee        cv-qualifiers
//                of pointee

Instead, write

const char *ca = *(static_cast<const char* const*>(str));

---

The (char const**) cast works here because it is equivalent to a static_cast followed by a const_cast (as per [expr.cast]/(4.3)) - ie it was equivalent to

const char *ca = *(const_cast<const char**>(static_cast<const char* const*>(str)));