用非const指针定义对const数据的指针的引用有什么问题？

Question

int main()
{
    const int* x;
    int* pa = x;//removes const, so UB.

    const int*& pb = pa;//error
    int* pd = pb;//error
    return 0;
}

I know that it's not possible to define a pointer to non-const data with a pointer to const data , because it would automatically cancel the constness out allowing me to modify the value.
But what is wrong with the second initialization? I know that a reference is an alias of something and how it works, but still don't get what actually is happening there at all. I guess that explanation of the second error will, hopefully, enlighten me the third error.
Can anyone shed some light? Thanks!

Answer 1

const int x = 1;
int* p;
const int*& r = p;

Imagine we had gotten this far. This last line will give the error you're questioning, but let's assume it works. Now r and p refer to the same pointer object. But now we can do this:

r = &x;

This makes r point at the const object x , which you might think is fine, but it will also make p point at it. Since p is an int* (not const ), we can now use p to modify x :

*p = 2;

Now we've changed the value of a const object. The error in question prevents us from doing this.

So basically, the reasoning for this error is that being able to bind a reference to pointer to const to a pointer to non- const would give you a way to get the non- const pointer to point at a const object. That's bad.

See the Why am I getting an error converting a Foo** → Foo const** C++ FAQ to learn about the same issue but with pointers instead of references. The reasoning is the same.

Answer 2

This is just another instance of the rule that you cannot bind a temporary to a non-const refence.

Eg

X f();

X& r = f(); // illegal
X const& cr = f(); // OK

You can convert a pointer to int to a pointer to const int but the result of that conversion is a new pointer with a different type. You cannot bind an rvalue (such as the result of this conversion) to a non-const reference, only to a const reference, eg

const int* const& pb = pa; // pb is not bound directly to pa but to
                           // the result of converting pa to const int*