What is wrong with this reference?

This is the sample program:

#include<cstdio>

int main()
{
        int*const a=new int;
        const int*const&b=a;
        printf("%p %p\n",&a,&b);
        delete a;
        return 0;
}

When running, it produces:

0x7fff1dc49fc8 0x7fff1dc49fb8

That means the memory locations of a and b are different i.e. a and b is
different object! I want to make something that *a is modifiable but *b is
not (to be used inside a class) but the following code generates a
compile-time error:

#include<cstdio>

int main()
{
        int*a=new int;
        const int*&b=a;
        printf("%p %p\n",&a,&b);
        delete a;
        return 0;
}

test.cpp:6: error: invalid initialization of reference of type ???const int*&???
from expression of type ???int*???

The following code runs perfect:

#include<cstdio>

int main()
{
        int*a=new int;
        const int&b=a;
        printf("%p %p\n",&a,&b);
        return 0;
}

What is the problem in the first code (I am using g++ 4.2.4)?