Little question about char convertion

[A2501]

Hello!
I've been using sprintf to convert a group of integers to a char sequence.
like this:
char st[];
int a=2,b=3;
sprintf(st,"%d and %d",a,b);
cout<<st;

it works good, but when trying to use char *st; instead of the array an error is thrown.
"Error while dumping state (probably corrupted stack)"
if sprintf receives a char * as first argument, shouldn't it work too?