Is this a valid for loop?

I'm currently learning c++ and i cant get my head around this syntax for the for loop.

I'm aware that for(<T>: <V>) (for-each) and the standard for(init; cond; incr) but i haven't come across the following before

for (char ch; cin>>ch && !isdigit(ch); )

If someone could shed some light onto it it would be greatly appreciated!