Never seen before C++ for loop

I was converting a C++ algorithm to C#. I came across this for loop:

for (u = b.size(), v = b.back(); u--; v = p[v]) 
b[u] = v;

It gives no error in C++, but it does in C# (cannot convert int to bool). I really can't figure out this for loop, where is the condition?

Can someone please explain?

PS. Just to check, to adapt a VECTOR to a LIST does b.back() correspond to b[b.Count-1] ?