luis
c += c++ is equivalent to:
c = c + c++
Before the operation is carrried out all of the operands are fully evaluated, c is equal to 0, and the result of the postfix increment is the original value of the operand so the value of c++ is also 0.
The reason it prints 0 then instead of 1 (or 2 as you thought) is becasue even the though the c++ increment sthe value by 1 that value is immediately replaced by the 0 that is assigned to c.
hoep that helps, if you are having confusion on the postfix operator there are plenty of threads on it in this forum, just do a
search for it.