C++ :: Increment And Decrement - No Loops

Parts of this program are missing. The last few lines are confusing, since the variable 'a' gets incremented then decremented. But there are no loops. I understand that the value of 'a' is passed to 'c' before 'a' is changed in both cases.

But where, and when, do the changes take place? Is the decrement ever processed? Is there a better way to write these lines?

Code:
main(){ int a = 21;int b = 10;int c ;
c = a++;
cout << "Line 6 - Value of c is :" << c << endl ;
c = a--;
cout << "Line 7 - Value of c is :" << c << endl ;
return 0;}