postfix.cpp: In function 'int main()':
postfix.cpp:25: error: '(op[i] <= '9')' cannot be used as a function
postfix.cpp:33: warning: suggest parentheses around assignment used as truth value
postfix.cpp:94: error: could not convert 'stk.Stack::empty' to 'bool'
postfix.cpp:94: error: in argument to unary !