How to create a interfaces in c++

what do you mean by "interfaces"? The Java feature for classes, or something else?

GUI maybe for calculate

Doggy :3 , it's not gui, but simplest way would be to have third input in the form of char data type.. that input should be second input...if user gives input as 2+5, you add these two numbers... check whether second input of type character is other than +,-,*,/,% display message to user that invalid operator