Even if you try exceptionally hard to use the correct types in your code, at some point you will find that you will have to convert between types. For example, you may be using library functions that return a value of a particular type, or you may be reading in data from an external source that is a different type to your routine.
With built-in types, there are standard rules about conversion between different types, some of which will be automatic. For example, if you have an expression like a + b, and a and b are different types, then, if it is possible, the compiler will automatically convert one variable's value to the type of the other and the + operator for that type will be called.
In other cases, you may need to force one type to another type so that the right operator is called and this will require a cast of some kind. C++ allows you to...