Fill-In Functions

Occasionally you will see functions like addTime written with a different interface (different arguments and return values). Instead of creating a new object every time addTime is called, we could require the caller to provide an "empty" object where addTime can store the result. Compare the following with the previous version:

One advantage of this approach is that the caller has the option of reusing the same object repeatedly to perform a series of additions. This can be slightly more efficient, although it can be confusing enough to cause subtle errors. For the vast majority of programming, it is worth a spending a little run time to avoid a lot of debugging time.

Notice that the first two parameters can be declared const, but the third cannot.