Member Function or Not?

We have seen Money's operator+ function overloaded as both a member function and as a non-member function. Which way is better?

A subtle issue

This code from last time works just fine regardless of whether operator+ is a member function or not. Do you see why this is true? Money total, popcorn, candy; // get popcorn sales amount cout << "Enter the amount of the popcorn sales: "; popcorn.Input( ); // get candy sales amount cout << "Enter the amount of the candy sales: "; candy.Input( ); // output total sales total = popcorn + candy; total.Output( ); cout << endl; What about this code? Money total, cost; cout << "Enter item cost: " "; cost.Input( ); // add $10 shipping and handling total = cost + 10; cout << "Total cost: "; total.Output( ); The statement total = cost + 10; seems like a natural thing to want to do. But 10 is not a Money object. How can this compile and execute?

The statement total = cost + 10; becomes the function call cost.operator+( 10 );. Since 10 is an integer, the compiler looks for a function named operator+ whose single argument is an integer since this would be an exact match for the function call. No such function exists.

Since cost is a Money object, the compiler next looks to see if there is a Money constructor which takes a single integer argument. Since such a constructor exists, the compiler converts 10 to a Money object and calls Money's operator+ function.

This code works regardless of whether operator+ is a member function or non-member function.

This example illustrates an important point --

Any class constructor that has a single argument can be used by the compiler to automatically convert that argument to an object of that class

A bigger issue

Now, let's change the line total = cost + 10; to total = 10 + cost; and look at what the compiler tries to do.

If operator+ is not a member function, 10 is converted to a Money object and operator+ can be called with it's two Money parameters.

If operator+ is a member function of Money, however, we have a problem because 10 (which is an int) cannot be a calling object. Conversions to type Money works only for arguments, not for calling objects.

So what does this mean for our original question? Should operator+ be a member function or not?

So what's the final decision?

Last Modified: Monday, 28-Aug-2006 10:15:53 EDT