What is the difference between using a return value and a pass-by-reference parameter?

Answer 1

The main difference is that return values can be used in compound statements (such as assignments), whereas output parameters cannot. An output parameter is a non-const parameter that is passed by reference or as a pointer variable.

As a general rule, every function should return something through the return value, even if only an error level where zero would typically indicate success (but not always). A function that does not return anything (returns void) is not strictly a function, it is better described as a procedure. However, if a function can be guaranteed never to fail (provides trivial functionality) and has no need to return any other value to the caller, then returning void is a logical option. Just because a function should return a value doesn't mean that it must return a value.

However, the return value is limited by the fact that it only permits one value to be returned by a function. Output parameters allow a function to return several values at once and can be used in conjunction with the return value. If there is no need for the return value, it should be used to indicate the error level of the function wherever it would be appropriate. However it is not unusual for a function to accept a reference as an output parameter and to also return that same reference via the return value.

Note that although you could return multiple values via a struct or class type, this should only be considered if you have several functions that can make use of the struct or class type. However, in most cases it would be simpler to make those functions members of the struct or class itself.