Multiplying an Enum in C++

Question

I have some code that is multiplying an enum by an integer:

QuantLib::Date date2 = date + 12 * QuantLib::Months;

Where QuantLib::Months is defined as:

enum TimeUnit { Days,
                Weeks,
                Months,
                Years
};

This gives me the desired result of date2 being one year on from date. However, I'm not able to comprehend how this is being achieved.

I had thought that this would not compile. Now I feel that I'm arriving at a "twelve months" object, which the is then handled by the QuantLib::Date '+' operator overload, but I've never seen this style before.

I have come from a C# background, so there may be something I'm not aware of at work here. Can anyone explain what is going on? Any reference documentation would be appreciated.

Answer 1

One of the following is in effect here:

1. In C++, an enumeration type can be implicitly converted to an integral type. If this is happening here, date + 12 * QuantLib::Months would be the same as date + 12 * 2.

2. It is also possible to overload operators for enumeration types. In that case, it might be that the library defines an operator* (int, QuantLib::TimeUnit) which returns something compatible with the + you're doing.

I don't know QuantLib, but I'd guess #2 is what's happening. QuantLib documentation corroborates this (thanks to @DaliborFrivaldsky for the link).