static_cast on integer to enum conversion

Question

There is some function that takes in an enum as argument

void myfunc(myEnum input);

As I understand, if I have to give an integer to this function, it is advised to explicitly cast it to enum, the reason being all integers may not be valid enum values.

As per MSDN

"The static_cast operator can explicitly convert an integral value to an enumeration type. If the value of the integral type does not fall within the range of enumeration values, the resulting enumeration value is undefined."

and as per the C++ standards 5.2.9 Static cast -> 10

"A value of integral or enumeration type can be explicitly converted to an enumeration type. The value is unchanged if the original value is within the range of the enumeration values (7.2). Otherwise, the resulting value is unspecified (and might not be in that range)."

So what's the point using static_cast in this scenario? Is there some option that would raise exceptions on values outside the enum range (other than writing explicit code for that)?

The reason for casting is because the language requires it - unlike in C, there is no implicit conversion from integer to enumeration types. Or are you asking why the language was designed like that? — Mike Seymour
– Mike Seymour, Commented Jul 7, 2014 at 12:49
@Mike Seymour Yes, I was wondering why haven't they put a checking mechanism? enum to int is implicitly convertible, so if there is no checking and the onus is on me, make int to enum the same way. — ontherocks
– ontherocks, Commented Jul 7, 2014 at 15:01
Automatic run-time checks are something C++ generally avoids, giving the programmer the choice of whether to pay for them. Requiring an explicit cast forces you to think about whether a check is needed, as well as making it clear to future readers that something a bit unusual is happening. Allowing implicit conversions would silently hide the potential error. — Mike Seymour
– Mike Seymour, Commented Jul 7, 2014 at 15:51

ComicSansMS · Accepted Answer · 2014-07-07 12:54:19Z

As usual, the compiler is just trying to keep you from shooting yourself in the foot. That's why you cannot just pass an int to a function expecting an enum. The compiler will rightfully complain, because the int might not match any valid enum value.

By adding the cast you basically tell the compiler 'Shut up, I know what I am doing'. What you are communicating here is that you are sure that the value you pass in is 'within the range of the enumeration values'. And you better make sure that is the case, or you are on a one-way trip to undefined-behavior-land.

If this is so dangerous, then why doesn't the compiler add a runtime check for the integer value? The reason is, as so often with C++, performance. Maybe you just know from the surrounding program logic that the int value will always be valid and you absolutely cannot waste any time on stupid runtime checks. From a language-design point of view, this might not be the most reasonable default to chose, especially when your goal is writing robust code. But that's just how C++ works: A developer should never have to pay for functionality that they might not want to use.

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