Correct way to convert an enum to value - c++

Question

I work on a project that has a lot of enums defined like this:
enum BundleSize
{
BUNDLE_SIZE_5 = 5,
BUNDLE_SIZE_10 = 10,
BUNDLE_SIZE_107 = 107,
};
I wrote a function to get the value from it like:
int convertBundleSizeEnumToVal(BundleSize b)
{
switch(b)
{
case BUNDLE_SIZE_5: return 5; break;
case BUNDLE_SIZE_10: return 10; break;
case BUNDLE_SIZE_107: return 107; break;
default: // handle appropriately and error out
}
}
I realize I don't really need a converter (method 1 and 2 both work below):
BundleSize b = getRandomBundleSize();
printf("The size is %d\n", convertBundleSizeEnumToVal(b)); // method 1
printf("The size is %d\n", b); // method 2
But I feel that method 1 is still the "correct" way to do this, because in the future something like this might come along:
enum BundleSize
{
BUNDLE_SIZE_5 = 5,
BUNDLE_SIZE_5_POINT_5 = 6,
BUNDLE_SIZE_10 = 10,
BUNDLE_SIZE_107 = 107,
};
Which now effectively breaks method 2 without an obvious runtime error, whereas method 1 will atleast catch the problem in the default switch case.
I'm curious about what people think is the right way to handle these conversions.

Answer 1

You can use the static_cast operator:
static_cast<int>(b)
IMO, this is "correct". You shouldn't be defining functions to convert every possible enum variant to an integer, especially with large enums.

Answer 2

Actually, you are using simple enum, so you can simply use your enum variable as a value.
For enum class (strict version of enum) you should use static_cast to convert your enum to value.

Answer 3

You can simplify your function to:
int convertBundleSizeEnumToVal(BundleSize b)
{
switch(b)
{
case BUNDLE_SIZE_5:
case BUNDLE_SIZE_10:
case BUNDLE_SIZE_107:
return b;
default: // handle appropriately and error out
}
}
Method 2, is broken because the underlying type of enum is not necessarily int, and so the format specifier %d may be wrong and behaviour of the program may be undefined.
You can cast the enum value to int first, and it will be correct as long as you don't use higher values than can be represented by int. You can use either static_cast or explicit cast. Or you can use an implicit conversion:
int value = getRandomBundleSize();
printf("The size is %d\n", b);
Regarding which you should use, well it depends on what you need. If you think that enum values added later should not be converted by convertBundleSizeEnumToVal but should result in an error, then method 1 is indeed superior.
If you wish all enum values to be converted, then the function is error prone, as you would have to remember to add a case each time a new value is added. In this case method 2 is superior.