Explain what "when necessary" means in Enum.7.

[BartVandewoestyne]

Enum.7 reads "Specify the underlying type of an enumeration only when necessary" but to me (and most likely others) it is not 100% clear what "when necessary" exactly means. In the code example, one comment writes "underlying type saves space" and the note in Enum.7 mentions that specifying the underlying type is necessary in forward declarations of enumerations. So my current conclusion would be that "when necessary" can mean only two things:

1. You want to save space,
2. You want to forward declare the enumeration.

Am I correct and are these the only two cases? Or are there other circumstances that would require specifying the underlying type to be necessary? If 'yes', then it would be nice if Enum.7 was expanded with a list of circumstances under which this is necessary.

[jwakely]

For an unscoped enumeration type, specifying the underlying type means that all values of the underlying type are valid values of the enumeration. So this is OK:

enum X : int { x0, x1, x2 };
X x = static_cast<X>(4);

Without the underlying type that is undefined, because the only valid values are 0,1,2,3.

[beinhaerter]

A case where I am specifying the underlying type is, when I have an enum that I am using in a struct for data received over a network socket. The memory layout of the data is fixed, it is part of the API. To be sure that the enum size matches the data size I am specifying std::uint8_t oder std::uint16_t for example.

[hsutter]

Editors call: Okay, we could add "for example, specifying the underlying type is necessary to forward-declare an enum or enum class, or to ensure that values of type that can have a specified bit-precision"

[hsutter]

Closed with PR #2000 .