Do C++ Enums Start at 0

Do C++ enums Start at 0?

Per that standard [dcl.enum]

The enumeration type declared with an enum-key of only enum is an unscoped enumeration, and its enumerators are unscoped enumerators. The enum-keys enum class and enum struct are semantically equivalent; an enumeration type declared with one of these is a scoped enumeration, and its enumerators are scoped
enumerators. The optional identifier shall not be omitted in the declaration of a scoped enumeration. The type-specifier-seq of an enum-base shall name an integral type; any cv-qualification is ignored. An opaqueenum-declaration declaring an unscoped enumeration shall not omit the enum-base. The identifiers in an enumerator-list are declared as constants, and can appear wherever constants are required. An enumeratordefinition with = gives the associated enumerator the value indicated by the constant-expression. If the first
enumerator has no initializer, the value of the corresponding constant is zero. An enumerator-definition without an initializer gives the enumerator the value obtained by increasing the value of the previous enumerator by one.

^{Emphasis mine}

So yes, if you do not specify a start value, it will default to 0.

I would really like to see an answer that confirms this for C++, but I'd also like to know if an ordinal 0 holds even if I specify a value in the middle of an enum:

This also works. It will start at 0 and increment up along the way. Then after the enum you assign the value to it will begin to increase by one from that value for subsequent enumerator.

Are default enum values in C the same for all compilers?

Yes. Unless you specify otherwise in the definition of the enumeration, the initial enumerator always has the value zero and the value of each subsequent enumerator is one greater than the previous enumerator.

Should an Enum start with a 0 or a 1?

Framework Design Guidelines:

✔️ DO provide a value of zero on simple enums.

Consider calling the value something like "None." If such a value is not appropriate for this particular enum, the most common default value for the enum should be assigned the underlying value of zero.

Framework Design Guidelines / Designing Flag Enums:

❌ AVOID using flag enum values of zero unless the value represents "all flags are cleared" and is named appropriately, as prescribed by the next guideline.

✔️ DO name the zero value of flag enums None. For a flag enum, the value must always mean "all flags are cleared."

Are the new typesafe enums defined to begin at 0?

Yes, they are. It's in §7.2/2:

[...] If the first enumerator has no initializer, the value of the corresponding constant is zero. An enumerator-definition without an initializer gives the enumerator the value obtained by increasing the value of the previous enumerator by one.

This section covers both enum class declarations and regular enum declarations

Is the default value for enum class always 0?

According to the Enums should have zero value:

The default value of an uninitialized enumeration, just like other
value types, is zero. By default, if the value of the first
enumeration member is not set in the declaration, its value is zero.

Unless you specify otherwise in the definition of the enumeration, the initial enumerator always has the value zero and the value of each subsequent enumerator is one greater than the previous enumerator.

You do not need to specify a starting value of 0, it defaults to 0.

Why the default enum value is 0 and not the minimum one?

We can only conjecture about why an aspect of the .NET framework was designed a certain way. For the most straightforward explanation, I'd like to highlight this remark from the MSDN documentation:

An enumeration is a set of named constants whose underlying type is
any integral type except Char. If no underlying type is explicitly
declared, Int32 is used.

Note that a .NET enumeration is essentially an extension of an integral type. The default value for integral types is 0, so it's reasonable (if somewhat inconvenient in the cases you've illustrated) for enumerations to inherit that behaviour.

What is the default value for enum variable?

It is whatever member of the enumeration represents the value 0. Specifically, from the documentation:

The default value of an enum E is the value produced by the expression (E)0.

As an example, take the following enum:

enum E
{
    Foo, Bar, Baz, Quux
}

Without overriding the default values, printing default(E) returns Foo since it's the first-occurring element.

However, it is not always the case that 0 of an enum is represented by the first member. For example, if you do this:

enum F
{
    // Give each element a custom value
    Foo = 1, Bar = 2, Baz = 3, Quux = 0
}

Printing default(F) will give you Quux, not Foo.

If none of the elements in an enum G correspond to 0:

enum G
{
    Foo = 1, Bar = 2, Baz = 3, Quux = 4
}

default(G) returns literally 0, although its type remains as G (as quoted by the docs above, a cast to the given enum type).

What is the rule for assignment of the integer value of enum?

Quoting from cppreference.com

If the first enumerator does not have an initializer, the associated value is zero. For any other enumerator whose definition does not have an initializer, the associated value is the value of the previous enumerator plus one.

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