Does this C++ casting to enum have problems?

Question

Given the following code:

enum Options
{
    Surf     = 0x01,
    Out      = 0x02
};

Options all = (Options) ( Surf | Out);

1. Does this casting have problems?
2. If this casting make sense, then why?

Based on my understanding, Options only defines two variables. How does the value 0x03 makes sense?

Answer 1

Does this casting have problems?

No.

If this casting make sense, then why? Based on my understanding, Options only defines two variables, how the value 0x03 makes sense?

The enumeration type Options has two named enumerators, but the range of values that it represents is large enough that it can be used as a bitfield containing each of the enumerators.

In short: yes, it is valid and well-defined to use an enumeration for a bitfield like this.

As requested in the comments to this answer, the formal language permitting this can be found in the C++ Standard (C++03 7.2/6):

For an enumeration where e_min is the smallest enumerator and e_max is the largest, the values of the enumeration are the values of the underlying type in the range b_min to b_max, where b_min and b_max are, respectively, the smallest and largest values of the smallest bit-field that can store e_min and e_max.

It is possible to define an enumeration that has values not defined by any of its enumerators.

There is some debate as to whether or not this is good style. Certainly it can be argued that it is often assumed that an enumeration object can only store one of its enumerators, so code like this could be confusing and error-prone.

On the other hand, I would argue that it is usually quite obvious when an enumeration is intended for use as a bitfield. Usually such an enumeration is named with an Options or Flags suffix or something similar. Likewise, if each of the enumerators has a set value that is clearly a single set bit, that enumeration is probably intended for use as a bitfield.