How to assert whether a value is within an enums range

Question

I am reading a value from a byte stream and want to assert that this value is within an enums range. I was wondering if there's a more generic approach than the one below.

int nval = ReadValue();
Assert(nval);

bool Assert(nval)
{
 if(nval == zero || nval == one || nval==two || (nval>=reserved_low && nval <= reserved_high))
  return true;
 return false
}
    enum Foo
    {
     zero = 0,
     one = 1,
     two = 2,
     reserved_low = 3,
     reserved_high = 255,
    }

seems to me that you only need `nval>=zero && nval <= reserved_high` — EdChum, Aug 22 '16 at 11:20
You can specify additional constants in the enum `Foo_min = zero, Foo_max = reserved_high`. — StoryTeller - Unslander Monica, Aug 22 '16 at 11:21
Please, search your [question] on SO before ask.(http://stackoverflow.com/questions/2102582/how-can-i-count-the-items-in-an-enum) — , Aug 22 '16 at 11:29

Bathsheba · Answer 1 · 2016-08-22T11:29:13.660

1

Not in C++03, no.

Your way is pretty optimal (and certainly readable), although you could recognise that, given all your enums are in the range 0 to 255, return !(nval & ~0xff); would do the job.

You might be able to squeeze out a bit more performance using a switch and compare nval>=reserved_low && nval <= reserved_high in the default case.

edited Aug 22 '16 at 11:29

answered Aug 22 '16 at 11:21

Bathsheba

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I wouldn't have thought that those optimizations would make much of a difference; the original code is pretty simple, so the compiler would probably do a better job of optimizing it than we would. – TartanLlama Aug 22 '16 at 11:31
It would be interesting to see if the compiler did collapse it to `!(nval & ~0xff)` – Bathsheba Aug 22 '16 at 11:32
The above version (when fixed) just gets a `cmpl` followed by a `setbe` from [GCC](https://godbolt.org/g/KS4GcZ). – TartanLlama Aug 22 '16 at 11:33

score 1 · Answer 2 · answered Aug 22 '16 at 14:01

Here is a similar but slightly cleaner trick:

enum Foo {
 zero = 0,
 one = 1,
 two = 2,
 reserved_low = 3,
 reserved_high = 255,
 foo_count
};

bool Assert(nval) {
  if(nval > zero && nval < foo_count) {
    return true;
  }
  return false;
}

Because enum increments each entry with 1, count will now be set to value 256. If you would add extra valid values later on count will automatically be updated.

How to assert whether a value is within an enums range

2 Answers2