According to Meyers, scoped enums are better than unscoped enums. Unscoped enums are C++98-style enums. Below is an example from OpenFoam.

//src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinReader.H
enum phase
{
    solid,
    liquid,
    gas
};

why is this ineffective? why is this unscoped? Normally, scope is contained within {}. But not in this case. solid, liquid, gas are not contained within those braces. They overflow and pollute the namespace of the class. That’s why I can’t have statement like the following.

bool solid = false;

I should be expecting the following error.

chemistryReaders/chemkinReader/chemkinReader.H:81:26: error: 'bool Foam::chemkinReader::solid' conflicts with a previous declaration
   81 |             bool solid = false;

Contain the scope

The magic keyword class will make the enum a scoped enum.

//src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinReader.H
enum enum phase
{
    solid,
    liquid,
    gas
};

This lets me to use the variables solid, liquid, gas in the namespace where phase lives.

Strongly type

unscoped enums could easily be passed off as ints and floats. But not anymore. One has to explicity cast a scoped enum to use it in the any other context. This can stop a lot of unforeseen errors.

References

Meyers, S. (2014). Effective Modern C++: 42 Specific Ways to Improve Your Use of C++11 and C++14. O’Reilly Media.