How to use enums in C++

Suppose we have an enum like the following:

enum Days {Saturday, Sunday, Tuesday, Wednesday, Thursday, Friday};

I want to create an instance of this enum and initialize it with a proper value, so I do:

Days day = Days.Saturday;

Now I want to check my variable or instance with an existing enum value, so I do:

if (day == Days.Saturday)
{
    std::cout << "Ok its Saturday";
}

Which gives me a compilation error:

error: expected primary-expression before ‘.’ token

So to be clear, what is the difference between saying:

if (day == Days.Saturday) // Causes compilation error

and

if (day == Saturday)

?

What do these two actually refer to, in that one is OK and one causes a compilation error?

This code is wrong:

enum Days {Saturday, Sunday, Tuesday, Wednesday, Thursday, Friday};
Days day = Days.Saturday;
if (day == Days.Saturday)

Because Days is not a scope, nor object. It is a type. And Types themselves don't have members. What you wrote is the equivalent to std::string.clear. std::string is a type, so you can't use . on it. You use . on an instance of a class.

Unfortunately, enums are magical and so the analogy stops there. Because with a class, you can do std::string::clear to get a pointer to the member function, but in C++03, Days::Sunday is invalid. (Which is sad). This is because C++ is (somewhat) backwards compatable with C, and C had no namespaces, so enumerations had to be in the global namespace. So the syntax is simply:

enum Days {Saturday, Sunday, Tuesday, Wednesday, Thursday, Friday};
Days day = Saturday;
if (day == Saturday)

Fortunately, Mike Seymour observes that this has been addressed in C++11. Change enum to enum class and it gets its own scope; so Days::Sunday is not only valid, but is the only way to access Sunday. Happy days!

This will be sufficient to declare your enum variable and compare it:

enum Days {Saturday, Sunday, Tuesday, Wednesday, Thursday, Friday};
Days day = Saturday;
if (day == Saturday) {
    std::cout << "Ok its Saturday";
}

Much of this should give you compilation errors.

// note the lower case enum keyword
enum Days { Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday };

Now, Saturday, Sunday, etc. can be used as top-level bare constants,and Days can be used as a type:

Days day = Saturday;   // Days.Saturday is an error

And similarly later, to test:

if (day == Saturday)
    // ...

These enum values are like bare constants - they're un-scoped - with a little extra help from the compiler: (unless you're using C++11 enum classes) they aren't encapsulated like object or structure members for instance, and you can't refer to them as members of Days.

You'll have what you're looking for with C++11, which introduces an enum class:

enum class Days
{
    SUNDAY,
    MONDAY,
    // ... etc.
}

// ...

if (day == Days::SUNDAY)
    // ...

Note that this C++ is a little different from C in a couple of ways, one is that C requires the use of the enum keyword when declaring a variable:

// day declaration in C:
enum Days day = Saturday;

You can use a trick to use scopes as you wish, just declare enum in such way:

struct Days 
{
   enum type
   {
      Saturday,Sunday,Tuesday,Wednesday,Thursday,Friday
   };
};

Days::type day = Days::Saturday;
if (day == Days::Saturday)