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?
Answer
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!