Stackoverflow 精彩答案搬运之 Enum类型

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How to use Enums in C++

Problem:

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)

Tips:

Fortunately, your complaint has been addressedin 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!

翻译就算了，因为真的太直白了，如果看不明白可以用划词翻译软件对照着看。