如何在C ++中使用枚举 [英] How to use enums in C++

问题描述

假设我们有一个枚举，如下所示：

Suppose we have an enum like the following:

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

我想创建一个这个枚举的实例并用适当的值初始化它，所以我做：

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

Days day = Days.Saturday;

现在我想使用现有的枚举检查我的变量或实例 value，所以我做：

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";
}

哪个给我一个编译错误：

Which gives me a compilation error:

错误：'。'标记之前的预期主表达式

error: expected primary-expression before ‘.’ token

所以要说清楚，说：

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

和

if (day == Saturday)

？

这两个实际上是指什么，一个是OK，一个导致编译错误？

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

推荐答案

错误：

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

由于天不是范围，目的。这是一种类型。而类型本身没有成员。你写的是相当于 std :: string.clear 。 std :: string 是一种类型，因此您不能使用。。您在课程的实例上使用。。

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.

不幸的是，枚举是神奇的所以类比在那里停止。因为有了一个类，你可以执行 std :: string :: clear 来获取一个指向成员函数的指针，但是在C ++ 03中，天::星期日无效。 （这是悲伤的）。这是因为C ++向后兼容C，C没有命名空间，所以枚举必须在全局命名空间中。所以语法很简单：

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)

幸运的是，西摩Seymour 观察到这已经在C ++ 11中得到解决。将枚举更改为枚举类，并获得自己的范围;所以 Days :: Sunday 不仅是有效的，而是只有访问星期日。快乐的时光！

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!

这篇关于如何在C ++中使用枚举的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！