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};

我想创建一个枚举

Days day = Days.Saturday;

现在我想用现有的枚举检查我的变量或实例 value，所以我这样做：

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";
}

这给我一个编译错误：

错误：'。'之前的预期主表达式

error: expected primary-expression before ‘.’ token

清除，说明之间有什么区别：

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

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

和

if (day == Saturday)

？

这两个实际上指的是什么，一个是OK，一个会导致编译错误？

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

推荐答案

是错误的：

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

因为天不是范围，目的。它是一种类型。类型本身没有成员。你写的是相当于 std :: string.clear 。 std :: string 是一个类型，因此不能使用。。您在类的实例上使用。

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，枚举是不可思议的所以类比停在那里。因为有了一个类，你可以通过 std :: string :: clear 来获取成员函数的指针，但在C ++ 03中， Days ::星期天无效。 （这是悲伤）。这是因为C ++（有点）向后兼容C，C没有命名空间，因此枚举必须在全局命名空间中。所以语法简单：

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)

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