C++ 类中的枚举

Enums inside of a C++ class

提问by Blade3

I am trying to use a class that has an enum type declared inside the class like so:

我正在尝试使用一个类，该类在类中声明了一个枚举类型，如下所示：

class x {
public:
    x(int);
    x( const x &);
    virtual ~x();
    x & operator=(const x &);
    virtual double operator()() const;

    typedef enum  {
        LINEAR = 0,      /// Perform linear interpolation on the table
        DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
    } XEnumType; 
};

I need to declare an instance of this class and initialize the enum type. I come from C# and normally see enums declared OUTSIDE of a class, not INSIDE like it is here. How do I initialize the enum type. For example, I want to do something like this:

我需要声明这个类的一个实例并初始化枚举类型。我来自 C#，通常会看到在类的外部声明的枚举，而不是像这里一样的 INSIDE。如何初始化枚举类型。例如，我想做这样的事情：

x myX(10);   
myX.XEnumType = Linear;

Obviously this doesn't work. How would I do this?

显然这行不通。我该怎么做？

回答by Dan F

First you need to declare a variable that is of the type XEnumTypewithin your class Then you can access the actual enumeration values using the class name for scope: x::LINEARor x::DIPARABOLIC

首先，您需要声明一个XEnumType属于您的类中类型的变量，然后您可以使用作用域的类名访问实际的枚举值：x::LINEAR或x::DIPARABOLIC

class x{
 //Your other stuff

XEnumType myEnum;
};

int main(void)
{
    x myNewClass();
    x.myEnum = x::LINEAR;
}

回答by Johannes Schaub - litb

First: Don't use typedef. Instead, put the name of the enumeration in its head

第一：不要使用typedef. 相反，将枚举的名称放在其头部

enum XEnumType {
    LINEAR = 0,      /// Perform linear interpolation on the table
    DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
};

In a nutshell, doing like you did will behave mostlythe same, but in arcane corner cases will be different. The syntax you used will behave very different from the syntax I used above only in C.

简而言之，像你那样做的行为大致相同，但在神秘的角落情况下会有所不同。您使用的语法将与我上面仅在 C 中使用的语法非常不同。

Second: That just defines a type. But you want to define an object of that enumeration. Do so:

第二：那只是定义了一个类型。但是您想定义该枚举的对象。这样做：

XEnumType e;

In summary:

总之：

class x {
    /* ... stays the same ... */

    enum XEnumType {
        LINEAR = 0,      /// Perform linear interpolation on the table
        DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
    }; 

    XEnumType e;
};

void someFunction() {
    x myX(10);
    myX.e = x::LINEAR;
}

回答by mcorley

enum XEnumType {
    LINEAR, DIPARABOLIC
};

class x {    
    public:    
      x(int);    
      x( const x &);    
      virtual ~x();    
      x & operator=(const x &);    
      virtual double operator()() const;    
      XEnumType my_enum;
};

Usage:

用法：

x myX(10);
myX.my_enum = LINEAR;

回答by Heisenbug

You declared a new type : XEnumType. You have to create a field of that type inside x class. . For example:

您声明了一个新类型：XEnumType。您必须在 x 类中创建该类型的字段。. 例如：

class x {

public:

x(int);

x( const x &);

virtual ~x();

x & operator=(const x &);

virtual double operator()() const;

typedef enum  {
    LINEAR = 0,      /// Perform linear interpolation on the table
    DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
} XEnumType; 
public:
XenumType type;
};

Then you can access to it that way:

然后你可以通过这种方式访问​​它：

x foo(10);
foo.type = LINEAR;

回答by Nim

the line

线

typedef enum  {
    LINEAR = 0,      /// Perform linear interpolation on the table
    DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
} XEnumType; 

defines a typecalled XEnumType, actually this is redundant anyway - prefer something like:

定义了一个名为的类型XEnumType，实际上这是多余的 - 更喜欢这样的：

enum XEnumType
{
  LINEAR = 0,      /// Perform linear interpolation on the table
  DIPARABOLIC = 1  /// Perform parabolic interpolation on the table
};

Now you need to define a member of this type in your class

现在你需要在你的类中定义一个这种类型的成员

XEnumType _eType;

In your constructor, then you can initialize to whatever

在你的构造函数中，然后你可以初始化为任何

x::x(int ) : _eType(x::LINEAR) {}

回答by Martin Hennings

Let me first assume some preconditions:

让我先假设一些先决条件：

• Class xis from a third-party library and thus cannot be changed.
• Class xdefines some integer constants with the help of an enum.
• Class xis supposed to be initialized with either one of the constants LINEARor DIPARABOLIC.

• 类x来自第三方库，因此无法更改。
• 类x在枚举的帮助下定义了一些整数常量。
• 类x应该使用常量之一LINEAR或DIPARABOLIC.

Your problem is that these constant values are declared within class x. So to initialize an instance of xyou need to specify the scope:

您的问题是这些常量值是在 class 中声明的x。所以要初始化一个x你需要指定范围的实例：

Instead of

代替

x myX(10);   
myX.XEnumType = Linear;

try

尝试

x myX(x::LINEAR);

By specifying x::you provide the scope of the constant.

通过指定x::您提供常量的范围。