如何在 C++ 中将 typedef 枚举转换为使用枚举

Question

I'm trying to explore "using" in C++, and currently I would like to convert typedef into using.

For example,

typedef enum tag_EnumType : int {
    A,
    B,
    C,
} EnumType;

And I tried to convert it as following

using EnumType = enum tag_EnumType : int {
    A,
    B,
    C,
};

However, the compilation failed. Could anyone help? Thanks in advance.

The truth is, I'm working on a giant project. Some of typedef enum could be rewritten as using, but others couldn't. Does anyone know the reason? Is it related with namespace or sth?

Answer 1

typedef enum is a C way of creating enums. If you want it the C++ way, you better write:

 enum EnumType : int {
    A,
    B,
    C,
};

In C++11 and above scoped enums are also available, that way you prevent name collisions.

 enum class EnumType : int {
    A,
    B,
    C,
};

Using statements are for other typedefs, some examples:

using IntVector = std::vector<int>;
using Iterator = IntVector::Iterator;
using FuncPtr = int(*)(char, double);

Answer 2

As with the typedef struct construction common in C, it is simply not needed in C++. Tag names and typedef names are not in separate namespaces.

All you need is

enum EnumType : int {
    A,
    B,
    C,
};

and the name EnumType will refer to the enum without need of prefixing enum , except if you want to disambiguate from non-type names.

---

using is normally used only to alias a type, not declare a new one, eg:

using EnumTypeAlias = EnumType;

Technically it is allowed to alias an unnamed struct or enum , but I see no reason for doing that:

using EnumTypeAlias = enum { A, B, C };

---

I would also generally advice to use scoped enums , which behave differently in some ways from C-style enums, see this question .