How solve compiler enum redeclaration conflict

问题

Consider the following C++ enumerations:

enum Identity
{
    UNKNOWN   = 1,
    CHECKED   = 2,
    UNCHECKED = 3
};

enum Status
{
    UNKNOWN    = 0,
    PENDING    = 1,
    APPROVED   = 2,
    UNAPPROVED = 3
};

The Compiler conflicted the both UNKNOWN items and threw this error:

error: redeclaration of 'UNKNOWN'

I am able to solve this error changing one of the UNKNOWN to UNKNOWN_a, but I would like to do not change the names.

How can I solve this conflict without changing the enum items name?

回答1:

You can use scoped enumerations for this. This requires C++11 or higher support.

enum class Identity
{
       UNKNOWN = 1,
       CHECKED = 2,
       UNCHECKED =3
};

enum class Status
{
       UNKNOWN = 0,
       PENDING = 1,
       APPROVED = 2,
       UNAPPROVED =3
};

int main ()
{
    Identity::UNKNOWN;
    Status::UNKNOW;
}

Live Example

回答2:

If using C++11 is not feasible(It really should by now, I mean, it's already 2015), consider using namespaces:

namespace Identity {
enum {
       UNKNOWN = 1,
       CHECKED = 2,
       UNCHECKED =3
};
}

namespace Status {
enum {
       UNKNOWN = 0,
       PENDING = 1,
       APPROVED = 2,
       UNAPPROVED =3
};
}

But, really, enum class is much better.

回答3:

Use scoped enums (C++ 11) - enum classes. They will not pollute the outer scope with duplicate names.

But, you'll need to access the enumerated values with a scope resolution operator - Identity::UNKNOWN, which is not a bad thing.

回答4:

This is how I would usually declare such enums (if I don't need something more fancy, like automatic conversions of the enum names to strings, serialization/deserialization etc.):

struct Identities
{
    enum Type
    {
        UNKNOWN   = 1,
        CHECKED   = 2,
        UNCHECKED = 3
    };
};

typedef Identities::Type Identity;

struct States
{
    enum Type
    {
        UNKNOWN    = 0,
        PENDING    = 1,
        APPROVED   = 2,
        UNAPPROVED = 3
    };
};

typedef States::Type Status;

// usage
Identity identity = Identities::UNKNOWN;
Status status = States::UNKNOWN;

Works in every C++ version and is type safe as well. Namespaces can be also used instead of structs (but I normally use structs).

来源：https://stackoverflow.com/questions/34319637/how-solve-compiler-enum-redeclaration-conflict

标签

c++

enums

compiler-errors

enumeration

redeclaration