Enum in Cpp and Python

Enum is very commonly used both in cpp and python. In this blog, we will introduce enumeration in cpp and python.

Cpp
- Unscoped enumeration
- Scoped enumeration
Python
Reference

Cpp

Unscoped enumeration

Old style of enumeration declaration does not have the concept of scope which means it is globally accessed.

#include <iostream>
using namespace std;

enum FileType {
    File = 0,
    Directory = 1,
    Link = 2,
    FileTypeCount,
};

enum Access {
    Read = 1,
    Write = 2,
    Exec = 4,
};

int main() {
    cout << FileTypeCount << endl;
    cout << Read << " " << (Read == 1) << endl;
    return 0;
}

Scoped enumeration

We add additional keyword class or struct after enum.

#include <iostream>
using namespace std;

enum FileType {
    File = 0,
    Directory = 1,
    Link = 2,
    FileTypeCount,
};

enum Access {
    Read = 1,
    Write = 2,
    Exec = 4,
};

enum class NewAccess {
    Read = 8,
    Write = 16,
    Exec = 32,
};

ostream& operator<<(ostream& os, NewAccess na) {
    switch (na) {
        case NewAccess::Read: cout << "Read" << endl; break;
        case NewAccess::Write: cout << "Write" << endl; break;
        case NewAccess::Exec: cout << "Exec" << endl; break;
    }
    return os;
}


int main() {
    cout << FileTypeCount << endl;
    cout << Read << " " << (Read == 1) << endl;
    cout << NewAccess::Read << endl;
    // No unexplicit conversion here
    cout << (static_cast<int>(NewAccess::Read) == 8) << endl;
    return 0;
}

Python

Normally in python, I will use the class variable as an enumeration.

class Access:
    Read = 1
    Write = 2
    Exec = 4

But if we want get the name of the enumeration, we have to add them ourselves.

from collections import namedtuple
class Item(namedtuple("Item", "name value")):
    pass

class Access:
    Read = Item("Read", 1)
    Write = Item("Write", 2)
    Exec = Item("Exec", 4)

In Python 3, we have a new module named enum.

from enum import Enum

class Access(Enum):
    Read = 1
    Write = 2
    Exec = 4

print(Access.Read.name)
print(Access.Read.value)

Permission = Enum("Permission", "Allow Decline")

There are unique and auto in enum.

unique: a decorator to make sure not conflict values
auto: automatically generate values for each enumeration

An enum is iterable and each enumeration item can be used as the key of a dict. By default an enum is not comparable besides is and is not. A subclass IntEnum supprt

Cpp

Unscoped enumeration

Scoped enumeration

Python

Reference