DEV Community

loading...

Modern C++: enum class|struct

Swastik Baranwal
A noob
・2 min read

Following my series from less known features of Modern C++.

I saw this post the other day and I agreed with all his reasoning why enum in TypeScript aren't safe. (Read this article for more information.)

To solve these problems C++ 11 introduces enum class or enum struct. If you are lazy to read that post then the old enum had the following problems:

  • Conventional enums implicitly convert to int, causing errors when someone does not want an enumeration to act as an integer.
  • Conventional enums export their enumerators to the surrounding scope, causing name clashes.
  • The underlying type of an enum cannot be specified, causing confusion, compatibility problems, and makes forward declaration impossible.

Syntax

enum [class|struct] [identifier] [:type] {list};

Both class and struct have the same meaning here (for no damn reason).

Usage

#include <iostream>

enum struct Num {One, Two, Three, Four, Five}; // default type is int

int main() {

    Num one = Num::One; // using :: operator for scope values

 std::cout << static_cast<int>(one) << std::endl; // using static_cast to obtain   the numerical value

   auto match_enum = [](Num one) {
       switch(one) {

        case Num::One:
         std::cout << "Num is one" << std::endl;
        break;

        case Num::Two:
         std::cout << "Num is two" << std::endl;
        break;

        case Num::Three:
         std::cout << "Num is one" << std::endl;
        break;

        default:
         std::`ncout << "Invalid" << std::endl;
        break;
       }

    };

  match_enum(one); // Using Num::One

  match_enum(Num(1)); // Casting the value to Num enum

}

Just like enum the values should be of literal type. There will be name clashes if we have enum and enum class|struct of the same name.

enum V/S enum class|struct

  • enum elements are accessed by . and enum class|struct are accessed by ::.
  • There can be clashes with other enum whereas enum class|struct are scoped so there are no clashes.
  • enum can be converted into other enum but this isn't the case for enum struct|class.
#include <iostream>

enum class Day {Mon, Tues, Wed, Thur, Fri, Sat, Sun};

//enum Day {Mon, Tues, Wed, Thur, Fri, Sat, Sun}; // Error : Previous defination at line 2

enum Days {Mon, Tues, Wed, Thur, Fri, Sat, Sun};


int main() {

    Days day_scoped = Mon;
    int num = day_scoped; // Enum converted into int
    std::cout << num << std::endl;

    Day days;
    days = Day::Mon;
 //   int num2 = Day; // Error: Scoped Enum cannot be converted to int implicitly
    int num_scoped = static_cast<int>(days); // Valid: Scoped Enum converted to int explicitly
    std::cout << num_scoped << std::endl;


    return 0;
}

If we un-commented the some code above then the program will raise errors.

Code

All the code used in this article is available here.

If there is some mistake or confusion then please tell me by commenting below!

Discussion (0)