using enums : feature from C++ 20

C++ 20

In this article, we are going to learn about the C++ 20 feature of using enums for scoped or strongly typed enums. This feature allows programmers to short hand the syntax for the code using the enums.

In C++ 20 , we got this new improvement for the enums that we can use the using keyword with enums in a given scope. This will help programmers to write the enums values in a shorter syntax, instead of writing the full enum syntax with scope resolution
operator ::.

Let us see this with the help of below example.

Before C++ 20 improvements:

Bring an enum’s members into scope to improve readability. 

enum class Number { One, Two, Three, Four };

int string_to_int(Number Num) {
  switch (Num) {
    case Number::One:   return 1;
    case Number::Two:   return 2;
    case Number::Three:  return 3;
    case Number::Four:   return 4;
  }
}

After C++ 20 improvements:

enum class Number { One, Two, Three, Four };

int string_to_int(Number Num) {
  switch (Num) {
   using enum Number;
    case Number::One:   return 1;
    case Number::Two:   return 2;
    case Number::Three:  return 3;
    case Number::Four:   return 4;
  }
}

An example with Structures:

enum class Number { One, Two, Three, Four };

struct PrimeNumbers {
  using enum Number;             
};
int main() 
{
  PrimeNumbers No;
  No.One;                     
  No::One;                    
}

An example with Classes:

class PrimeNumbers {
				enum class Number { 
				One, 
				Two, 
				Three, 
				Four 
				};
    using enum Number;
};

int main()
{
    PrimeNumbers::Number No = PrimeNumbers::One;
}

Another example:

enum class Number { One, Two, Three, Four };
int main() {
    using Number::One, Number::Three;
    auto value = One;
    return value != Three;
}

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