C++ – How to Convert Strongly Typed Enum to Int Automatically

c++c++11strongly-typed-enum

#include <iostream>

struct a {
  enum LOCAL_A { A1, A2 };
};
enum class b { B1, B2 };

int foo(int input) { return input; }

int main(void) {
  std::cout << foo(a::A1) << std::endl;
  std::cout << foo(static_cast<int>(b::B2)) << std::endl;
}

The a::LOCAL_A is what the strongly typed enum is trying to achieve, but there is a small difference : normal enums can be converted into integer type, while strongly typed enums can not do it without a cast.

So, is there a way to convert a strongly typed enum value into an integer type without a cast? If yes, how?

Best Answer

As others have said, you can't have an implicit conversion, and that's by-design.

If you want you can avoid the need to specify the underlying type in the cast.

template <typename E>
constexpr typename std::underlying_type<E>::type to_underlying(E e) noexcept {
    return static_cast<typename std::underlying_type<E>::type>(e);
}

std::cout << foo(to_underlying(b::B2)) << std::endl;

Related Solutions

C++ – Conversion from int to Enum Class Type

Yes, you can convert both ways: int to enum class and enum class to int. This example should be self explanatory:

enum class Color{Red = 1, Yellow = 2, Green = 3, Blue = 4};
std::cout << static_cast<int>(Color::Green) << std::endl; // 3
// more flexible static_cast - See Tony's comment below
std::cout << static_cast<std::underlying_type_t<Color>>(Color::Green) << std::endl; // 3
std::cout << (Color::Green == static_cast<Color>(3)) << std::endl; // 1
std::cout << (Color::Green == static_cast<Color>(2)) << std::endl; // 0

You can try it yourself here.

[EDIT] Since C++23, we'll have available std::to_underlying (at <utility>), what will allow us to write:

std::cout << std::to_underlying(Color::Green) << std::endl; // 3

Best Answer

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C++ – Conversion from int to Enum Class Type

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