enum class as array index

Question

I made an enum as:

enum class KeyPressSurfaces {
    KEY_PRESS_SURFACE_DEFAULT,
    KEY_PRESS_SURFACE_UP,
    KEY_PRESS_SURFACE_DOWN,
    KEY_PRESS_SURFACE_LEFT,
    KEY_PRESS_SURFACE_RIGHT,
    KEY_PRESS_SURFACE_TOTAL
};

and later on I attempt to define an array as I typed below, but I received the error, size of array 'KEY_PRESS_SURFACES' has non-integral type 'KeyPressSurfaces'

SDL_Surface*KEY_PRESS_SURFACES[KeyPressSurfaces::KEY_PRESS_SURFACE_TOTAL];

I understand the error fine, but I don't know where to move the KeyPressSurfaces to qualify the constant in the enum.

I also realize I could just use an enum and not an enum class, but I feel like this should work, and I want to learn how to do this.

Any response/advice?

Answer 1

Scoped enums (enum class) are not implicitly convertible to integers. You need to use a static_cast:

SDL_Surface*KEY_PRESS_SURFACES[static_cast<int>(KeyPressSurfaces::KEY_PRESS_SURFACE_TOTAL)];

Answer 2

You can convert your enum to int using template function and you will get more readable code:

#include <iostream>
#include <string>
#include <typeinfo>

using namespace std;

enum class KeyPressSurfaces: int {
    KEY_PRESS_SURFACE_DEFAULT,
    KEY_PRESS_SURFACE_UP,
    KEY_PRESS_SURFACE_DOWN,
    KEY_PRESS_SURFACE_LEFT,
    KEY_PRESS_SURFACE_RIGHT,
    KEY_PRESS_SURFACE_TOTAL
};

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


int main() {
    KeyPressSurfaces val = KeyPressSurfaces::KEY_PRESS_SURFACE_UP;
    int valInt = to_underlying(val);
    std::cout << valInt << std::endl;
    return 0;
}

I fount to_underlying function here

Answer 3

Remove the class keyword or cast explicitly to an integral type.

Answer 4

Building on the other responses, another alternative is a simple templated class that wraps a c-style array. With the EnumArray example below, any enum class with a kMaxValue can be used as the index.

In my opinion, the improved readability is worth the introduction of a template.

template <class IndexType, class ValueType>
class EnumArray {
 public:  
  ValueType& operator[](IndexType i) { 
    return array_[static_cast<int>(i)];
  }

  const ValueType& operator[](IndexType i) const {
    return array_[static_cast<int>(i)];
  }

  int size() const { return size_; }

 private:
  ValueType array_[static_cast<int>(IndexType::kMaxValue) + 1];

  int size_ = static_cast<int>(IndexType::kMaxValue) + 1;
}; 

enum class KeyPressSurfaces {
    KEY_PRESS_SURFACE_DEFAULT,
    KEY_PRESS_SURFACE_UP,
    KEY_PRESS_SURFACE_DOWN,
    KEY_PRESS_SURFACE_LEFT,
    KEY_PRESS_SURFACE_RIGHT,
    KEY_PRESS_SURFACE_TOTAL,
    kMaxValue = KEY_PRESS_SURFACE_TOTAL
};

int main() {
    EnumArray<KeyPressSurfaces, int> array;
    array[KeyPressSurfaces::KEY_PRESS_SURFACE_DEFAULT] = 5;
    std::cout << array[KeyPressSurfaces::KEY_PRESS_SURFACE_DEFAULT] << std::endl;
    return 0;
}