How to use enums as flags in C++?

Question

The "correct" way is to define bit operators for the enum, as:

enum AnimalFlags
{
    HasClaws   = 1,
    CanFly     = 2,
    EatsFish   = 4,
    Endangered = 8
};

inline AnimalFlags operator|(AnimalFlags a, AnimalFlags b)
{
    return static_cast<AnimalFlags>(static_cast<int>(a) | static_cast<int>(b));
}

Etc. rest of the bit operators. Modify as needed if the enum range exceeds int range.

Note (also a bit off topic): Another way to make unique flags can be done using a bit shift. I, myself, find this easier to read.

enum Flags
{
    A = 1 << 0, // binary 0001
    B = 1 << 1, // binary 0010
    C = 1 << 2, // binary 0100
    D = 1 << 3  // binary 1000
};

It can hold values up to an int so that is, most of the time, 32 flags which is clearly reflected in the shift amount.

For lazy people like me, here is templated solution to copy&paste:

template<class T> inline T operator~ (T a) { return (T)~(int)a; }
template<class T> inline T operator| (T a, T b) { return (T)((int)a | (int)b); }
template<class T> inline T operator& (T a, T b) { return (T)((int)a & (int)b); }
template<class T> inline T operator^ (T a, T b) { return (T)((int)a ^ (int)b); }
template<class T> inline T& operator|= (T& a, T b) { return (T&)((int&)a |= (int)b); }
template<class T> inline T& operator&= (T& a, T b) { return (T&)((int&)a &= (int)b); }
template<class T> inline T& operator^= (T& a, T b) { return (T&)((int&)a ^= (int)b); }

Note if you are working in Windows environment, there is a DEFINE_ENUM_FLAG_OPERATORS macro defined in winnt.h that does the job for you. So in this case, you can do this:

enum AnimalFlags
{
    HasClaws = 1,
    CanFly =2,
    EatsFish = 4,
    Endangered = 8
};
DEFINE_ENUM_FLAG_OPERATORS(AnimalFlags)

seahawk.flags = CanFly | EatsFish | Endangered;

What type is the seahawk.flags variable?

In standard C++, enumerations are not type-safe. They are effectively integers.

AnimalFlags should NOT be the type of your variable. Your variable should be int and the error will go away.

Putting hexadecimal values like some other people suggested is not needed. It makes no difference.

The enum values ARE of type int by default. So you can surely bitwise OR combine them and put them together and store the result in an int.

The enum type is a restricted subset of int whose value is one of its enumerated values. Hence, when you make some new value outside of that range, you can't assign it without casting to a variable of your enum type.

You can also change the enum value types if you'd like, but there is no point for this question.

EDIT: The poster said they were concerned with type safety and they don't want a value that should not exist inside the int type.

But it would be type unsafe to put a value outside of AnimalFlags's range inside a variable of type AnimalFlags.

There is a safe way to check for out of range values though inside the int type...

int iFlags = HasClaws | CanFly;
//InvalidAnimalFlagMaxValue-1 gives you a value of all the bits 
// smaller than itself set to 1
//This check makes sure that no other bits are set.
assert(iFlags & ~(InvalidAnimalFlagMaxValue-1) == 0);

enum AnimalFlags {
    HasClaws = 1,
    CanFly =2,
    EatsFish = 4,
    Endangered = 8,

    // put new enum values above here
    InvalidAnimalFlagMaxValue = 16
};

The above doesn't stop you from putting an invalid flag from a different enum that has the value 1,2,4, or 8 though.

If you want absolute type safety then you could simply create a std::set and store each flag inside there. It is not space efficient, but it is type safe and gives you the same ability as a bitflag int does.

C++0x note: Strongly typed enums

In C++0x you can finally have type safe enum values....

enum class AnimalFlags {
    CanFly = 2,
    HasClaws = 4
};

if(CanFly == 2) { }//Compiling error

I find the currently accepted answer by eidolon too dangerous. The compiler's optimizer might make assumptions about possible values in the enum and you might get garbage back with invalid values. And usually nobody wants to define all possible permutations in flags enums.

As Brian R. Bondy states below, if you're using C++11 (which everyone should, it's that good) you can now do this more easily with enum class:

enum class ObjectType : uint32_t
{
    ANIMAL = (1 << 0),
    VEGETABLE = (1 << 1),
    MINERAL = (1 << 2)
};


constexpr enum ObjectType operator |( const enum ObjectType selfValue, const enum ObjectType inValue )
{
    return (enum ObjectType)(uint32_t(selfValue) | uint32_t(inValue));
}

// ... add more operators here. 

This ensures a stable size and value range by specifying a type for the enum, inhibits automatic downcasting of enums to ints etc. by using enum class, and uses constexpr to ensure the code for the operators gets inlined and thus just as fast as regular numbers.

For people stuck with pre-11 C++ dialects

If I was stuck with a compiler that doesn't support C++11, I'd go with wrapping an int-type in a class that then permits only use of bitwise operators and the types from that enum to set its values:

template<class ENUM,class UNDERLYING=typename std::underlying_type<ENUM>::type>
class SafeEnum
{
public:
    SafeEnum() : mFlags(0) {}
    SafeEnum( ENUM singleFlag ) : mFlags(singleFlag) {}
    SafeEnum( const SafeEnum& original ) : mFlags(original.mFlags) {}

    SafeEnum&   operator |=( ENUM addValue )    { mFlags |= addValue; return *this; }
    SafeEnum    operator |( ENUM addValue )     { SafeEnum  result(*this); result |= addValue; return result; }
    SafeEnum&   operator &=( ENUM maskValue )   { mFlags &= maskValue; return *this; }
    SafeEnum    operator &( ENUM maskValue )    { SafeEnum  result(*this); result &= maskValue; return result; }
    SafeEnum    operator ~()    { SafeEnum  result(*this); result.mFlags = ~result.mFlags; return result; }
    explicit operator bool()                    { return mFlags != 0; }

protected:
    UNDERLYING  mFlags;
};

You can define this pretty much like a regular enum + typedef:

enum TFlags_
{
    EFlagsNone  = 0,
    EFlagOne    = (1 << 0),
    EFlagTwo    = (1 << 1),
    EFlagThree  = (1 << 2),
    EFlagFour   = (1 << 3)
};

typedef SafeEnum<enum TFlags_>  TFlags;

And usage is similar as well:

TFlags      myFlags;

myFlags |= EFlagTwo;
myFlags |= EFlagThree;

if( myFlags & EFlagTwo )
    std::cout << "flag 2 is set" << std::endl;
if( (myFlags & EFlagFour) == EFlagsNone )
    std::cout << "flag 4 is not set" << std::endl;

And you can also override the underlying type for binary-stable enums (like C++11's enum foo : type) using the second template parameter, i.e. typedef SafeEnum<enum TFlags_,uint8_t> TFlags;.

I marked the operator bool override with C++11's explicit keyword to prevent it from resulting in int conversions, as those could cause sets of flags to end up collapsed into 0 or 1 when writing them out. If you can't use C++11, leave that overload out and rewrite the first conditional in the example usage as (myFlags & EFlagTwo) == EFlagTwo.