Template for type safe c++11 enum class flags

Question

I am trying to create type safe C++ flags using templates. I also want to distinguish between a flag and flag s (being zero, one or many flags).

The solution below works good, except for EnumFlag<T> operator | (T, T) EnumFlag<T> operator | (T, T) which causes all | -operations on enums to return type EnumFlag . This breaks lots of code. Any tricks to fix this? In my code I do the following, however hard coding Option here is not an option. How to make this generic?

EnumFlag<typename std::enable_if<std::is_same<T, Option>::value, T>::type> operator | (T l, T r)

Changing this to...

EnumFlag<T> operator | (T l, T r)

...of cause breaks everything. I would like something like this (not compilabel code). Or any other better idea!

EnumFlag<typename std::enable_if<std::already_expanded<EnumFlag<T>>::value, T>::type> operator | (T l, T r)

Complete compilable code:

EnumFlag.h

#ifndef __classwith_flags_h_
#define __classwith_flags_h_

#include <type_traits>

enum class Option
{
    PrintHi = 1 << 0,
    PrintYo = 1 << 1,
    PrintAlot = 1 << 2
};

template <typename T>
class EnumFlag
{
public:
    using UnderlayingType = typename std::underlying_type<T>::type;

    EnumFlag(const T& flags)
        : m_flags(static_cast<UnderlayingType>(flags))
    {}

    bool operator & (T r) const
    {
        return 0 != (m_flags & static_cast<UnderlayingType>(r));
    }

    static const T NoFlag = static_cast<T>(0);

private:
    UnderlayingType  m_flags;
};
template<typename T>
EnumFlag<typename std::enable_if<std::is_same<T, Option>::value, T>::type> operator | (T l, T r)
{
    return static_cast<T>(static_cast<typename EnumFlag<T>::UnderlayingType>(l) | static_cast<typename EnumFlag<T>::UnderlayingType>(r));
}

class ClassWithFlags
{
public:
    using Options = EnumFlag < Option >;

    void doIt(const Options &options);
};

#endif

EnumFlag.cpp

#include "EnumFlag.h"

#include <iostream>

void ClassWithFlags::doIt(const Options &options)
{
    if (options & Option::PrintHi)
    {
        std::cout << "Hi" << std::endl;
    }
    if (options & Option::PrintYo)
    {
        std::cout << "Yo!" << std::endl;
    }
}

int main()
{
    ClassWithFlags classWithFlags;
    classWithFlags.doIt(Option::PrintHi | Option::PrintAlot);
}

> DEMO <

The actual code will contain a lot more operators, however this is enough to illustrate the problem.

One less intrusive solution is this (but still too intrusive)

template<typename T>
typename std::underlying_type<T>::type operator | (T l, T r)
{
    return (static_cast<typename std::underlying_type<T>::type>(l) | static_cast<typename std::underlying_type<T>::type>(r));
}

Not god enough, then EnumFlag(const std::underlying_type<T> &flags) must exist and I lose type safty. Also, I would like for the global operator overloads to only be created for the types actually needed. Macros is also no god because I want ta allow declaration of EnumFlag s inside classes. The global overloads can not be there, hence I need two macros calls at different locations to create on EnumFlag .

The solution must be pure C++11/stl.

Answer 1

Anthony Williams have a good acticle with ready code: "Using Enum Classes as Bitfields" .

Maybe it's just what youre looked for. In difference from other solutions which I saw, macroes aren't used here - just pure templates.

Simple example demonstrating his solution:

#include "bitmask_operators.hpp"

enum class A{
    x=1,y=2
       };

enum class B:unsigned long {
    x=0x80000000,y=0x40000000
        };

template<>
struct enable_bitmask_operators<A>{
    static const bool enable=true;
};

template<>
struct enable_bitmask_operators<B>{
    static const bool enable=true;
};

enum class C{x,y};


int main(){
    A a1=A::x | A::y;
    A a2=a1&A::y;
    a2^=A::x;
    A a3=~a1;

    B b1=B::x | B::y;
    B b2=b1&B::y;
    b2^=B::x;
    B b3=~b1;

    // C c1=C::x | C::y;
    // C c2=c1&C::y;
    // c2^=C::x;
    // C c3=~c1;
}

bitmask_operators.hpp source code .

Answer 2

Here is how I would do it, hope you will find it useful.

The main problem is how to distinguish between enum flags and all other types. I would use an extra template type, eg flag_type , which would have a member value only for our enum flags:

template<typename T>
typename flag_type<T>::value operator | (T, T)
{
    /* implementation */
}

Default flag_type is empty

template<typename T>
struct flag_type {};

whereas for enum flags it will contain an EnumFlag type which is returned from operator| . Here is a macro which does it:

#define DECLARE_FLAG_TYPE(__type)                             \
template<>                                                    \
struct flag_type<__type> { using value = EnumFlag<__type>; }

Then, we can use it to define enum flags in both outer scope and in classes:

enum class Option
{
    One   = 1 << 0,
    Two   = 1 << 1,
    Three = 1 << 2
};
DECLARE_FLAG_TYPE(Option);

class Class
{
public:
    enum class Option
    {
        Four = 1 << 0,
        Five = 1 << 1
    };
};
DECLARE_FLAG_TYPE(Class::Option);

Answer 3

This solution doesn't require any additional macros\\using to enum class declaration at all:

enum class MyEnum { Value1 = 1 << 0, Value2 = 1 << 1 };
using MyEnums = flags<MyEnum>; // actually this line is not necessary

auto mask = Value1 | Value2; // set flags Value1 and Value 2
if (mask & Value2) { // if Value2 flag is set
    doSomething();
}

https://github.com/grisumbras/enum-flags