Give a generic enum type as template argument

问题

In short:

Is there a way I can feed a General templated class with something that only represent an enum type? Something like:

template <typename T> struct General {};
struct EnumSpecific : General<any_enum_type> {};

<int> is too much / does not work in my case.

My specific case:

A templated Holder class handles any kind of data in a generic way.
An abstract General class implements specific algorithms relying on the Holders' behaviour.
Template specifications of General (like IntSpecific, DoubleSpecific, StringSpecific, MoreSophisticatedTypeSpecific ..) define how to deal with some concrete Holder types.
How can I properly define an EnumSpecific specification?

Here is boilt down code making my problem occur:

// A templated value holder:
template <typename T>
class Holder {
public:
    Holder(T const& t) : _value(t) {};
    // generic methods
    void generics() {};
    // methods concerning the value:
    void set(T const& t /*, setInfo */) {
        // .. check for an actual change, notify buddies of the change..
        _value = t;
    };
    T value(/*readInfo*/) {
        // .. do stuff depending on how / why the value is read..
        return _value;
    };
private:
    T _value;
};
// (in reality, all `generics` methods come from a parent, untemplated class)

// A generic process involving such `Holder`s:
template <typename T>
class General {
public:
    typedef bool /* or anything */ KnownReturnTypes;
    General(Holder<T>* const a
          , Holder<T>* const b)
        : _a(a)
        , _b(b)
    {};
    void methods() {
        // Use common behavior of all `Holder`'s
        _a->generics();
        // .. or methods that rely on the actual values:
        KnownReturnTypes knr( valuedMethods() );
        if (knr) {} else {}
        // ...
    };
    // Use polymorphism to adapt to each situation..
    virtual KnownReturnTypes valuedMethods() = 0;
protected:
    Holder<T>* _a;
    Holder<T>* _b;
};

// Example of specialization for integral types (there might be others)
class IntSpecific : General<int> {
public:
    IntSpecific(Holder<int>* const a
              , Holder<int>* const b)
        : General<int>(a, b)
    {};
    // implement the valuedMethods:
    virtual KnownReturnTypes valuedMethods() {
        return _a->value() > _b->value(); // dummy
    }
};

// Specialization for enum types:
// * * * class EnumSpecific : General<any_enum_type> { // does not exist * *
class EnumSpecific : General<int> {
public:
    EnumSpecific( Holder<int>* const a
                , Holder<int>* const b)
        : General<int>(a, b)
    {};
    // only use properties and methods offered by an enum type:
    virtual KnownReturnTypes valuedMethods() {
        return _a->value() == _b->value(); // dummy
    }
};

// One particular case
typedef enum {One, Two, Three} Enum;
typedef Holder<Enum> EnumHolder;


int main() {

    // Check that `IntSpecific` works fine.
    Holder<int>* i( new Holder<int>(3) );
    Holder<int>* j( new Holder<int>(5) );
    IntSpecific is(i, j); // ok.

    // Try the `EnumSpecific`
    EnumHolder* a( new EnumHolder { One } );
    EnumHolder* b( new EnumHolder { Two } );
    EnumSpecific es(static_cast<Holder<int>*>(a)    // invalid cast
                  , static_cast<Holder<Enum>*>(b)); // unexpected type
    // This is because the compiler doesn't know enough about what
    // EnumSpecific actually *is*. How to tell him more about it?


    return EXIT_SUCCESS;
}

What should I feed the template argument with in EnumSpecific : General<??> to make things clear for the compiler?

Do I need to use some kind of enum_type concept and more sophisticated tools from generic programming?

回答1:

We can accomplish this with std::enable_if and std::is_enum. As a sample this is a class that will take an enum type as a template parameter.

#include <type_traits>
enum Enum { FOO, BAR};

template<typename T, typename std::enable_if<std::is_enum<T>::value>::type* = nullptr>
class Test {};

int main()
{
    Test<Enum> a; // ok
    Test<double> b; // error
}

来源：https://stackoverflow.com/questions/32316875/give-a-generic-enum-type-as-template-argument

标签

c++

templates

generics

enums

concept