How to check if enum value is valid?

Question

I am reading an enum value from a binary file and would like to check if the value is really part of the enum values. How can I do it?

#include <iostream>

enum Abc
{
    A = 4,
    B = 8,
    C = 12
};

int main()
{
    int v1 = 4;
    Abc v2 = static_cast< Abc >( v1 );

    switch ( v2 )
    {
        case A:
            std::cout<<"A"<<std::endl;
            break;
        case B:
            std::cout<<"B"<<std::endl;
            break;
        case C:
            std::cout<<"C"<<std::endl;
            break;
        default :
            std::cout<<"no match found"<<std::endl;
    }
}

Do I have to use the switch operator or is there a better way?

EDIT

I have enum values set and unfortunately I can not modify them. To make things worse, they are not continuous (their values goes 0, 75,76,80,85,90,95,100, etc.)

Answer 1

Maybe use enum like this:

enum MyEnum
{
A,
B,
C
};

and to check

if (v2 >= A && v2 <= C)

If you don't specify values for enum constants, the values start at zero and increase by one with each move down the list. For example, given enum MyEnumType { ALPHA, BETA, GAMMA }; ALPHA has a value of 0, BETA has a value of 1, and GAMMA has a value of 2.

Answer 2

enum value is valid in C++ if it falls in range [A, B], which is defined by the standard rule below. So in case of enum X { A = 1, B = 3 }, the value of 2 is considered a valid enum value.

Consider 7.2/6 of standard:

For an enumeration where emin is the smallest enumerator and emax is the largest, the values of the enumeration are the values of the underlying type in the range bmin to bmax, where bmin and bmax are, respectively, the smallest and largest values of the smallest bit-field that can store emin and emax. It is possible to define an enumeration that has values not defined by any of its enumerators.

There is no retrospection in C++. One approach to take is to list enum values in an array additionally and write a wrapper that would do conversion and possibly throw an exception on failure.

See Similar Question about how to cast int to enum for further details.

Answer 3

The only way I ever found to make it 'easy', was to create (macro) a sorted array of the enums and checking with that.

The switch trick fail with enums because an enum may have more than one enumerator with a given value.

It's an annoying issue, really.

Answer 4

Speaking about a language, there is no better way, the enum values exist compile time only and there is no way to enumerate them programatically. With a well thought infrastructure you may still be able to avoid listing all values several times, though. See Easy way to use variables of enum types as string in C?

Your sample can then be rewritten using the "enumFactory.h" provided there as:

#include "enumFactory.h"

#define ABC_ENUM(XX) \
    XX(A,=4) \
    XX(B,=8) \
    XX(C,=12) \

DECLARE_ENUM(Abc,ABC_ENUM)

int main()
{
    int v1 = 4;
    Abc v2 = static_cast< Abc >( v1 );

    #define CHECK_ENUM_CASE(name,assign) case name: std::cout<< #name <<std::endl; break;
    switch ( v2 )
    {
        ABC_ENUM(CHECK_ENUM_CASE)
        default :
            std::cout<<"no match found"<<std::endl;
    }
    #undef CHECK_ENUM_CASE
}

or even (using some more facilities already existing in that header):

#include "enumFactory.h"

#define ABC_ENUM(XX) \
    XX(A,=4) \
    XX(B,=8) \
    XX(C,=12) \

DECLARE_ENUM(Abc,ABC_ENUM)
DEFINE_ENUM(Abc,ABC_ENUM)

int main()
{
    int v1 = 4;
    Abc v2 = static_cast< Abc >( v1 );
    const char *name = GetString(v2);
    if (name[0]==0) name = "no match found";
    std::cout << name << std::endl;
}

Answer 5

In C++ 11 there is a better way if you are prepared to list your enum values as template parameters. You can look at this as a good thing, allowing you to accept subsets of the valid enum values in different contexts; often useful when parsing codes from external sources.

A possible useful addition to the example below would be some static assertions around the underlying type of EnumType relative to IntType to avoid truncation issues. Left as an exercise.

#include <stdio.h>

template<typename EnumType, EnumType... Values> class EnumCheck;

template<typename EnumType> class EnumCheck<EnumType>
{
public:
    template<typename IntType>
    static bool constexpr is_value(IntType) { return false; }
};

template<typename EnumType, EnumType V, EnumType... Next>
class EnumCheck<EnumType, V, Next...> : private EnumCheck<EnumType, Next...>
{
    using super = EnumCheck<EnumType, Next...>;

public:
    template<typename IntType>
    static bool constexpr is_value(IntType v)
    {
        return v == static_cast<IntType>(V) || super::is_value(v);
    }
};

enum class Test {
    A = 1,
    C = 3,
    E = 5
};

using TestCheck = EnumCheck<Test, Test::A, Test::C, Test::E>;

void check_value(int v)
{
    if (TestCheck::is_value(v))
        printf("%d is OK\n", v);
    else
        printf("%d is not OK\n", v);
}

int main()
{
    for (int i = 0; i < 10; ++i)
        check_value(i);
}

Answer 6

Yet another way to do it:

#include <algorithm>
#include <iterator>
#include <iostream>

template<typename>
struct enum_traits { static constexpr void* values = nullptr; };

namespace detail
{

template<typename T>
constexpr bool is_value_of(int, void*) { return false; }

template<typename T, typename U>
constexpr bool is_value_of(int v, U)
{
    using std::begin; using std::end;

    return std::find_if(begin(enum_traits<T>::values), end(enum_traits<T>::values),
        [=](auto value){ return value == static_cast<T>(v); }
    ) != end(enum_traits<T>::values);
}

}

template<typename T>
constexpr bool is_value_of(int v)
{ return detail::is_value_of<T>(v, decltype(enum_traits<T>::values) { }); }

////////////////////
enum Abc { A = 4, B = 8, C = 12 };

template<>
struct enum_traits<Abc> { static constexpr auto values = { A, B, C }; };
decltype(enum_traits<Abc>::values) enum_traits<Abc>::values;

enum class Def { D = 1, E = 3, F = 5 };

int main()
{
    std::cout << "Abc:";
    for(int i = 0; i < 10; ++i)
        if(is_value_of<Abc>(i)) std::cout << " " << i;
    std::cout << std::endl;

    std::cout << "Def:";
    for(int i = 0; i < 10; ++i)
        if(is_value_of<Def>(i)) std::cout << " " << i;
    std::cout << std::endl;

    return 0;
}

The "ugly" part of this approach IMHO is having to define:

decltype(enum_traits<Abc>::values) enum_traits<Abc>::values

If you are not opposed to macros, you can wrap it inside a macro:

#define REGISTER_ENUM_VALUES(name, ...) \
template<> struct enum_traits<name> { static constexpr auto values = { __VA_ARGS__ }; }; \
decltype(enum_traits<name>::values) enum_traits<name>::values;