For example:
enum class MyEnum { A, B };
static_assert(A == 0 && B == 1); // error: expected constructor, destructor, or type conversion before '(' token
How do I achieve this?
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2You're also missing the string literal part.Qaz– Qaz2013-10-16 15:50:12 +00:00Commented Oct 16, 2013 at 15:50
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5 Answers
The whole purpose of adding enum class to the language was to make enumeration strongly typed and scoped. This means:
You cannot use just
Awithout qualification.You have to useMyEnum::A.You cannot treat it like
int— strongly-typed enum cannot be compared with integral type without explicit cast.
So you have to do something like this:
static_assert(to_integral(MyEnum::A)== 0 && to_integral(MyEnum::B)==1,
"your message");
And take the implementation of to_integral from this answer : it is a generic implementation, so you don't have to assume or figure out what the underlying type of MyEnum is.
Alternatively, you can define operator== for MyEnum. Make sure it is constexpr:
constexpr bool operator==(MyEnum x, int y) { return to_integral(x) == y; }
constexpr bool operator==(int x, MyEnum y) { return y == x; }
Now you can write this:
static_assert(MyEnum::A== 0 && MyEnum::B ==1, "your message");
Just for the sake of completeness, I copy-paste the implementation of to_integral from my other answer:
#include <type_traits> //must include it
template<typename E>
constexpr auto to_integral(E e) -> typename std::underlying_type<E>::type
{
return static_cast<typename std::underlying_type<E>::type>(e);
}
Hope that helps.
Comments
Your code have two problems:
static_asserttakes two arguments- You can't compare
Aand0, because:MyEnumis not justenum, butclass enum. So, firstly, you have to writeMyEnum::A- You can't compare
MyEnum::Awith0, becauseMyEnumis a strongly-typed enum. You have to cast it tointbefore comparing.
But if you are using strongly typed enumerations, I guess, you don't need to compare it with ints.
Comments
Try to access through the enum, and cast the enum values:
enum class MyEnum { A, B };
static_assert((int)MyEnum::A == 0 && (int)MyEnum::B == 1, "message");
Comments
You have four problems. You need
enum class MyEnum { A, B };
static_assert(MyEnum::A == MyEnum(0) && MyEnum::B == MyEnum(1), "invalid values");
which fixes
- the scope of the enum values (
Avs.MyEnum::A) - converts the integer values to enums correctly so the comparison is valid
- adds the missing error message for
static_assert
What's the fourth problem?
You need to compile in C++11 mode (this is what the others are missing!). Add -std=c++11 to the compiler's command line. How do I know? Because of the error message you got, this only happens when compiling in C++03 mode.
Comments
static_assert takes two arguments, the second being a string to output if the assertion fails.
Should be something like,
static_assert(A == 0 && B == 1, "Enum values are not as expected");
