C++11 type trait to differentiate between enum class and regular enum
I'm writing a promotion template alias similar to boost::promote but for C++11. The purpose of this is to avoid warnings when retrieving arguments from varidic functions. eg
template <typename T>
std::vector<T> MakeArgVectorV(int aArgCount, va_list aArgList)
{
std::vector<T> args;
while (aArgCount > 0)
{
args.push_back(static_cast<T>(va_arg(aArgList, Promote<T>)));
--aArgCount;
}
return args;
}
The Promote template alias promotes the type following the default argument promotion for variadic arguments: 1) An integer that's smaller than an int is promoted to int 2) A float is promoted to double
My problem is that a standard C++ enum can be promoted but a C++11 enum class is not promoted (compiler does not generate a warning). I want Promote to work with a regular enum but ignore a C++11 enum class.
How can I tell the difference between an enum class and an enum in my Promote template alias?
Here is a possible solution:
#include <type_traits>
template<typename E>
using is_scoped_enum = std::integral_constant<
bool,
std::is_enum<E>::value && !std::is_convertible<E, int>::value>;
The solution exploits a difference in behavior between scoped and unscoped enumerations specified in Paragraph 7.2/9 of the C++11 Standard:
The value of an enumerator or an object of an unscoped enumeration type is converted to an integer by integral promotion (4.5). [...] Note that this implicit enum to int conversion is not provided for a scoped enumeration. [...]
Here is a demonstration of how you would use it:
enum class E1 { };
enum E2 { };
struct X { };
int main()
{
// Will not fire
static_assert(is_scoped_enum<E1>::value, "Ouch!");
// Will fire
static_assert(is_scoped_enum<E2>::value, "Ouch!");
// Will fire
static_assert(is_scoped_enum<X>::value, "Ouch!");
}
And here is a live example.
ACKNOWLEDGEMENTS:
Thanks to Daniel Frey for pointing out that my previous approach would only work as long as there is no user-defined overload of operator + .
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