Is it possible to create a boost::mpl::list_c from a variadic enum using static_cast?

Question

The following code does not compile:

#include <boost/mpl/list_c.hpp>
enum class DataType : int {
  t1 = 0,
  t2 = 1,
  t3 = 2
};
int main()
{
  boost::mpl::list_c<DataType, DataType::t1, DataType::t2> lc2;
}

I have to cast my values, like this (which compiles):

#include <boost/mpl/list_c.hpp>
enum class DataType : int {
  t1 = 0,
  t2 = 1,
  t3 = 2
};
int main()
{
  boost::mpl::list_c<int, static_cast<int>(DataType::t1), static_cast<int>(DataType::t2)> lc2;
}

I can also create the list from a variadic list of types, like this (also compiles):

#include <boost/mpl/list_c.hpp>
template<int... ints>
void create_list() {
  boost::mpl::list_c<int, ints...> lc;
}
int main()
{
  create_list<1,2,3>();
}

However, how do I create the list from a variadic list of types like below (doesn't compile):

#include <boost/mpl/list_c.hpp>
enum class DataType : unsigned char {
  t1 = 0,
  t2 = 1,
  t3 = 2
};
template<DataType... datatypes>
void create_list() {
  boost::mpl::list_c<DataType, datatypes...> lc;
}
int main()
{
  create_list<DataType::t1, DataType::t2>();
}

This gives the following compiler error:

 In instantiation of 'void create_list() [with DataType ...datatypes = {(DataType)0u, (DataType)1u}]':
13:43:   required from here
9:46: error: could not convert template argument '(DataType)0u' to 'long int'
9:46: error: could not convert template argument '(DataType)1u' to 'long int'

I also cannot cast when calling the create_list function, like this:

create_list<static_cast<unsigned char>(DataType::t1), static_cast<unsigned char>(DataType::t2)>();

since, my variadic type list has been passed by my client.

There is no implicit conversion from `enum class` to its underlying type: http://stackoverflow.com/a/8357366/218774 — J. Calleja, May 17 '17 at 21:33
That wasn't the point of the question. I could have been clearer in hindsight. — Jibbity jobby, May 17 '17 at 22:49

Jibbity jobby · Answer 1 · 2017-05-17T20:15:00.487

0

Got it:

#include <boost/mpl/list_c.hpp>
enum class DataType : unsigned char {
  t1 = 0,
  t2 = 1,
  t3 = 2
};
template<DataType... datatypes>
void create_list() {
  boost::mpl::list_c<unsigned char, static_cast<unsigned char>(datatypes)...> lc;
}
int main()
{
  create_list<DataType::t1, DataType::t2>();
}

edited May 17 '17 at 20:15

answered May 17 '17 at 18:51

Jibbity jobby

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score 0 · Answer 2 · answered May 17 '17 at 19:01

0

A list_c is a convenience builder for a sequence of integral_c

ref: http://www.boost.org/doc/libs/1_64_0/libs/mpl/doc/refmanual/list-c.html

an integral_c<T, TN> is documented as being supported where T is an integral type

An enum class is not an integral type. I think that ends the discussion.

ref: http://www.boost.org/doc/libs/1_64_0/libs/mpl/doc/refmanual/integral-c.html

answered May 17 '17 at 19:01

Richard Hodges

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@PeterBingham I did. Please read the documentation again. Your answer casts the enum to an integral type and therefore makes your code conformant with the template's design. – Richard Hodges May 17 '17 at 21:32
I've clarified the question title. – Jibbity jobby May 17 '17 at 21:46

Is it possible to create a boost::mpl::list_c from a variadic enum using static_cast?

2 Answers2