[odb-users] Can I use C++ 17 std::optional members with ODB?

[Boris Kolpackov]

Mann, David W (B62) CTR USN NAVSURFWARCEN DAH VA (USA) <david.w.mann5.ctr at us.navy.mil> writes:

> std::optional<double> m_testDouble;
> 
> The error was error: unable to map C++ type '::std::optional< double >' used
> in data member 'm_testDouble' to a SQLite database type.
> 
> Is there a way I can work around this?

Yes, we should actually add built-in support for this (I will try to
add it before the next 2.5.0 pre-release).

In the meantime, you can place the following specialization into a
header, say, odb-optional-traits.hxx:

------------------------------------------------------------------
#pragma once

#include <optional>
#include <type_traits>

#include <odb/wrapper-traits.hxx>

namespace odb
{
  template <typename T>
  class wrapper_traits<std::optional<T>>
  {
  public:
    using wrapped_type = T;
    using wrapper_type = std::optional<T>;

    // T can be const.
    //
    using unrestricted_wrapped_type = typename std::remove_const<T>::type;

    static const bool null_handler = true;
    static const bool null_default = true;

    static bool
    get_null (const wrapper_type& o)
    {
      return !o;
    }

    static void
    set_null (wrapper_type& o)
    {
      o = wrapper_type ();
    }

    static const wrapped_type&
    get_ref (const wrapper_type& o)
    {
      return *o;
    }

    static unrestricted_wrapped_type&
    set_ref (wrapper_type& o)
    {
      if (!o)
        o = unrestricted_wrapped_type ();

      return const_cast<unrestricted_wrapped_type&> (*o);
    }
  };
}
------------------------------------------------------------------

And then add the following two ODB compiler options when compiling
your headers:

--odb-epilogue '#include "odb-optional-traits.hxx"'
--hxx-prologue '#include "odb-optional-traits.hxx"'