boost::variant limitation to 20 elements because of mpl limitations

[cppljevans]

The comment here:

https://github.com/BoostGSoC17/dataframe/blob/master/include/df.cpp#L44

mentions a problem with boost::variant which may be solved by the implementation
mentioned in this thread:

https://groups.google.com/forum/#!msg/boost-developers-archive/35ghGGPePuM/4W2hlvFRDgAJ

In addition, there's another workaround that uses multiple inheritance and std::aligned_union.
(Actually, I've no looked at the method in boost-developers-archive; so, it may be
the same implementation as I was thinking of).

[cppljevans]

The variant workaround mentioned above is found here:

https://github.com/cppljevans/composite_storage/blob/master/MapAsVariant.cpp

If you study that "long enough", you might see a simpler implementation of:

allowed_inner_types Type()

HINT. Instead of:

https://github.com/cppljevans/composite_storage/blob/master/MapAsVariant.cpp#L112-#L139

add a get_at_val with the template args reversed. The get_at_val()
would return the Key at the given ValueType, and from that, you could get the needed
allowed_inner_type enumerator. WARNING: not tested.

Or, I think maybe:

https://github.com/cppljevans/composite_storage/blob/master/MapAsVariant.cpp#L108

does something similar and you might find useful.

HTH.

[RArora28]

Hi,
Thank you for the suggestion, I will talk to my mentor about the same.