Enable import/export of memory layouts

[j-stephan]

(Follow-up to a conversation we had today)

In a CMS presentation by @fwyzard we found an interesting use case: defining memory layouts outside of a C++ source code. In this example, the layout is pre-defined in an XML file, but other formats are certainly possible.

Example:

<class name="SomeSoAStruct">
    <field name="raw_mem" />
    <field name="bytes" />
    <field name="x" comment="[vector]" />
    <field name="y" comment="[vector]" />
    <field name="z" comment="[vector]" />
    <field name="a" comment="[scalar]" />
</class>

The import of the file should result in a corresponding LLAMA type. Would this be possible in LLAMA? And would an import/export functionality fit its scope?

[fwyzard]

To give some background on the example: that is (an abridged version of) the syntax that ROOT uses to declare dictionaries for persistent types: https://linux.die.net/man/1/genreflex .

[bernhardmgruber]

[...] defining memory layouts outside of a C++ source code. [...] The import of the file should result in a corresponding LLAMA type. Would this be possible in LLAMA?

This is not at all possible and will not be possible until C++ can read external files and meta-program based on these (for which there are proposals, see e.g. P2237 section 9). The scope of LLAMA is restricted by design to what is possible within standard C++.

However, since we can meta-program on LLAMA data structures, we could easily generate the XML structure you have shown. I could put together an example on how this could look like.