ArrayBuilder (C++) - How to go back / move the index

[lobis]

I'm using ArrayBuilder on some C++ code to build awkward arrays.

I'm struggling to go back on an append / begin_list call. From some offline conversation with @jpivarski I thought it should be possible to "move the pointer / index" back and keep building (without any overhead, because you are not really deleting anything) but I'm not finding how to do this.

template<class PRIMITIVE>
using NumpyBuilder = awkward::LayoutBuilder::Numpy<PRIMITIVE>;

template<class PRIMITIVE, class BUILDER>
using ListOffsetBuilder = awkward::LayoutBuilder::ListOffset<PRIMITIVE, BUILDER>;

ListOffsetBuilder<int, NumpyBuilder<int>> builder;

builder.begin_list();
builder.append(1);
builder.append(2);
builder.being_list();

// I no longer want this list and wish to keep on building

Thanks!

[agoose77]

@lobis I never spent a huge amount of time thinking about LayoutBuilder, but AFAICR we don't implement an "undo". It wouldn't be that hard to do (I don't think). Is there a reason you can't avoid the list altogether?

[jpivarski]

Oh, yeah—this came out of a conversation that @lobis and I had a few days ago. LayoutBuilders don't have an undo, but easily could. (Same for ArrayBuilder, but this is for a LayoutBuilder application that wants to produce arrays without large entries, without knowing ahead of time whether the next entry will be large.)