mpgen: support primitive std::optional struct fields

ryanofsky commented at 10:27 pm on February 20, 2026: collaborator

Currently C++ structs with primitive std::optional members (ints, bools, floats) cannot easily by mapped to Cap’n Proto structs because Cap’n Proto does not provide a way to leave primitive fields unset, so there isn’t a natural way to represent std::nullopt values. This PR makes it possible to map C++ structs with fields like:

0    std::optional<int> foo;

to Cap’n Proto structs by using extra Bool fields prefixed with “has” for primitive optional members:

0    foo [@3](/bitcoin-core-multiprocess/contributor/3/) :Int32;
1    hasFoo [@4](/bitcoin-core-multiprocess/contributor/4/) :Bool;

Boolean “has” fields were already supported by the code generator and used to pass primitive std::optional parameters and return values, so this PR just extends it work with all struct fields, not just fields in params and result structs.

Note: Motivation for this change is dealing with the CreatedTransactionResult::change_pos field introduced to the wallet interface in bitcoin-core/gui#807. This also could have been useful in https://github.com/bitcoin/bitcoin/pull/33965#issuecomment-3937522269

mpgen refactor: Move field handling code to FieldList class

This is a move-only change that should be easy to review with --color-moved. No
behavior is changing.

5a52ea01a9

DrahtBot commented at 10:27 pm on February 20, 2026: none

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

#242 (proxy-types: add CustomHasField hook to map Cap’n Proto values to null C++ values by Sjors)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

mpgen refactor: add AccessorType function

This doesn't change generated code at all, just noves functionality out of
Generate method into a helper method so it can be reused in the next commit.

d4ee75d5db

mpgen: support primitive std::optional struct fields

Currently optional primitive fields like `std::optional<int>` are not well
supported as struct members.

Non-primitive optional fields like `std::optional<std::string>` and optional
struct fields are well-supported because Cap'n Proto allows non-primitive
fields to be unset, but primitive fields are always considered set so there is
natural way to represent null values.

Libmultiprocess does already support primitive optional method parameters and
result values, by allowing the .capnp files to declare extra boolean parameters
prefixed with "has" and treating the extra boolean parameters as indicators of
whether options are set or unset. This commit just this functionality to work
for struct members as well.

For example a C++ `std::optional<int> param` parameter can be represented by
'param :Int32, hasParam :Bool` parameters in a .capnp file and libmultiprocess
will use both Cap'n Proto fields together to represent the C++ value. Now C++
struct fields can be represented the same way (see unit changes test for an
example).

26fdbad22b

Sjors commented at 12:26 pm on February 24, 2026: member

It would be useful to demonstrate that other implementations can straightforwardly handle this, e.g. with a functional tests in a Bitcoin Core branch (or PR).

in src/mp/gen.cpp:448 in 9f99c7d8e0

446                 add_accessor(field_name);
447                 dec << "    using " << Cap(field_name) << "Accessor = Accessor<" << base_name
448                     << "_fields::" << Cap(field_name) << ", FIELD_IN | FIELD_OUT";
449-                if (BoxedType(field.getType())) dec << " | FIELD_BOXED";
450+                if (field.optional) dec << " | FIELD_OPTIONAL";
451+                if (field.requested) dec << " | FIELD_REQUESTED";

Sjors commented at 1:29 pm on February 24, 2026:

In 9f99c7d8e0f5bbdbf5262094b6b9fe515c19bf05 mpgen: support primitive std::optional struct fields: IIUC all changes in this file, except these two lines, could be a separate refactor commit. LLM came up with this: https://github.com/Sjors/libmultiprocess/commit/eced98fb71db1e54a138f02c3843cbe49080f0ce

ryanofsky commented at 10:47 am on February 25, 2026:

re: #243 (review)

In 9f99c7d mpgen: support primitive std::optional struct fields: IIUC all changes in this file, except these two lines, could be a separate refactor commit.

Thanks, I added a separate refactoring commit similar makes changes in this commit smaller and eliminates the need to add these two lines.

ryanofsky force-pushed on Feb 25, 2026

ryanofsky commented at 11:02 am on February 25, 2026: collaborator

It would be useful to demonstrate that other implementations can straightforwardly handle this, e.g. with a functional tests in a Bitcoin Core branch (or PR).

My next rebase of https://github.com/bitcoin/bitcoin/pull/10102 will use this for the change_pos field as mentioned in the PR description, but that’s probably a less obvious application than a mining interface change accepting optional BlockCreateOptions. Technically https://github.com/bitcoin/bitcoin/pull/33966 could do that but I think because of complexity of that PR, it’s best if that PR only updates the C++ mining interface and leaves extending the Cap’n Proto interface as a followup.

In the meantime I updated the PR description with a more specific example of how this can be used and I think the unit test should provide a good test of functionality.

Updated 9f99c7d8e0f5bbdbf5262094b6b9fe515c19bf05 -> 4de44299ea6d981f79c41fbbd76b61d902dd3580 (pr/optint.1 -> pr/optint.2, compare) adding a refactoring commit deduplicating more code between parameter & struct field mapping implementations and reducing size of the last commit

Updated 4de44299ea6d981f79c41fbbd76b61d902dd3580 -> 26fdbad22bacaa28868fa0ad3c7d4de8e058dd16 (pr/optint.2 -> pr/optint.3, compare) tweaking $Proxy.skip implementation to continue defining accessors for skipped struct fields (needed for https://github.com/bitcoin/bitcoin/pull/10102)

ryanofsky force-pushed on Mar 1, 2026

mpgen: support primitive std::optional struct fields #243

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