logos-cpp-sdk/cpp-generator/docs/project.md

Logos Code Generator — Project Description

Project Structure

cpp-generator/
├── main.cpp                        # Entry point — `--umbrella`/`--general-only` mode, dispatch to legacy or experimental
├── CMakeLists.txt                  # Build config
├── compile.sh                      # Standalone build script
├── metadata_dependencies.h         # What a metadata.json `dependencies[]` array declares
├── generator_lib.h/cpp             # Shared emitter library: type mapping, wrapper + umbrella emission
├── lidl_to_json.h/cpp              # ModuleDecl → the JSON surface generator_lib consumes
├── legacy/                         # Original generator (plugin-introspection modes only)
│   ├── main.cpp                    # legacy_main() — plugin/metadata modes
│   └── legacy_main.h              # Forward declaration
├── experimental/                   # C++/Qt-specific generator backends
│   ├── lidl_compat.h              # Bridges the backends onto logos-lidl's std AST
│   ├── lidl_emit_common.h/cpp     # LIDL type → Qt/std type-name mapping
│   ├── lidl_gen_client.h/cpp      # Typed client stub generation (+ Doxygen /// docs)
│   ├── lidl_gen_cdylib.h/cpp      # cdylib module-impl C-ABI export generation
│   └── impl_header_parser.h/cpp   # C++ impl header → lidl::ModuleDecl
│   # The lexer/parser/AST/serializer/validator now live in the standalone
│   # logos-lidl repo (linked via find_package(logos-lidl)); the Qt glue
│   # emitters live in logos-qt-sdk's logos-qt-generator.
└── docs/                          # This documentation

Components

Entry Point (main.cpp)

Checks for --from-header or --lidl flags before creating QCoreApplication. If neither is present, falls through to legacy_main().

LIDL frontend — logos-lidl (consumed as a library)

The lexer, parser, AST, serializer, and validator are no longer embedded here — they live in the standalone logos-lidl repo, the language-neutral (Qt-free) common frontend every Logos SDK shares (C++ here, Rust in logos-rust-sdk, …). cpp-generator links it via find_package(logos-lidl) and reaches it through experimental/lidl_compat.h.

logos-lidl exposes (namespace lidl):

• lidl::parse(std::string) → ParseResult (ModuleDecl + error/line/column)
• lidl::serialize(ModuleDecl) → std::string
• lidl::validate(ModuleDecl) → ValidationResult
• the AST: TypeExpr (Kind: Primitive/Array/Map/Optional/Named, name, elements), ParamDecl, FieldDecl, MethodDecl (name, params, returnType, description, jsonReturn, resultReturn), EventDecl (name, params, description), TypeDecl, ModuleDecl. (logos-lidl also exposes an AST↔JSON bridge and a C ABI that the Rust SDK consumes over FFI — not used by this generator.)

The .lidl grammar (defined in logos-lidl):

module     = "module" IDENT "{" body "}"
body       = (metadata | type_def | method_def | event_def)*
metadata   = "version" STRING | "description" STRING | "category" STRING
           | "depends" "[" (IDENT ("," IDENT)*)? "]"
type_def   = "type" IDENT "{" field* "}"
field      = "?"? IDENT ":" type_expr
method_def = "method" IDENT "(" params ")" "->" type_expr ("description" STRING)?
event_def  = "event" IDENT "(" params ")" ("description" STRING)?
params     = (IDENT ":" type_expr ("," IDENT ":" type_expr)*)?
type_expr  = IDENT | "[" type_expr "]" | "{" type_expr ":" type_expr "}"
           | "?" type_expr

Validation (in logos-lidl) checks: empty module name, duplicate type/method/event names, builtin type shadowing, unknown named type references, duplicate parameter names. Serialization round-trips ModuleDecl back to .lidl text (incl. the trailing description "…" clause).

Compat shim (lidl_compat.h)

Bridges the existing Qt-flavored backends onto logos-lidl's std AST so they compile unchanged:

• brings the lidl:: AST types into the global scope the backends use (via using)
• qs(std::string) → QString plus a QTextStream << std::string overload, so emission of AST string fields just works
• name-compatible shims lidlParse / lidlSerialize / lidlValidate over lidl::parse/serialize/validate

Type Mapping (lidl_emit_common.h/cpp)

• lidlTypeToQt(TypeExpr) / lidlTypeToStd(TypeExpr) — LIDL type → Qt / std type-name strings
• lidlIsStdConvertible(TypeExpr) — whether a type has a pure-C++ (Qt-free) representation
• lidlToPascalCase(name) — converts snake_case to PascalCase

Optionality

?T is two-state: a value of T, or empty. Never three-state — "one LIDL type ↔ one type per language" leaves nowhere for a third state, because every target has exactly one empty inhabitant.

Two spellings, one meaning. A record field may be written ? name: T (the flag) or name: ?T (the type kind); the spec binds them to the same declaration, so they MUST emit identical code. Backends never answer this themselves — logos-lidl's fieldIsOptional(f) / fieldValueType(f) (re-exported by lidl_compat.h) are the one place the two are reconciled. Reading f.optional or f.type.kind == Optional on its own is a bug.

Wire rule. Absent and explicit null are the same state on decode and different on encode:

	empty is spelled
decode, optional slot	absent or null → empty
decode, required slot	absent and null are both still errors
encode, named slot (a record field)	the key is omitted
encode, positional slot (argument, return, event param)	null — there is no key to omit, and arity must never change

A round trip therefore canonicalises: a peer that sent "f": null gets the key back omitted. A present-but-wrong-typed value is still an error — optional widens the domain by exactly one inhabitant, it does not switch type checking off.

Per surface:

Surface	?T	Notes
cdylib / std (lidlTypeToStd, lidl_gen_cdylib)	std::optional<T>	encoded by logos-protocol's Codec<std::optional<T>>; key omission is the record emitter's job (a codec never sees the slot)
?any / ?{tstr: any} / ?[any]	LogosMap / LogosList	collapses: nlohmann::json already carries null, so wrapping it would make the slot three-state
Qt (lidlTypeToQt, lidl_gen_client)	QVariant	two-state (an invalid QVariant is Qt's empty inhabitant) but untyped — Qt has no optional template
legacy consumer, record field (lidl_to_json + generator_lib)	QVariant (Qt) / std::optional<T> (Lp)	same answer as the client-stub and cdylib backends respectively; the alias collapse above applies on Lp
legacy consumer, positional slot (param, return, event param)	QVariant (Qt) / LogosMap (Lp)	still flattened — see Known Limitations
header-first (impl_header_parser)	std::optional<T> ↔ ?T	std::optional<std::optional<T>> has no LIDL type; it collapses to ?T and is reported on stderr

Client stubs (lidl_gen_client.h/cpp)

• lidlMakeHeader(ModuleDecl) / lidlMakeSource(ModuleDecl) — typed <Module> client wrapper; each method (and its …Async twin) carries a Doxygen /// comment generated from the method's description
• lidlGenerateMetadataJson(ModuleDecl) — generates metadata.json content

cdylib backend (lidl_gen_cdylib.h/cpp)

Emits the Qt-free half of a universal C++ cdylib module:

• lidlCdylibSupported(ModuleDecl) — gate to the std-convertible (Qt-free) type subset
• lidlMakeModuleImplExports(...) — the logos_module_impl.h C-ABI export wrapper around the universal impl class (compiled into the module's cdylib; dispatches via nlohmann::json)
• lidlMakeEventsSourceCdylib(...) — typed logos_events: bodies marshalling into nlohmann::json

Per-build API-style choice (generator_lib.{h,cpp})

The codegen exposes one wrapper class per module — <Module> — with signatures that match the API style picked at the consumer's build time. The two styles are mutually exclusive (no composite output):

--api-style	Wrapper signatures
qt (default)	QString / QStringList / QVariantList / QVariantMap / int / LogosResult
lp	std::string / std::vector<std::string> / LogosMap / LogosList / int64_t / StdLogosResult, over the Qt-free logos-protocol C ABI

(A third value, std — std signatures over a QVariant / LogosAPIClient body — was retired; the generator now rejects --api-style=std instead of aliasing it.)

Both styles emit:

• A <Module> client class with sync method shapes + matching <method>Async(...) overloads.
• The std variant additionally inlines Qt↔std conversion in its .cpp so the caller's translation unit needs zero Qt headers.

The umbrella logos_sdk.h is also generated per-build and aggregates every dep into a flat LogosModules struct — no nested view:

struct LogosModules {
    LogosAPI*       api;
    SomeDep         some_dep;              // one accessor per `metadata.json#dependencies` entry
    // ...
};

Only the modules explicitly listed as dependencies are exposed. The runtime's core_manager is intentionally NOT in LogosModules — apps that need to manage the core do so via liblogos' C API, not via a typed RPC wrapper.

A dependencies[] element is either a bare name or an object carrying that name alongside the constraints an installer resolves it by — the two declare the same dependency and generate the same code:

"dependencies": [
    "dep_a",
    { "name": "dep_b", "version": "=1.2.3" },
    { "name": "dep_c", "version": "^2.0", "signer": "did:jwk:abc" }
]

Read the array through dependencyNames() (metadata_dependencies.h) rather than element by element. The umbrella is emitted by several passes over the same array — includes, constructor initialisers, members — and a pass that decides on its own what an element names can decide differently from its neighbours, yielding a member whose type was never included. That aggregate no longer compiles, and nothing catches it until a module builds against it. One reader, one answer.

ApiStyle enum + new helpers in generator_lib:

• enum class ApiStyle { Qt, Lp } — passed to every wrapper-emitting function.
• File-local mapParamTypeStd / mapReturnTypeStd — the std-side type-mapping table the lp surface exposes. Hidden from generator_lib.h (not part of the public surface).
• makeHeader(moduleName, className, methods, apiStyle, events) / makeSource(moduleName, className, headerBaseName, methods, apiStyle, events) — single entry points that branch on apiStyle internally to emit the right include block, signature shape, and conversion bridges. events is loaded from a <name>.lidl sidecar via --events-from; when non-empty, the wrapper also gets one typed on<EventName>(callback) adapter per declared event (callback arg types follow apiStyle).
• makeUmbrellaHeaderFromDeps(deps, interfaceNames, apiStyle, originName) / makeUmbrellaSourceFromDeps(deps, interfaceNames) — the logos_sdk.{h,cpp} aggregate above. They return the text; main.cpp's runUmbrellaMode writes it. That split is what lets the aggregate be asserted on directly, without a filesystem.

Flag plumbing:

1. metadata.json#interface == "universal" (or "cdylib") → mkLogosModule.nix adds -DLOGOS_API_STYLE=lp to extraCmakeFlags. Anything else ("legacy", "provider", absent) leaves the default qt.
2. LogosModule.cmake reads ${LOGOS_API_STYLE} (default qt) and forwards --api-style=${LOGOS_API_STYLE} to the logos-cpp-generator --general-only invocation that writes the umbrella. Each module's Nix build emits two header derivations (<name>.headers-qt and <name>.headers-lp) via buildHeaders.nix — one logos-cpp-generator --api-style=… run per style, at the dep's build time. A consumer's buildPlugin.nix picks dep.headers-${apiStyle} and copies its include/ straight into the build sandbox; no codegen runs at consume time. Nix's laziness means only the variant a downstream actually depends on is realised.
3. parseApiStyleFlag() in generator_lib parses --api-style once (rejecting the retired std); main.cpp's runUmbrellaMode threads the resulting ApiStyle into makeUmbrella*FromDeps, and legacy/main.cpp threads it through generateFromPlugin / writeUmbrellaHeader (the QPluginLoader path). No per-style filenames are ever emitted; each module gets a single <name>_api.h + <name>_api.cpp pair regardless of style.

Provider Generation (logos-qt-generator)

The Qt provider glue (lidl_gen_provider.{h,cpp}) is emitted by logos-qt-sdk's logos-qt-generator, not this binary — it consumes the same logos-lidl frontend (+ the shared lidl_emit_common / impl_header_parser / lidl_compat.h / metadata_dependencies.h helpers, distributed under share/lidl-frontend). Documented here for reference. Adding a header to what impl_header_parser.cpp includes means adding it to that install list too (nix/bin.nix) — the qt-generator compiles that source out of the installed directory, so a header left behind breaks its build, not ours.

• lidlMakeProviderHeader(ModuleDecl, implClass, implHeader) — generates Qt glue header
  • Emits nlohmannToQVariant() helper when any method has jsonReturn = true
  • Always emits an onInit(LogosAPI*) override that, via SFINAE'd helpers in logos_module_context.h, (a) copies the three runtime-injected properties (modulePath, instanceId, instancePersistencePath) into the impl, (b) constructs a per-module LogosModules aggregate and threads its pointer through the same base, and (c) installs the typed-event callback (maybeSetEmitEvent) consumed by <name>_events.cpp method bodies. Impls that don't inherit LogosModuleContext compile unchanged — the helper overloads collapse to no-ops. The full LogosAPI is never exposed past the provider boundary.
  • Always emits #include "logos_sdk.h" and a std::unique_ptr<LogosModules> m_logosModules member; ownership lives on the provider, the context base sees only a non-owning void* reinterpreted in LogosModuleContext::modules() (which depends on the impl's TU having included logos_sdk.h).
• lidlMakeProviderDispatch(ModuleDecl) — generates callMethod/getMethods dispatch. getMethods() emits the full interface: each method tagged type: "method", then each module.events entry tagged type: "event" (name, signature, parameters, escaped description; no returnType/isInvokable). There is no separate getEvents() — folding events into getMethods() keeps the provider vtable ABI-stable.
• lidlMakeEventsSource(ModuleDecl, implClass, implHeader) — generates <name>_events.cpp: Qt-MOC-style method bodies for prototypes declared in the impl's logos_events: block. Each body marshals typed args into a QVariantList and calls this->emitEventImpl_("<name>", &args) on the LogosModuleContext base.

Impl Header Parser (impl_header_parser.h/cpp)

• parseImplHeader(headerPath, className, metadataPath, err) — parses C++ header + metadata.json into ModuleDecl
• State machine: LookingForClass → InClass → InPublic/InPrivate/InLogosEvents
• The literal logos_events: token (defined in logos_module_context.h as #define logos_events public) opens an events section; bare prototypes inside become EventDecl{name, params, description} entries appended to ModuleDecl.events (the description is the doc comment immediately above the declaration, captured via joinDocLines exactly as for methods)
• Skips: constructors, destructors, typedefs, using, friend, enum, struct, std::function declarations
• Recognizes LogosMap and LogosList return types (nlohmann::json aliases) and sets MethodDecl.jsonReturn = true
• Recognizes std::optional<T> → ?T (see Optionality). Anything it does not recognize still falls back to the opaque any, silently — that fallback is why an optional was unexpressible header-first until it was named explicitly
• Template-aware parameter splitting (handles std::vector<std::string> correctly)

CLI Usage

From C++ impl header (primary use case for universal modules)

logos-cpp-generator --from-header src/my_module_impl.h \
    --backend qt \
    --impl-class MyModuleImpl \
    --impl-header my_module_impl.h \
    --metadata metadata.json \
    --output-dir ./generated_code

Generates: my_module_qt_glue.h, my_module_dispatch.cpp

From LIDL file — provider glue

logos-cpp-generator --lidl my_module.lidl \
    --backend qt \
    --impl-class MyModuleImpl \
    --impl-header my_module_impl.h \
    --output-dir ./generated_code

From LIDL file — client stubs

logos-cpp-generator --lidl my_module.lidl \
    --output-dir ./generated_code \
    --module-only

Legacy modes (unchanged)

logos-cpp-generator /path/to/plugin.so --output-dir ./generated
logos-cpp-generator --metadata metadata.json --general-only --output-dir ./generated

Consumer wrapper with typed event accessors

The --events-from <path> flag points the legacy <plugin>.dylib --module-only codegen at a LIDL sidecar shipped alongside the dep's pre-built headers. When set, the generated <name>_api.{h,cpp} gains one typed on<EventName>(callback) accessor per declared event (callback arg types match --api-style):

logos-cpp-generator /path/to/plugin.dylib \
    --module-only --api-style lp \
    --events-from /path/to/dep/share/logos/my_module.lidl \
    --output-dir ./generated

In Nix builds this is wired automatically: buildHeaders.nix looks for <pluginLib>/share/logos/<name>.lidl (which buildPlugin.nix's installPhase placed there) and threads it through.

Building

The generator is built as part of logos-cpp-sdk:

ws build logos-cpp-sdk     # builds everything including the generator

The generator binary is available as logos-cpp-generator in module build environments (provided by logos-module-builder's nativeBuildInputs).

Testing

The backends are tested in tests/experimental/, the legacy emitters in tests/generator/:

ws test logos-cpp-sdk      # runs all tests including experimental

The frontend tests (lexer/parser/validator/serializer) moved to the logos-lidl repo along with the code; only the C++/Qt-specific backends are tested here:

Test file	What it tests
test_lidl_type_mapping.cpp	lidlTypeToQt, lidlTypeToStd, lidlIsStdConvertible, lidlToPascalCase, optionality on both surfaces
test_lidl_gen_client.cpp	Client stub generation: sync/async methods, events, metadata JSON, edge cases, both optional spellings agreeing
test_lidl_gen_cdylib.cpp	cdylib eligibility + emission: bytes at depth, records, typed maps, optionality (key omission, arity, ?any collapse)
test_impl_header_parser.cpp	Header parsing: type mapping, access specifiers, skipping private/protected, error cases, std::optional<T>

(The lexer/parser/AST/serializer/validator round-trip + description tests live in logos-lidl's own tests/test_lidl.cpp.)

In tests/generator/, alongside the wrapper-emitter tests:

Test file	What it tests
test_make_umbrella.cpp	The LogosModules aggregate: both dependency forms on both API styles, that every member's type is included, dropped nameless entries, empty deps

Fixture files in tests/experimental/fixtures/:

• sample_impl.h — module with all supported type variations
• sample_metadata.json — metadata with dependencies
• object_deps_metadata.json — dependencies declared in both forms, with resolution constraints
• complex_impl.h — module with multiple access specifier sections
• empty_class_impl.h — class with no public methods
• empty_metadata.json — minimal metadata
• optional_impl.h / optional_metadata.json — std::optional<T> header-first, incl. an optional over a declared record

Known Limitations

• The impl header parser is lightweight (regex + state machine). It does not handle:

  • Multi-line method declarations
  • Default parameter values
  • Method definitions in the header (only declarations ending with ;)
  • Nested classes
  • Template methods
  • std::function members are silently skipped (never treated as methods)

• LIDL does not support generic/parameterized types or inheritance

• --from-header emits the cdylib backend here (the qt glue backend moved to logos-qt-generator); the Rust backend lives in logos-rust-sdk's lidl-gen, generating over logos-lidl's C ABI

• Client stub generation (lidlMakeHeader/lidlMakeSource) is only available from LIDL files, not from --from-header

• Optionality is still untyped in positional slots on the legacy consumer path. The consumer wrappers real modules get come from legacy/main.cpp → generateInterfaceWrappers → lidl_to_json → generator_lib, and that JSON boundary carries a single Qt type-name string per slot. Record fields now carry an optional flag alongside the value type, so both LIDL spellings emit identical, typed code (QVariant on Qt, std::optional<T> on Lp). A method parameter, a return type and an event parameter still do not: they arrive as QVariant (Qt) / LogosMap (Lp) — the right shape (an invalid QVariant / a JSON null is the empty inhabitant) with no type, so a caller gets no compile-time check and cannot tell ?tstr from ?uint or recover a ?Record's struct. There is no spelling divergence there — a positional slot has no name to hang a flag on, so it only ever had the type-kind form — and closing it changes generated method signatures, i.e. a source break for every existing call site. Until then the generator prints a Note: naming every still-flattened slot, so an affected build is never silent. OptionalSpellings.PositionalSlotsAreStillFlattened pins the current behaviour so closing the gap is a deliberate change.

• Nesting, map key types and descriptions still do not cross that boundary either — the flag added for optionality is per-field, not a general widening.

• lidlRecordCollidesWithBytesTag reads through an optional (via fieldValueType), so a single-_bytes-field record is refused under both spellings. It used to read f.type, which refused ? _bytes: tstr and let _bytes: ?tstr through — the same declaration, two answers. ?bstr is unaffected either way: the tag lives in the value, not the slot.

• A provider REJECTION reaches …Async's callback only as a log line (but …AsyncResult's callback gets it properly). A provider that refuses a call answers the canonical {"code":"dispatch_failed", "message":…, "origin":…} object as its RESULT, not as a transport error, and the Qt return table would convert it like any other value — erasing it (_result.toList() on that map is []). The Qt consumer emitter therefore detects it (logosDispatchRejection, emitted once per wrapper) and folds it into the error channel of every surface that HAS one:

  • sync — the logos::CallError* out-parameter, so mod.echoUintList(v, &err) can tell a rejection from an empty return;
  • …AsyncResult — logos::AsyncResult<T>::error, so r.ok() is false and r.error.code == "dispatch_failed" exactly as on the sync path.

  The historical …Async overload is the one exception: its callback is std::function<void(T)>, and adding an error parameter would change a generated public surface (which logos-qt-sdk's qt-generator --backend consumer veneer mirrors 1:1). It is left untouched, so there an async rejection is reported with qWarning and the callback still receives the default-converted value. …AsyncResult exists precisely because giving async an error channel was an API addition rather than a code-generation fix — a caller that needs to SEE the rejection uses it.