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

Logos Code Generator — Project Description

Project Structure

cpp-generator/
├── main.cpp                        # Entry point — dispatches to legacy or experimental
├── CMakeLists.txt                  # Build config
├── compile.sh                      # Standalone build script
├── legacy/                         # Original generator (unchanged from master)
│   ├── main.cpp                    # legacy_main() — plugin/metadata/provider-header modes
│   ├── generator_lib.h/cpp         # Shared utilities, type mapping, header parser
│   └── legacy_main.h              # Forward declaration
├── experimental/                   # New LIDL + impl-header generator
│   ├── lidl_ast.h                 # AST types (TypeExpr, ModuleDecl, MethodDecl, etc.)
│   ├── lidl_lexer.h/cpp           # LIDL tokenizer
│   ├── lidl_parser.h/cpp          # LIDL recursive descent parser
│   ├── lidl_validator.h/cpp       # Semantic validation
│   ├── lidl_serializer.h/cpp      # AST → LIDL text pretty-printer
│   ├── lidl_gen_client.h/cpp      # Client stub generation + helpers
│   ├── lidl_gen_provider.h/cpp    # Provider glue + dispatch generation
│   └── impl_header_parser.h/cpp   # C++ header → ModuleDecl parser
└── 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().

AST (lidl_ast.h)

Shared data model used by all pipelines:

• TypeExpr — type expression with Kind (Primitive, Array, Map, Optional, Named), name, and elements
• ParamDecl — parameter name + type
• MethodDecl — method name, params, return type, description (doc comment above the declaration, emitted into getMethods()), jsonReturn flag (true when impl returns LogosMap/LogosList)
• EventDecl — event name + params
• FieldDecl — struct field name, type, optional flag
• TypeDecl — named struct type with fields
• ModuleDecl — complete module: name, version, description, category, depends, types, methods, events

All types have operator== for testing.

Lexer (lidl_lexer.h/cpp)

Tokenizes LIDL source. Token types: Module, TypeKw, Method, Event, Version, Description, Category, Depends, Ident, StringLit, symbols ({, }, (, ), [, ], :, ,, ->, ?), Eof, Error. Tracks line/column for error reporting.

Parser (lidl_parser.h/cpp)

Recursive descent parser. Grammar:

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
event_def  = "event" IDENT "(" params ")"
params     = (IDENT ":" type_expr ("," IDENT ":" type_expr)*)?
type_expr  = IDENT | "[" type_expr "]" | "{" type_expr ":" type_expr "}"
           | "?" type_expr

Validator (lidl_validator.h/cpp)

Checks: empty module name, duplicate type/method/event names, builtin type shadowing, unknown named type references, duplicate parameter names within methods.

Serializer (lidl_serializer.h/cpp)

Converts ModuleDecl back to LIDL text. Used for roundtrip testing (parse → serialize → parse → compare).

Type Mapping (lidl_gen_client.h/cpp)

• lidlTypeToQt(TypeExpr) — maps LIDL types to Qt type strings
• lidlToPascalCase(name) — converts snake_case to PascalCase
• lidlMakeHeader(ModuleDecl) — generates client API header
• lidlMakeSource(ModuleDecl) — generates client API source
• lidlGenerateMetadataJson(ModuleDecl) — generates metadata.json content

Per-build API-style choice (legacy/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
std	std::string / std::vector<std::string> / LogosMap / LogosList / int64_t / StdLogosResult

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.

ApiStyle enum + new helpers in generator_lib:

• enum class ApiStyle { Qt, Std } — passed to every wrapper-emitting function.
• File-local mapParamTypeStd / mapReturnTypeStd / stdParamToQVariant / qVariantToStdReturn — std-side type-mapping + Qt↔std conversion expressions. 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). The std-style wrapper grows the necessary ensureReplica() plumbing on demand.

Flag plumbing:

1. metadata.json#interface == "universal" → mkLogosModule.nix adds -DLOGOS_API_STYLE=std 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-std) 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. legacy/main.cpp parses --api-style once and threads the resulting ApiStyle through generateFromPlugin, writeUmbrellaHeader{,FromDeps}. No _api_std.{h,cpp} files are ever emitted; each module gets a single <name>_api.h + <name>_api.cpp pair regardless of style.

Provider Generation (lidl_gen_provider.h/cpp)

• lidlTypeToStd(TypeExpr) — maps LIDL types to C++ std type strings
• lidlIsStdConvertible(TypeExpr) — checks if a type has a pure C++ representation
• 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
• 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.
• lidlGenerateProviderGlue(lidlPath, ...) — full pipeline from .lidl file. Also emits <name>_events.cpp and a <name>.lidl sidecar (via lidlSerialize) when the module has any events; both ride the dep's headers-* outputs to power consumer-side typed on<X>() accessors.

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} entries appended to ModuleDecl.events
• 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
• 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
logos-cpp-generator --provider-header src/provider.h --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 std \
    --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

Tests are in tests/experimental/:

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

Test coverage:

Test file	What it tests
test_lidl_lexer.cpp	Tokenization: keywords, identifiers, symbols, strings, escapes, comments, errors, line/column tracking
test_lidl_parser.cpp	Parsing: metadata, methods, events, types, type expressions (array, map, optional, all primitives), error cases
test_lidl_validator.cpp	Validation: duplicates, shadowing, unknown types, duplicate params
test_lidl_serializer.cpp	Serialization: all constructs, roundtrip (parse → serialize → parse → compare)
test_lidl_type_mapping.cpp	lidlTypeToQt, lidlTypeToStd, lidlIsStdConvertible, lidlToPascalCase
test_lidl_gen_provider.cpp	Provider header + dispatch generation: class names, includes, macros, wrapper methods, conversions, events
test_lidl_gen_client.cpp	Client stub generation: sync/async methods, events, metadata JSON, edge cases
test_impl_header_parser.cpp	Header parsing: type mapping, access specifiers, skipping private/protected, error cases

Fixture files in tests/experimental/fixtures/:

• sample_impl.h — module with all supported type variations
• sample_metadata.json — metadata with dependencies
• complex_impl.h — module with multiple access specifier sections
• empty_class_impl.h — class with no public methods
• empty_metadata.json — minimal metadata

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
• Only the qt backend is implemented for --from-header; future backends (CBOR, Rust) are planned
• Client stub generation (lidlMakeHeader/lidlMakeSource) is only available from LIDL files, not from --from-header