feat: add C++ header-only exporter for embedded/MCU use

docs/cpp-header.md

@@ -0,0 +1,144 @@
+# C++ Header Exporter
+
+The `cpp-header` exporter generates a single header-only `.hpp` file that encodes the VSS tree as `constexpr` C++ data structures. It is intended for embedded and MCU targets where no filesystem or JSON parser is available at runtime. The output requires only `<cstddef>` and `<cstdint>`.
+
+## Exporter Specific Arguments
+
+### `--namespace`
+
+C++ namespace that wraps all generated symbols. Defaults to `vss`.
+
+### `--include-branches` / `--no-include-branches`
+
+When `--include-branches` is set, branch nodes are emitted alongside leaf signals. By default only leaf signals are included.
+
+## Output Structure
+
+The generated header always contains:
+
+- A `VssSignal` struct with string fields for `path`, `type`, `datatype`, `cpp_type`, `unit`, `description`, `min_value`, `max_value`, `default_value`, and a null-terminated `allowed_values` pointer (or `nullptr`).
+- One `constexpr VssSignal` per leaf signal. If the signal has `allowed:` values, a null-terminated `constexpr const char*[]` is emitted immediately above it.
+- A `constexpr VssSignal kSignals[]` aggregate containing every signal in pre-order.
+- `constexpr std::size_t kSignalCount` derived from `kSignals`.
+
+VSS datatypes are mapped to C++ types as follows:
+
+| VSS datatype | C++ type |
+|---|---|
+| `uint8` / `int8` | `uint8_t` / `int8_t` |
+| `uint16` / `int16` | `uint16_t` / `int16_t` |
+| `uint32` / `int32` | `uint32_t` / `int32_t` |
+| `uint64` / `int64` | `uint64_t` / `int64_t` |
+| `float` | `float` |
+| `double` | `double` |
+| `boolean` | `bool` |
+| `string` | `const char*` |
+| `T[]` (array) | `const T*` |
+
+## Example
+
+Input model:
+
+```yaml
+# model.vspec
+Vehicle:
+  type: branch
+  description: High-level vehicle data.
+
+Vehicle.Speed:
+  type: sensor
+  datatype: float
+  unit: km/h
+  min: 0
+  max: 250
+  description: Vehicle speed.
+
+Vehicle.Powertrain.Type:
+  type: attribute
+  datatype: string
+  allowed: [combustion, hybrid, electric]
+  description: The powertrain type of the vehicle.
+```
+
+Generator call:
+
+```bash
+vspec export cpp-header --vspec model.vspec --output vss.hpp
+```
+
+Generated file (excerpt):
+
+```cpp
+// SPDX-FileCopyrightText: Copyright (c) 2024 Contributors to COVESA
+// SPDX-License-Identifier: MPL-2.0
+
+// Auto-generated from the Vehicle Signal Specification.
+// Do not edit manually.
+
+#pragma once
+
+#include <cstddef>
+#include <cstdint>
+
+namespace vss {
+
+struct VssSignal {
+    const char* const path;
+    const char* const type;
+    const char* const datatype;
+    const char* const cpp_type;
+    const char* const unit;
+    const char* const description;
+    const char* const min_value;
+    const char* const max_value;
+    const char* const* const allowed_values;  // null-terminated, or nullptr
+    const char* const default_value;
+};
+
+constexpr VssSignal Vehicle_Speed = {
+    "Vehicle.Speed",
+    "sensor",
+    "float",
+    "float",
+    "km/h",
+    "Vehicle speed.",
+    "0",
+    "250",
+    nullptr,
+    nullptr,
+};
+
+constexpr const char* Vehicle_Powertrain_Type_kAllowed[] = {
+    "combustion",
+    "hybrid",
+    "electric",
+    nullptr,
+};
+constexpr VssSignal Vehicle_Powertrain_Type = {
+    "Vehicle.Powertrain.Type",
+    "attribute",
+    "string",
+    "const char*",
+    nullptr,
+    "The powertrain type of the vehicle.",
+    nullptr,
+    nullptr,
+    Vehicle_Powertrain_Type_kAllowed,
+    nullptr,
+};
+
+constexpr VssSignal kSignals[] = {
+    Vehicle_Speed,
+    Vehicle_Powertrain_Type,
+};
+
+constexpr std::size_t kSignalCount = sizeof(kSignals) / sizeof(kSignals[0]);
+
+}  // namespace vss
+```
+
+The header is self-contained and can be compiled without any vss-tools dependency:
+
+```bash
+g++ -std=c++17 -c vss.hpp
+```

docs/vspec.md

@@ -50,6 +50,7 @@ vspec export json --vspec spec/VehicleSignalSpecification.vspec --output vss.jso
 
 - [apigear](./apigear.md)
 - [binary](../binary/README.md)
+- [cpp-header](./cpp-header.md)
 - [ddsidl](./ddsidl.md)
 - [plantuml](./plantuml.md)
 - [go](./go.md)

src/vss_tools/cli.py

@@ -49,6 +49,7 @@ def cli(ctx: click.Context, log_level: str, log_file: Path):
         "yaml": "vss_tools.exporters.yaml:cli",
         "tree": "vss_tools.exporters.tree:cli",
         "samm": "vss_tools.exporters.samm:cli",
+        "cpp-header": "vss_tools.exporters.cpp_header:cli",
         "go": "vss_tools.exporters.go:cli",
         "ros2interface": "vss_tools.exporters.ros2interface:cli",
         "s2dm": "vss_tools.exporters.s2dm:cli",