# Step 2: element-base-interface

## Read First

Read these files before editing:

- `/AGENTS.md`
- `/docs/AGENT_RULES.md`
- `/docs/ARCHITECTURE.md`
- `/docs/implementation-plans/analysis-model-objects-implementation-plan.md`
- `/include/fesa/core/ElementDefinition.hpp`
- `/CMakeLists.txt`

## Task

Add the abstract element model interface.

Allowed files:

- Create `/include/fesa/element/Element.hpp`
- Create `/tests/element/element_base_test.cpp`
- Modify `/tests/core/domain_bootstrap_test.cpp` only if the current test executable remains the single test runner
- Modify `/CMakeLists.txt`

Required interface:

- virtual destructor.
- `ElementId id() const noexcept`.
- `ElementType type() const noexcept`.
- `std::size_t nodeCount() const noexcept`.
- `std::span` is not allowed because this project targets C++17.
- Use const reference return for connectivity data.
- no stiffness, residual, stress, state, or equation API in this phase.

## Tests To Write First

Write `/tests/element/element_base_test.cpp` before production changes. Use a small test subclass to prove:

- base pointer dispatch works.
- id/type/connectivity/property id are readable through base API.
- deleting through `std::unique_ptr<Element>` is valid.

Run targeted build and confirm RED because `Element.hpp` is missing.

## Acceptance Criteria

Run:

```powershell
python scripts/validate_workspace.py
ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R model-object
```

Update step 2 status.

## Do Not

- Do not implement MITC4 stiffness or numerical element behavior.