OCCT/.github/copilot-instructions.md

OCCT AI Assistant Guidelines

This document provides comprehensive guidance for AI assistants (GitHub Copilot, Claude, ChatGPT, and others) working with the Open CASCADE Technology (OCCT) C++17+ 3D CAD/CAM/CAE library.

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1. Core Directives

IMPORTANT: These are the most critical rules. Follow them strictly.

1. Memory Management is Paramount:

   • ALWAYS use occ::handle<ClassName> for any class inheriting from Standard_Transient (e.g., Geom_*, Poly_*, AIS_*, V3d_*). This is for reference-counted objects.
   • NEVER use raw pointers (ClassName*) for these types, as it will cause memory leaks.
   • Correct: occ::handle<Geom_Circle> aCircle = new Geom_Circle(...);
   • Wrong: Geom_Circle* aCircle = new Geom_Circle(...);
   • Note: The legacy Handle(ClassName) macro still exists but occ::handle<> is preferred in new code.

2. Check Operation Status:

   • After using an API that performs a geometric operation (e.g., BRepAlgoAPI_Fuse, BRepBuilderAPI_MakeEdge), ALWAYS check if the operation was successful using the IsDone() method before accessing the result.
   • Correct:

     BRepAlgoAPI_Fuse aFuser(theShape1, theShape2);
     if (aFuser.IsDone())
     {
       const TopoDS_Shape& aResult = aFuser.Shape();
     }
     else
     {
       // Handle error
     }
     

3. Strict Naming and File Conventions:

   • Adhere to the strict Package_ClassName convention.
   • Place new files in the correct directory: src/Module/Toolkit/Package/.
   • After adding a file, ALWAYS update the corresponding src/Module/Toolkit/Package/FILES.cmake file.

4. Use Modern C++ Idioms:

   • Use range-based for loops and structured bindings where applicable.
   • Use the modern TopExp_Explorer constructor style.
   • Correct: for (TopExp_Explorer anExp(theShape, TopAbs_FACE); anExp.More(); anExp.Next()) { ... }
   • Wrong: for (TopExp_Explorer anExp; anExp.Init(theShape, TopAbs_FACE); anExp.More(); anExp.Next()) { ... }

5. Safe Type Casting:

   • When downcasting topological shapes, ALWAYS use the TopoDS helper functions to avoid errors.
   • Correct: const TopoDS_Face& aFace = TopoDS::Face(anExp.Current());
   • Wrong: const TopoDS_Face& aFace = (const TopoDS_Face&)anExp.Current();

6. Handle Downcasting:

   • Use occ::down_cast<DerivedClass>(BaseHandle) to safely downcast handles.
   • Correct:

     const occ::handle<Geom_Circle> aCircle = occ::down_cast<Geom_Circle>(aCurveHandle);
     // Alternative (legacy): occ::handle<Geom_Circle>::DownCast(aCurveHandle)
     

   • Wrong:

     // Do not use C-style casts on handles
     Geom_Circle* aCircle = (Geom_Circle*)aHandle.Get();
     

7. Validate Handles for Null Safety:

   • ALWAYS check that an occ::handle<ClassName> is not null before dereferencing it:

     if (!theHandle.IsNull())
     {
       // use theHandle
     }
     

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2. Project Overview

Open CASCADE Technology (OCCT) is a comprehensive C++ software development platform for 3D surface and solid modeling, CAD data exchange, and visualization. It's a modern C++17+ library providing services for CAD/CAM/CAE applications.

Architecture & Source Organization

• src/: Source code, organized by a Module/Toolkit/Package hierarchy.
• tests/: Draw Harness test files, organized by functionality.
• adm/: Administrative and build configuration files.
• samples/: Example applications.

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3. Code Conventions

NEVER use non-ASCII symbols in code or comments.

Naming Patterns

Element Type	Pattern	Example
Classes	Package_ClassName	TopoDS_Shape, BRep_Builder
Public Methods	MethodName	IsDone(), Shape()
Private Methods	methodName	myInternalMethod()
Method Parameters	theParameterName	theShape, theTolerance
Local Variables	aVariableName	aBox, anExplorer
Class Member Fields	myFieldName	myShape, myIsDone
Struct Member Fields	FieldVariableName	Point, Value
Global Variables	THE_GLOBAL_VARIABLE	THE_DEFAULT_PRECISION

Primitive Types

Use standard C++ primitive types in all code. The old Standard_* typedefs are deprecated and must not be used:

Use This	Instead of (Deprecated)	Usage
int	Standard_Integer	Integer values
double	Standard_Real	Floating-point numbers
bool	Standard_Boolean	Boolean values
float	Standard_ShortReal	Single-precision floats
char	Standard_Character	Character values
uint8_t	Standard_Byte	Byte values
size_t	Standard_Size	Size values
void*	Standard_Address	Generic pointers
char16_t	Standard_ExtCharacter	Unicode characters
const char*	Standard_CString	C-style strings
true/false	Standard_True/False	Boolean literals

WARNING: All Standard_* type aliases in Standard_TypeDef.hxx are marked Standard_DEPRECATED and will be removed in future versions. Always use native C++ types directly.

Strings and Collections

ALWAYS use OCCT collection classes instead of STL containers:

OCCT Type	Instead of	Description
TCollection_AsciiString	std::string	ASCII string
TCollection_ExtendedString	std::wstring	Unicode string
NCollection_Array1<T>	std::vector<T>	Fixed-size array (1-based)
NCollection_Vector<T>	std::vector<T>	Dynamic array
NCollection_List<T>	std::list<T>	Doubly-linked list
NCollection_Sequence<T>	std::deque<T>	Indexed sequence
NCollection_Map<K>	std::unordered_set<K>	Hash set
NCollection_DataMap<K,V>	std::unordered_map<K,V>	Hash map
NCollection_IndexedMap<K>	-	Indexed hash set
NCollection_IndexedDataMap<K,V>	-	Indexed hash map

IMPORTANT: Package-Specific Type Aliases Removed

Old package-specific type aliases have been removed from OCCT. You must use NCollection_* templates directly with explicit types:

Removed Alias (Do Not Use)	Use Instead
TColStd_ListOfInteger	NCollection_List<int>
TColStd_Array1OfReal	NCollection_Array1<double>
TColStd_SequenceOfInteger	NCollection_Sequence<int>
TColStd_MapOfInteger	NCollection_Map<int>
TColStd_DataMapOfIntegerReal	NCollection_DataMap<int, double>
TopTools_ListOfShape	NCollection_List<TopoDS_Shape>
TopTools_MapOfShape	NCollection_Map<TopoDS_Shape>
TColgp_Array1OfPnt	NCollection_Array1<gp_Pnt>
TColgp_SequenceOfPnt	NCollection_Sequence<gp_Pnt>

Exception: TColStd_PackedMapOfInteger still exists as it has a specialized implementation.

If STL containers are absolutely necessary, use OCCT's allocators:

#include <NCollection_Allocator.hxx>
#include <NCollection_OccAllocator.hxx>
#include <NCollection_IncAllocator.hxx>

// Option 1: NCollection_Allocator - uses standard OCCT memory allocation
std::vector<gp_Pnt, NCollection_Allocator<gp_Pnt>> aPoints;
std::list<TopoDS_Shape, NCollection_Allocator<TopoDS_Shape>> aShapes;

// Option 2: NCollection_OccAllocator - with custom memory pool (preferred for performance)
occ::handle<NCollection_IncAllocator> anIncAlloc = new NCollection_IncAllocator();
NCollection_OccAllocator<gp_Pnt> anAllocator(anIncAlloc);
std::vector<gp_Pnt, NCollection_OccAllocator<gp_Pnt>> aPooledPoints(anAllocator);

Modern C++ Features

This is a C++17+ codebase. Use modern features where appropriate:

• Range-based for loops
• Structured bindings: for (const auto& [aKey, aValue] : aMap)
• std::optional for optional return values
• std::variant for type-safe unions
• if constexpr for compile-time conditions
• [[nodiscard]], [[maybe_unused]] attributes where appropriate

Use of auto Keyword

Use auto only in the following cases:

• Where syntax requires it (templates, structured bindings, lambdas)
• To omit the type of object in range-based for loops
• To omit the type of container iterators: auto anIter = aContainer.begin()

Avoid auto in these cases:

• To omit long type names (use type aliases instead)
• To omit "obvious" return types
• Simply to avoid typing the actual type

Readability is more important than brevity. Always prefer explicit types.

Const Correctness

Use const for variables that will not be modified after initialization:

const double aTolerance = 0.001;
const TopoDS_Shape aShape = aBuilder.Shape();

Use constexpr for values that can be computed at compile time:

constexpr int THE_MAX_ITERATIONS = 100;
constexpr double THE_PI = 3.14159265358979323846;

Prefer const references to avoid unnecessary copies:

void ProcessShape(const TopoDS_Shape& theShape);

for (const TopoDS_Face& aFace : aFaces)
{
  // ...
}

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4. Step-by-Step Workflow: Adding a New Class

This example demonstrates adding a new class BRepLib_MyNewClass to the BRepLib package in the TKTopAlgo toolkit.

1. Create Header and Source Files:

cd src/ModelingAlgorithms/TKTopAlgo/BRepLib
touch BRepLib_MyNewClass.hxx BRepLib_MyNewClass.cxx

2. Implement the Class: Add content following all code conventions (see sections 3 and 7).

3. Add Files to Package CMake: Edit src/ModelingAlgorithms/TKTopAlgo/BRepLib/FILES.cmake:

set(OCCT_BRepLib_FILES
  ...
  BRepLib_MyNewClass.hxx
  BRepLib_MyNewClass.cxx
)

4. Create a GTest:

cd src/ModelingAlgorithms/TKTopAlgo/GTests
touch BRepLib_MyNewClass_Test.cxx

5. Add GTest to CMake: Edit src/ModelingAlgorithms/TKTopAlgo/GTests/FILES.cmake:

set(OCCT_TKTopAlgo_GTests_FILES
  ...
  BRepLib_MyNewClass_Test.cxx
)

6. Build and Run:

cd build
cmake .. -DBUILD_GTEST=ON
cmake --build . --config Release
./bin/OpenCascadeGTest --gtest_filter="*MyNewClass*"

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5. Build and Test System

Build System (CMake)

• Primary build system: CMake 3.16+
• Build Directory: Always build out-of-source (e.g., build/)
• Quick Build:

  mkdir -p build && cd build
  cmake .. -DCMAKE_BUILD_TYPE=Release -DBUILD_GTEST=ON
  cmake --build . --config Release --parallel
  

• Environment: Before running OCCT executables, source the environment:
  • Linux/macOS: source build/env.sh
  • Windows: build\env.bat

Testing Frameworks

• Draw Harness: Tcl-based interactive testing in tests/. Run with DRAWEXE.
• GTest: C++ unit tests in src/.../GTests/. Enable with -DBUILD_GTEST=ON.

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6. Common Patterns & Key Packages

Common Operations

• Shape Creation: BRepPrimAPI_MakeBox, BRepPrimAPI_MakeCylinder
• Boolean Operations: BRepAlgoAPI_Fuse, BRepAlgoAPI_Cut, BRepAlgoAPI_Common
• Shape Exploration: TopExp_Explorer
• Transformations: gp_Trsf with BRepBuilderAPI_Transform

Key Packages

Package	Purpose	Module
gp	Geometric Primitives (Points, Vecs)	FoundationClasses
Geom	Geometric entities (Curves, Surfaces)	ModelingData
TopoDS	Topological Data Structures	ModelingData
TopExp	Exploring topological shapes	ModelingData
BRepAlgoAPI	High-level modeling algorithms	ModelingAlgorithms
BRepPrimAPI	Geometric primitives creation	ModelingAlgorithms
AIS	Application Interactive Services	Visualization

Common Headers

#include <Standard_Handle.hxx>           // occ::handle, occ::down_cast
#include <gp_Pnt.hxx>
#include <gp_Trsf.hxx>
#include <Geom_Circle.hxx>
#include <TopoDS.hxx>
#include <TopoDS_Shape.hxx>
#include <TopoDS_Face.hxx>
#include <TopExp_Explorer.hxx>
#include <BRep_Builder.hxx>
#include <BRepAlgoAPI_Fuse.hxx>
#include <BRepBuilderAPI_Transform.hxx>
#include <BRepPrimAPI_MakeBox.hxx>
#include <Standard_Failure.hxx>
#include <NCollection_List.hxx>          // NCollection_List<T>
#include <NCollection_DataMap.hxx>       // NCollection_DataMap<K,V>
#include <NCollection_IndexedDataMap.hxx>// NCollection_IndexedDataMap<K,V>

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7. Code Documentation Style

Header Files (.hxx) - Method Documentation

Use Doxygen-style comments with //! prefix:

//! Brief description of the method functionality.
//! Additional details if needed.
//! @param[in] theInputParam description of input parameter
//! @param[out] theOutputParam description of output parameter
//! @param[in,out] theInOutParam description of in/out parameter
//! @return description of return value
Standard_EXPORT bool MethodName(const InputType& theInputParam,
                                OutputType&      theOutputParam);

Rules:

• @param[in] for input parameters
• @param[out] for output parameters
• @param[in,out] for bidirectional parameters
• @return for return value description

Source Files (.cxx) - Method Separators

Each method MUST be preceded by:

1. A separator line of exactly 100 characters: // followed by 98 = signs
2. An empty line after the separator

//==================================================================================================

void MyClass::MyMethod(const TopoDS_Shape& theShape)
{
  // Implementation
}

//==================================================================================================

bool MyClass::AnotherMethod()
{
  // Implementation
}

DO NOT use old-style function guards:

// WRONG - Do not use this style:
//==================================================================================================
// purpose: Description of what the method does
// function: MyClass::MyMethod
//==================================================================================================

Technical Comments

Place implementation notes inside the method body:

• At the beginning for overall approach
• Inline where specific logic needs explanation

//==================================================================================================

void MyClass::ComplexMethod(const TopoDS_Shape& theShape)
{
  // Using iterative approach for better performance with large shape hierarchies.
  for (TopExp_Explorer anExp(theShape, TopAbs_FACE); anExp.More(); anExp.Next())
  {
    // Handle degenerate faces separately
    const TopoDS_Face& aFace = TopoDS::Face(anExp.Current());
    // ...
  }
}

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8. GTest Guidelines

Test Location and Naming

• Location: src/Module/Toolkit/GTests/
• File naming: ClassName_Test.cxx or PackageName_Test.cxx

Test Structure

#include <gtest/gtest.h>

#include <MyClass.hxx>

// Test fixture (optional)
class MyClassTest : public testing::Test
{
protected:
  void SetUp() override
  {
    // Setup code
  }
};

// Test with fixture
TEST_F(MyClassTest, MethodName_Scenario)
{
  // Arrange
  MyClass anObject;

  // Act
  const bool isSuccess = anObject.SomeMethod();

  // Assert
  EXPECT_TRUE(isSuccess);
}

// Standalone test
TEST(MyClassTest, BasicFunctionality)
{
  // Test implementation
}

Naming Convention

Pattern: TestFixture.MethodOrFeature_Scenario_ExpectedBehavior

Examples:

• DE_WrapperTest.Read_ValidSTEPFile_ReturnsTrue
• BRepAlgoAPI_FuseTest.TwoBoxes_ProducesValidShape
• gp_PntTest.Distance_SamePoint_ReturnsZero

Assertions

• EXPECT_* - non-fatal (test continues)
• ASSERT_* - fatal (test stops)

Common assertions:

• EXPECT_TRUE(condition) / EXPECT_FALSE(condition)
• EXPECT_EQ(actual, expected) / EXPECT_NE(actual, expected)
• EXPECT_NEAR(actual, expected, tolerance) - for floating-point
• EXPECT_THROW(statement, exception_type) - for exceptions

Running Tests

# Build
cmake ..
cmake --build .

# Run all
./bin/OpenCascadeGTest

# Run filtered
./bin/OpenCascadeGTest --gtest_filter="*MyClass*"

# List tests
./bin/OpenCascadeGTest --gtest_list_tests

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9. Key Project Files

File	Purpose
src/MODULES.cmake	Defines all modules
src/Module/TOOLKITS.cmake	Defines toolkits within a module
src/Module/Toolkit/PACKAGES.cmake	Defines packages within a toolkit
src/Module/Toolkit/Package/FILES.cmake	Lists source/header files (edit when adding files)
src/Module/Toolkit/GTests/FILES.cmake	Lists GTest source files
src/Module/Toolkit/EXTERNLIB.cmake	External library dependencies
build/env.sh / build/env.bat	Environment setup script