VTK/Wrapping hints

Sometimes the VTK wrappers need a little help in order to properly wrap certain C++ methods. Most often, this occurs when a method takes or returns a pointer. Some examples of the "unknowns" for a pointer that might have to be hinted are:

• Does the API allow the pointer to be NULL?
• Does it point to a single object, or an array?
• If an array, what is the size of the array?
• Will 'delete' or 'free()' have to be called to free the memory?
• If the pointer is 'char *', is it null-terminated? Is it ASCII, utf-8, or latin1?

Hinting with attributes

C++11 introduced attributes to provide hints for the compiler, such as the [[noreturn]] attribute to indicate that a method will never return:

[[noreturn]] void exit(int status);

We are allowed to define our own attributes, as long as we define a unique namespace, e.g. [[vtk::name]]. However, we don't want the attributes to be be "naked" in the code, because that would break pre-C++11 compilers. Instead, we conditionally define the attributes within macros (this is done within vtkSetGet.h, as for most VTK macros).

 #ifdef __VTK_WRAP__
 #define VTK_NEWINSTANCE [[vtk::newinstance]]
 #else
 #define VTK_NEWINSTANCE
 #endif

Once this macro is defined, we can use it in our method declarations.

The hint attributes that VTK provides are as follows:

[[vtk::newinstance]]

This indicates that the method returns a vtkObject that has just been created, and the caller must call Delete() on the returned value when done with it.

VTK_NEWINSTANCE
vtkObjectBase* NewInstance();

[[vtk::sizehint(expression)]]

This indicates that a pointer type, such as 'float *', points to an array of 'expression' values, where 'expression' is a C++ expression that evaluates to an integer.

VTK_SIZEHINT(3)
double* GetPoint();

// the function's parameters can be used in the hint expression
void InsertNextCell(vtkIdType n, VTK_SIZEHINT(n) vtkidType* ptIds);

// "this" is understood to be a pointer to the object
class vtkDataArray
{
  VTK_SIZEHINT(this->GetNumberOfComponents())
  double* GetTuple();

  void SetTuple(VTK_SIZEHINT(this->GetNumberOfComponents() double* tuple)
}

[[vtk::nevernull]]

This indicates that a pointer will never be null (if returned), or must never be null (if passed as a parameter).

void SetName(VTK_NEVERNULL const char *name);

[[vtk::unterminated]]

For "char *" or "const char *", this hint indicates that the string might not be terminated. Similarly, [[vtk::terminated]] indicates that the string is definitely terminated.

VTK_UNTERMINATED
const char* GetData();

[[vtk::takeref(method)]]

For a return value, this indicates that the caller should take ownership of the object and dispose of it by the given method, which can be "delete", "delete []", "free", etc. For a parameter, this indicates that the method will dispose of the object by the given method, and that the caller is no longer responsible for deletion.

VTK_TAKEREF(delete[])
char* CreateName();