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:
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();
This indicates that a returned pointer, such as 'float *', points to an array of 'n' values.
VTK_SIZEHINT(3) double* GetPoint();
This indicates that a pointer will never be null (if returned), or must never be null (if passed as a parameter).
VTK_NEVERNULL void SetName(VTK_NEVERNULL const char *name);
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", "Py_DECREF", "Py_XDECREF", 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();