How to access dynamic C++ array variable, belonging to an object, with Python?

Question

I have a C++ class with a variable that is a dynamic array. It is extremely simple, currently only for testing purposes:

class testClass {

public:
    int *x;

    testClass();
    ~testClass();
};

The variable x is initialised with some values, currently through the constructor. I am trying to write a python wrapper code for the C++ class through Cython that can access x . How can I do this?

Best thing would be to be able to access the variable without copying a lot of data, as x might be large. Can I access it as a numpy array?

Example, that it behave as numpy array, that could be read only for instance? I want to be able to use the data in x in other calculations with python, so a numpy array is preferred.

I suppose I could make a GET method that initialize a numpy array, passes it it to the GET method and populate it with the data from x with a loop, but this would copy the data, and does not seem very elegant. Hoping for a better solution.

I have tried with a static array, and found a solution that sort of worked. If x is static, I could do the following in a .pyx file:

cdef extern from "testClass.h":
    cdef cppclass testClass:
        testClass()
        int x[5]

cdef class pyTestClass:
    cdef testClass *thisptr

    def __cinit__(self):
        self.thisptr = new testClass()
    def __dealloc__(self):
        del self.thisptr

    property x:
        def __get__(self):
            return self.thisptr.x

If I access x in Python, I will get a Python list with the values back.

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How to access dynamic C++ array variable, belonging to an object, with Python?

Answer 1

Have a look at this example that explains how to expose arrays allocated in C to numpy without data copies. The relevant line to create, for instance, a 1D numpy array of integers in Cython with already allocated data is,

ndarray = np.PyArray_SimpleNewFromData(1, shape, np.NPY_INT, data_pointer) 

See the corresponding Numpy documentation .