In-place fft. How to alias fftw_complex* to double* in C++11

I have a class which represents a real 2D scalar field. As is suggested here, I allocated the memory in a contiguous block, i.e using a double* pointer and fftw_malloc.

Now I want the DFT of this real 2D scalar field. Following the documentation of the FFTW3 library I have written the following code.

#include <cmath>
#include "RGFun.hpp"

int main()
{
  int N = pow(2,6);
  RGFun A(N+2,N); //adding two extra columns as suggested for real transforms.
  
  int n0,n1;
  n0 = n1 = N;
  int len = n0*(n1/2 + 1);
  double* in = A.getMem();
  fftw_complex* out = (fftw_complex*) fftw_malloc(sizeof(fftw_complex)*len);
  
  fftw_plan p = fftw_plan_dft_r2c_2d(n0,n1,in,out,FFTW_FORWARD);
  fftw_execute(p);
  fftw_destroy_plan(p);
  fftw_free(in);
  fftw_free(out);
  return 0;
}

Now this code compiles but when executed gives segmentation fault. I read that for an in-place transform the in pointer must be equal to the out pointer. How can I do this in C++11?