Convert 2D Vector to FFTW_Complex 2D vector

In My program I have 2D Vector like (vector<vector<double>> Data(ROWS, vector<double>(COLS));),then I implement FFTW to take FFT from my vector with :

fftw_complex in[ROWS][COLS], out[ROWS][COLS];
fftw_plan g;


g = fftw_plan_dft_2d(ROWS, COLS, *in, *out, FFTW_FORWARD, FFTW_MEASURE);

How can I convert my vector to fftw_complex in[ROWS][COLS] for input fftw_plan_dft_2d function.