  Radio interferometers do not measure the sky brightness distribution directly
but rather a modified Fourier transform of it. Imaging algorithms, thus, need a
computational representation of the linear measurement operator and its
adjoint, irrespective of the specific chosen imaging algorithm. In this paper,
we present a C++ implementation of the radio interferometric measurement
operator for wide-field measurements which is based on "improved $w$-stacking".
It can provide high accuracy (down to $\approx 10^{-12}$), is based on a new
gridding kernel which allows smaller kernel support for given accuracy,
dynamically chooses kernel, kernel support and oversampling factor for maximum
performance, uses piece-wise polynomial approximation for cheap evaluations of
the gridding kernel, treats the visibilities in cache-friendly order, uses
explicit vectorisation if available and comes with a parallelisation scheme
which scales well also in the adjoint direction (which is a problem for many
previous implementations). The implementation has a small memory footprint in
the sense that temporary internal data structures are much smaller than the
respective input and output data, allowing in-memory processing of data sets
which needed to be read from disk or distributed across several compute nodes
before.
