  We present extensive ultraviolet (UV) and optical photometry, as well as
dense optical spectroscopy for type II Plateau (IIP) supernova SN 2016X that
exploded in the nearby ($\sim$ 15 Mpc) spiral galaxy UGC 08041. The
observations span the period from 2 to 180 days after the explosion; in
particular, the Swift UV data probably captured the signature of shock breakout
associated with the explosion of SN 2016X. It shows very strong UV emission
during the first week after explosion, with contribution of $\sim$ 20 -- 30% to
the bolometric luminosity (versus $\lesssim$ 15% for normal SNe IIP). Moreover,
we found that this supernova has an unusually long rise time of about 12.6
$\pm$ 0.5 days in the $R$ band (versus $\sim$ 7.0 days for typical SNe IIP).
The optical light curves and spectral evolution are quite similar to the
fast-declining type IIP object SN 2013ej, except that SN 2016X has a relatively
brighter tail. Based on the evolution of photospheric temperature as inferred
from the $Swift$ data in the early phase, we derive that the progenitor of SN
2016X has a radius of about 930 $\pm$ 70 R$_{\odot}$. This large-size star is
expected to be a red supergiant star with an initial mass of $\gtrsim$ 19 -- 20
M$_{\odot}$ based on the mass $--$ radius relation of the Galactic red
supergiants, and it represents one of the most largest and massive progenitors
found for SNe IIP.
