  It is widely believed that magnetic flux ropes are the key structure of solar
eruptions; however, their observable counterparts are not clear yet. We study a
flare associated with flux rope eruption in a comprehensive radiative
magnetohydrodynamic simulation of flare-productive active regions, especially
focusing on the thermodynamic properties of the plasma involved in the eruption
and their relation to the magnetic flux rope. The pre-existing flux rope, which
carries cold and dense plasma, rises quasi-statically before the eruption
onsets. During this stage, the flux rope does not show obvious signatures in
extreme ultraviolet (EUV) emission. After the flare onset, a thin `current
shell' is generated around the erupting flux rope. Moreover, a current sheet is
formed under the flux rope, where two groups of magnetic arcades reconnect and
create a group of post-flare loops. The plasma within the `current shell',
current sheet, and post-flare loops are heated to more than 10 MK. The
post-flare loops give rise to abundant soft X-ray emission. Meanwhile a
majority of the plasma hosted in the flux rope is heated to around 1 MK, and
the main body of the flux rope is manifested as a bright arch in cooler EUV
passbands such as AIA 171 \AA~channel.
