add 0.5 before casting (if x > 0) or subtract 0.5 (if x < 0), because the compiler will always truncate.
float x = 55; // stored as 54.999999...
x = x + 0.5 - (x<0); // x is now 55.499999...
int y = (int)x; // truncated to 55
C++11 also introduces std::round, which likely uses a similar logic of adding 0.5 to |x| under the hood (see the link if interested) but is obviously more robust.
A follow up question might be why the float isn't stored as exactly 55. For an explanation, see this stackoverflow answer.