I went for a night flight last night to keep myself current and polish my night landings. It was a beautiful clear night, good visibility but only 3 degrees on the ground when I took off and only about 1 degree when I landed.
As it was a beautiful clear night, it's highly unlikely that the frost formed in flight. For ice to form on the leading edges in flight, there has to be a concentration of super-cooled water droplets (like in a cloud) that the aircraft impacts as it flies through them.
Another way that ice can form in flight is when a super-cooled airframe flies through rain or drizzle.
As neither of these conditions pertained on the night in question, I would suggest that the frost formed after landing, while you were taxying in what was probably a high-humidity environment. The airframe was cooled during its recent flight and the water vapour in the air sublimated directly to ice.
There are two scenarios to consider:
1. Pre-flight.
As others have mentioned, taking off with any ice on the wings or tail is a mug's game. Thin frost may be acceptable on the fuselage, provided that any aerials, intakes, vents or probes (and the areas ahead of them) are clear. Heavy frost or ridges formed by previously-melted ice are unacceptable. If in doubt, the best way is a perfectly clean airframe.
2. In flight.
Dealing with ice accumulation in flight depends on the certification and equipment fitted to the aircraft. In a C.172, no ice accumulation is certified, therefore avoidance is the only sensible policy. If an inadvertent encounter with ice is experienced, the following may be a sensible course of action:
Full Carb Heat (Adjust Mixture as required)
Pitot heat ON
Windshield heat ON
Try to exit the icing (180* turn, climb, descent)
Be aware of MSA
Cross-check instruments
Use Alternate Static Source if required
If landing with residual ice, conduct a low-speed handling check at a safe altitude before making the approach. Be very careful when lowering flaps and be prepared to retract them if handling is degraded. Consider making a flapless landing.