That is not a switcher as the Op-amp and transistor are linear devices. What will happen is the transistor's base current will be driven to the point where the transistor's output voltage is equal to the op-amps Vref.
Except that the wrong transistor is being used. Remember that an NPN transistor passes current from collector to emitter when current flows from base to emitter.
The transistor should be an PNP with its emitter connected to the 12V. Then the op-amp sinks current out of the base to turn on the transistor.
This circuit is really just a linear voltage regulator using a pass transistor not a switcher. You could use a biased zener diode on the transistors base instead of a Vregulator and op-amp.
Or you can just put a PNP transistor across the linear regulator to increase the regulator's current. See the application section of linear regulator's data sheet that typically shows how to use a pass transistor. Try looking at the data sheet for an LM7805 regulator.
The other part that is missing is an output filter cap.