Nios II and Cyclone II confusion

sorry for confusing you

nios executes C/C++

but nios itself if coded in some kind of hdl as other functionality is done in vhdl or verilog hdl or schematic entry

if you have an altera fpga that has a nios ond some other functionality then you can swap to almost any other altera fpga. for example you need a bigger fpga or a faster one ...

C/C++ is your application that is executet by the nios cpu.

and to completly confuse you, some parts of you application can accelerated by the c to hardware functionality to speed up the stuff.