Options() is your default constructor. Structs and classes are equivalent in the sense that they can have methods and constructors. What’s after : is the initialisation list for Options(). It tells you that in your default constructor for Options():
num_particlesis initialised toNUM_PARTICLESule_lbpis initialised tofalseinfileinitialised to an empty stringoutfileinitialised to an empty string
Similar line of reasoning applies for State_.
solved Functions in Structures ? C++