game.cpp

int num_valid_controls = 0;
cata::optional<tripoint> vehicle_position;
cata::optional<vpart_reference> vehicle_controls;
for( const tripoint elem : m.points_in_radius( g->u.pos(), 1 ) ) {
    if( const optional_vpart_position vp = m.veh_at( elem ) ) {
        vehicle_controls = *vp.part_with_feature( "CONTROLS", true );
        if( vehicle_controls ) {
            num_valid_controls++;
            vehicle_position = elem;
        }
    }
}
if( num_valid_controls < 1 ) {
    add_msg( _( "No vehicle controls found." ) );
    return;
} else if( num_valid_controls > 1 ) {
    vehicle_position = choose_adjacent( _( "Control vehicle where?" ) );
    if( !vehicle_position ) {
        return;
    }
    const optional_vpart_position vp = m.veh_at( *vehicle_position );
    if( vp ) {
        vehicle_controls = *vp.part_with_feature( "CONTROLS", true );
        if( !vehicle_controls ) {
            add_msg( _( "The vehicle doesn't have controls there." ) );
            return;
        }
    } else {
        add_msg( _( "No vehicle there." ) );
        return;
    }
}
// If we hit neither of those, there's only one set of vehicle controls, which should already have been found.
if( vehicle_controls ) {
    veh = &vehicle_controls->vehicle();
    veh_part = vehicle_controls->part_index();
    if( veh->avail_part_with_feature( veh_part, "CONTROLS", true ) >= 0 ) {
        if( !veh->handle_potential_theft( dynamic_cast<player &>( u ) ) ) {
            return;
        }
        veh->use_controls( *vehicle_position );
    }
}