@foreach
Description:
Calls the provided function once for each field in the Structure.
The function must not modify the fields. Also see map_in_place and filter_and_map_in_place.
Parameters:
| this | |
| func | a function to call for each field | 
| user_data | private data | 
Returns:
| true if the supplied function returns true For each of the fields, false otherwise. |