Q: Help with compile time function

[Jari-Matti Mäkelä]

Myron Alexander wrote:

> Hello.
> 
> I want to check that a structure contains only the supported types for a
> fetch from the database. I want to check this at compile time and issue
> an assert. I have come up with the following:
> 
>> /**
>>  * Compile time check that structure field types match supported types
>>  for * structure fetch.
>>  *
>>  * Supported types :-
>>  *    int (int32), long (int64), char[] (utf-8), void[] (blob), float,
>>  double. *
>>  * Unsupported types :-
>>  *    wchar[] (utf-16)
>>  *    void*   (null)
>>  *
>>  * It does not make sense to support null as a value return type so it
>>  will * never be supported. The utf16 type is not supported at this time
>>  as I do * not know how to support it. Only partial support for utf16 in
>>  the system. */
>> static void chkStrucOkForFetch(T) () {
>>    foreach (i, f; FieldTypeTuple!(T)) {
>>       static assert (
>>          is (typeof(f) == int)    || // int32
>>          is (typeof(f) == long)   || // int64
>>          is (typeof(f) == char[]) || // utf-8
>>          is (typeof(f) == void[]) || // blob
>>          is (typeof(f) == float)  ||
>>          is (typeof(f) == double)
>>          , "Invalid field type for structure fetch. Field ("~str(i)~
>>            ") not one of int, long, char[], void[], float, double."
>>       );
>>    }
>> }
> 
> This code performs the check but I cannot get it to print the name of
> the structure and the field.
> 
> Please can you help me with the following:
> 
>    - How can I get the name of the structure (T) and field type at
> compile time?

You mean T.stringof and f.stringof ?

>    - If I call this from a normal run-time function, will it get
> executed at run-time as well as in compile-time?

I'm afraid that function above doesn't do anything on runtime. You need to
use the RTTI system to get the type at runtime.

> 
> Thanks and best regards,
> 
> Myron.

--