On Sunday, 25 June 2017 at 11:39:27 UTC, Andrew Chapman wrote:
Hi guys, I'm a little confused as to whether D supports
interfaces with templates. I can compile OK, but linking
reports an error like this:
Error 42: Symbol Undefined
_D12relationaldb10interfaces21RÇëÜDBIÇêÿ35__T7loadRowTS6prefix÷6P¶ZÇêáMFAyaAS3std7variant18Çâ└8VÇåìNVki20ZÇëÅZÇûð
Essentially my desired interface looks like this:
interface RelationalDBInterface {
public T loadRow(T)(string sql, Variant[] params);
}
An example implementation:
public T loadRow(T)(string sql, Variant[] params)
{
Prepared prepared = prepare(this.conn, sql);
prepared.setArgs(params);
auto row = prepared.queryRow();
if (row.isNull()) {
throw new Exception(this.classID ~ "::loadRow - Query
returned an empty row");
}
T item;
return row.toStruct!T(item);
}
And I would try to call it like this:
auto user = this.loadRow!User(sql, params);
Is it possible, or do I need to rethink the solution? The idea
is to pass around a RelationalDBInterface so I can later switch
from MySQL to Postgres or SQLite or whatever.
You cannot have unimplemented templates in interfaces (where
would they go in the virtual function table?), just return a
variant.
Implementations of interfaces must be classes not free functions
so
class MyDB : IRelationalDB
{
// implementation ...
}
which you then need to create a instance of
auto mydb = MyDB(...); // connection
auto user = mydb.loadRow!User(sql, params);
'this' is only valid inside an aggregate (struct or class).
