Re: [boost] SQL library with full embedded SQL-syntax

From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2007-03-15 14:38:59

>> Query<string,int> insert_thing(db, "insert into things(name,qty) values ($1,$2)");
>> SingletonQuery<int, string> count_things(db, "select sum(qty) from 
>> things where name=$1");
>> 
>> insert_thing("table",1);
>> insert_thing("chair",4);
>> int n_pens = count_things("pen");

> I'd say that your interface is another use case for variadic templates.

Agreed absolutely.

> http://www.generic-programming.org/~dgregor/cpp/variadic-templates.html
>
> How many columns can Query support?

I first wrote it with 3, and then hacked it to use the Boost 
preprocessor library so you can set a preprocessor symbol to define how 
many you want.  Of course the code is now unreadable...

> It's been my dream for a couple years now to try 
> and get all the folks interested in db access to work together to get a Boost 
> library and then subsequently a standards proposal completed.

Well here's my view on the subject, for what it's worth.  Few 
applications will be portable between databases due to the feature 
differences between those databases.  So the benefit of a standard API 
is minimal: it doesn't make your application portable, but it might 
make you more productive on your next project when it's on a different platform.

IMHO, where Boost can help is by providing the building blocks that can 
be used to build database-specific (and maybe application-specific) 
interfaces, as I have done.  My PostgreSQL library greatly benefits 
from the use of libraries including preprocessor, iterator_facade, 
shared_ptr, lexical_cast, type_traits, string algorithms, etc.

In my present experience, the weak spot with C++/SQL interfacing is 
with the representation of query results.  I want it to be a 
sequence<struct>, and sequence<tuple<...>> just isn't quite good enough 
because of the awkwardness of accessing a member of a tuple compared to 
a field in a struct.

struct thing {
   string name;
   int qty;
};

Query<???> get_things(.....);

sequence<thing> things = get_things();

This needs either some sort of introspection/relection, or an "IDL 
compiler" type of approach.  Similar to serialisation of course.  Now 
this is something that I would love to see Boost get involved with.  I 
have a vague recollection that Bjarne Stroustrup was involved with an 
introspection project at one time; I don't know if this came to anything.

> I'd love to see a detailed review from you of the SOCI 
> interface -- advantages, disadvantages, etc.

The 'operator,' syntax is not immediately appealing :-(

Phil.