Re: [boost] sqlpp11, 3rd iteration

Subject: Re: [boost] sqlpp11, 3rd iteration
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2014-08-19 03:39:53

On 19/08/2014 19:21, Roland Bock wrote:
> On 2014-08-19 04:17, Gavin Lambert wrote:
>> On 19/08/2014 09:55, Roland Bock wrote:
>>> Now, this contains an int64_t value. The address of this value is given
>>> to the backend in the method bind() when fetching each result row (no
>>> laziness here). It seems to me that I cannot replace int64_t by
>>> boost::optional<int64_t>. For instance, I cannot call get() to obtain
>>> the address of the value if the optional is not initialized (I would run
>>> into an assert).
[...]
>> Therefore it should be the backend's responsibility to fill in the
>> optional correctly
> If I had an optional in the result field, yes.
>> -- ie. you should be passing an address/reference to the entire
>> optional, not the internal integer.
> Some backends have functions like this (simplified):
>
> void get_int_field(int index, int* retval);
>
> How would I interact with such an interface if I had an optional<int>?

I think we've had a terminology clash.  By "backend" I thought you meant 
"the sqlpp11 class that knows how to talk to the native driver", not the 
native driver itself.  Of course the native driver probably won't know 
how to drive an optional, nor should it be expected to.

There are several layers, I assume:

1. User code
2. sqlpp11 database-independent frontend
3. sqlpp11 database-specific connector
4. native database library

Between layers 3 & 4 obviously you have to use whatever the native 
library supports, which is unlikely to be boost::optional (but still 
possible in some cases).  So you might have to provide a raw int64_t 
pointer to the database up front, and translate from a int64_t pointer 
and an "is this null" method call (or a bool*) to a boost::optional when 
it calls you back saying the complete row is ready.  (I'm assuming this 
is asynchronous, otherwise it's easier.)

But between layers 1 & 2 and 2 & 3 you'd only have boost::optionals.

>> Otherwise how does the backend return a NULL value?
> The backend is called with two parameters, one pointer for the value,
> the other for the is_null information.

So once you know that those have been filled in, you can translate it 
into a boost::optional to be returned to the higher layer.  It does mean 
the value has to be copied (unless boost::optional has had move 
assignment added since I last looked), but you'd be doing that anyway 
for std::string so this doesn't seem any worse than that.