Boost logo

Boost :

Subject: Re: [boost] sqlpp11, 3rd iteration
From: Roland Bock (rbock_at_[hidden])
Date: 2014-08-19 03:21:02

On 2014-08-19 04:17, Gavin Lambert wrote:
> On 19/08/2014 09:55, Roland Bock wrote:
>>> I'm guessing that the problem exists because select is lazily
>>> executed. C++ objects corresponding to the data aren't created (e.g.
>>> std::string, int, float, etc.). Instead some pointers to buffers are
>>> kept (in sqlpp::result_field_t?). And C++ objects are created and
>>> returned later from value() method or conversion operators each time
>>> one of them is called. Is that right?
>> Not quite. Lets look at integral.h and the partial specialization of
>> template<typename Db, typename FieldSpec>
>> result_field_t<integral, Db, FieldSpec>
>> {...};
>> 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).
> Presumably it is the backend that knows whether the column/result is
> NULL or not.
> 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>?
As suggested by Adam:

optional<int> member;
if (!member)
  member = 0
get_int_field(17, member.get());

> 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.



Boost list run by bdawes at, gregod at, cpdaniel at, john at