Re: [boost] sqlpp11: SQL for C++

Subject: Re: [boost] sqlpp11: SQL for C++
From: Roland Bock (rbock_at_[hidden])
Date: 2013-11-11 17:16:06

On 2013-11-11 22:41, Abel Sinkovics wrote:
> Hi,
>
> On 2013-11-11 21:31, Roland Bock wrote:
>> Metaparse requires const char[N] arguments, right? That would be a
>> rather atypical case for using a query interface, I'd say. Personally I
>> have never used queries without variable parameters except in examples
>> like the one above.
> Yes, it takes char[N] arguments, however, you can use a
> boost::format-like syntax (as mentioned by Christof) or a printf-like,
> type checked one.
>
>>
>>>> The string could be parsed by a template metaprogram and the right
>>>> classes could be built out of it. It could provide all the static
>>>> guarantees you have described above.
>>> Guessing from code here:
>>>
>>> https://github.com/rbock/sqlpp11/blob/master/include/sqlpp11/select.h#L574
>>>
>>>
>>>
>>> The sql string written to oss would be something like the argument
>>> to your QUERY function.  IOW, IIUC, there's no need for parsing a
>>> string to build the right classes.
>>>
>>> OTOH, the string passed to the actual database (via the db
>>> on select.h#L574) would have to be parsed, I assume, by dbms, which
>>> might issue some error message or return some error code if the
>>> sql string were not right.  I think Roland's code almost guarantee's
>>> the sql string would be correct.
>>>
>>> Is that about right Roland?
>> That is correct, Larry, nicely guessed from the code, indeed :-)
>>
>> The query is constructed via functions and objects to build an
>> expression which /can/ be evaluated as a string which is then being sent
>> to the database. This is also the current use case. But there have been
>> several ideas presented in this thread what else could be done
>> (evaluating XML or JSON or incoming streams). In those cases, it might
>> be better to transform the query into another representation.
>>
>> Regarding the correctness of the string: That's the goal, yes.
>
> If you don't want to transform the string, just validate it (and maybe
> do some variable substitution) you can approach it in a similar way
> the type-checked printf does it: it parses the string, does the
> validation at compile-time and then uses the original string at runtime.
>
> Code of it:
> https://github.com/sabel83/mpllibs/tree/master/mpllibs/safe_printf
>
> Example using it:
> https://github.com/sabel83/mpllibs/blob/master/libs/safe_printf/example/safe_printf/main.cpp
> (here check the C++11 one at the bottom)

I see use cases for printf and regex for instance, where the user
provides a textual representation of something at compile time. In those
cases, compile time validation of strings is a wonderful tool, and I
have highest respect for it.

But in the context of sqlpp11 I don't see how or why I should use it?
The library is constructing the query string at runtime. There is no
string to be validated at compile time. This is a major difference to
most other C++ SQL libraries.

Best regards,

Roland