Re: [boost] sqlpp11, 3rd iteration

Roland Bock wrote:

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On 2014-08-19 16:43, Adam Wulkiewicz wrote:

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I'm wondering, could it be possible to implement similar, compile-time interface in sqlpp, e.g. something similar to MPL? I'm asking because what the sqlpp library requires is very complicated:https://github.com/rbock/sqlpp11/blob/master/tests/Sample.h I'm aware that its purpose is to create a type reflecting required columns and to use them from the level of C++ but couldn't it be done simpler? The crucial part is the _member_t template. I need that one. The other stuff can be constructed whatever way you like. But if the name of the

column is `beta`, then I need a way to get this:

template<typename T> struct _member_t { T beta; T& operator()() { return beta; } const T& operator()() const { return beta; } };

This is the magic ingredient that makes tables, result rows and parameter sets of prepared statements to have members with a proper name.

The way it works is best to be observed in the table_t template, see https://github.com/rbock/sqlpp11/blob/master/include/sqlpp11/table.h

template struct table_t : public table_base_t, public ColumnSpec::_name_t ::template _member_t>...

Ok AFAIU a struct like _member_t should define some convenient member variable for the user and must define operator() for the library. But the rest could be automatically generated, couldn't it? Why not just pass a list of templates of classes adapted to MemberType concept (defined operator()) into the table/column/etc.? I'm thinking about something like the code below. I don't know exactly what's required so this is just an example of a technique rather than a solution ready-to-use in sqlpp. The idea is good. For the columns, you will have to add a few more

On 2014-08-19 18:56, Adam Wulkiewicz wrote: parameters, e.g. the value_type (mandatory), can_be_null, must_not_insert, must_not_update, null_is_trivial, trivial_is_null, maybe as per your suggestion a default value or a function for producing it. That default stuff might be tough in such a design. But thats manageable. And yes, the code would be shorter, although not that much, I suspect. The only problem I have with it is that now the column types are going to be about a hundred characters long. And users are going to operate on columns all the time. So error message have to be short. I would thus add a struct which inherits from the column template instance for each column, e..g. struct alpha: public column {}; I tried something similar a while back but failed, which is mainly due to lack of perseverance, I guess. Right now, I am happy with the current design because it is quite easy to change things, like introducing that default value or a function for handling attempts to read NULL. If you want to put everything into that one list of template parameters, it is much tougher, IMO. I mean how would you add a function for handling access to NULL value? You would need another class, I think. And you would have to group those tags into a tuple or type_set, because otherwise it would be ugly to add another optional parameter... [...]

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I see no way to create such a template other than having it in the code. You should of course not write it personally. You should use macros (yuk) or code generators similar to the ddl2cpp script in the repository. But you have to somehow create this code.

Unless you know some brilliant TMP technique for this? Personally I think that is a missing feature in C++. I'd call it named member mixin. Or something like that. Hmm. Or, in addition to names and values, we should be able to declare names in templates. That would be awesome!

Anyway, if the name thing can be solved without macros, I am all for a terser notation :-)

Maybe in one of the future standards... :)

Yeah, I should start working on that. I am really quite fascinated by the idea of having names as template parameters. Cheers, Roland