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

Subject: Re: [boost] sqlpp11: SQL for C++
From: Roland Bock (rbock_at_[hidden])
Date: 2013-11-11 10:19:39

On 2013-11-11 15:06, Larry Evans wrote:
> On 11/11/13 02:36, Roland Bock wrote:
>> On 2013-11-11 01:47, Larry Evans wrote:
>>> On 11/10/13 18:02, Larry Evans wrote:
>>>> On 11/09/13 16:03, Roland Bock wrote:
>>>>> Hi,
>>>>>
>>>>> over the last four or five years I developed several SQL libraries
>>>>> for
>>>>> C++. With C++11 I was finally able to create an SQL library that
>>>>> meets
>>>>> my own expectations and requirements. It is being used in
>>>>> production, I
>>>>> recently put it on github, and I would really like to hear from you
>>>>> guys
>>>>> whether this is something that could be interesting for you
>>>>> personally
>>>>> or for boost?
>>>>>
>>>>> https://github.com/rbock/sqlpp11
>>>>> https://github.com/rbock/sqlpp11/wiki
>>>>>
>>>> [snip]
>>>> The code here:
>>>>
>>>> https://github.com/rbock/sqlpp11/blob/master/include/sqlpp11/table_base.h
>>>>
>>>>
>>>>
>>>> contains:
>>>>
>>>>      template<typename Table, typename... ColumnSpec>
>>>>      struct table_base_t
>>>>      : public ColumnSpec::_name_t::template
>>>>        _member_t<column_t<Table, ColumnSpec>>...
>>>>      {...};
>>>>
>>>> which looks like it declares a table row, where the columns
>>>> come from ColumnSpec...
>>
>> In understanding how the library works, this is probably one of the
>> crucial parts.
>> As you guessed correctly, it declares the innards of a table class. The
>> somewhat convoluted looking inheritance adds columns as members to the
>> class. The
>>
>> column_t<Table, ColumnSpec>
>>
>> instantiates a column class with all required information like the value
>> type, whether it can be NULL, its textual representation, etc. The
>> ColumnSpec also contains a template like this:
>>
>> template<typename T>
>> struct _member_t
>> {
>>      T foo;
>> };
>>
>>
>> Inheriting from an instance of this template adds a member with the
>> respective name (foo in this example) to derived class.
>> Thus, the code you cited adds objects representing columns as
>> appropriately named members to the table class.
>>
>
> Ah.  I see.  So tab_sample, if declared as:
>
>   TabSample tab_sample;
>
> where TabSample is from:
>
>   https://github.com/rbock/sqlpp11/blob/master/tests/TabSample.h
>
> could be used in expressions like:
>
>   tab_sample.alpha;
>   tab_sample.beta;
>   tab_sample.gamma;
>
> Nice!  Makes meaningful column names.
>
Exactly! And this is transported into the result rows as well, e.g.

for (const auto& row: db.run(select(all_of(tab_sample)).from(tab_sample)))
{
   std::cerr << row.alpha << '\n';
   std::cerr << row.beta << '\n';
   std::cerr << row.gamma << '\n';
}

And they also have appriate types of course :-)

> However, instead of table_base_t, would maybe row_base_t
> be a better name since it's really a row in a table?

It is a set of columns, which more or less represents a table (if you
ignore indexes and foreign keys, etc). So I wouldn't say that it is a row.