Re: [boost] Review of Boost.MySql

12 May 2022

      On Wed, 11 May 2022 at 06:16, Dennis Luehring via Boost
 wrote:
...
Am 10.05.2022 um 22:33 schrieb Marcelo Zimbres Silva via Boost:
...
users could do
boost::mysql::row row;
    while (result.async_read_one(row, additional_info, yield[ec]))
avoiding unnecessary further copies. In principle it would be even
possible to parse directly in a custom data structure, for example,
when storing json string in the database.
i've developed a small C++17 example of how to work with
Input/Output transformers to write the statements more compact
and type-safe
the base idea is that the MySQL <-> C++ needs sometimes some sort
of conversions and to combine most of the infos at one point so its
easier or at least possible to optimize better - the current interface
looks very runtime allocation stylish :)
Thanks for sharing. I think I get the idea of "allowing the user to define
custom MySQL <=> C++ mappings".

I'm not really getting the interface that your input/output
transformers would expose, though. Let's take this one, for example:

template <>
struct OutputTransformer<NullAsEmptyString>
{
    using value_type = std::string;

    static std::string get_value( int /*index_*/ )
    {
        static std::string x = "hello";
        return x;
    };
    static void get_value( int index_, std::string& value_ )
    {
        value_ = get_value( index_ );
    };
};

I assume this is user-provided code. Where does the user get the
string value from? What does index_ mean in this context?

Let's also take a look at actually using the statements:

        My_select my_select( db_connection, "select a, b, c from test
where d == ?1" );

        {
            // fetch into ref tuple
            int some_int{};
            float some_float{};
            std::string some_string;
            my_select( { some_int, some_float, some_string }, { 123 } );
        }

How is that different from the following snippet?

resultset r = conn.query("select a, b, c from test where d == ?1" );
tuple row;
r.read_one(row);
...
the Input- and Output Transformers help to work with basic types
and also with SQL/MySQL special types like Null-string etc. - its not
possible to map MySQL types always 1:1 to C++ and back, sometimes you
want to
behave the transformation different
the Transformers aren't visible when used with basic types
its just an example to promote the Idea: https://pastebin.com/raw/vepbTAKL
the best combination would be some sort of fluent SQL interface like:
https://github.com/rbock/sqlpp11
This library is supposed to be a protocol driver library, so it
provides primitives
close to the MySQL protocol. sqlpp11 is great, but it's a higher level library.
I don't think it makes sense trying to incorporate this kind of features here.
It would make more sense for a higher level library like sqlpp11 to build
on top of Boost.MySQL, instead.
...
but i still think a plain string+binding interface like Boost.MySql
currently got is also needed
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Re: [boost] Review of Boost.MySql

Ruben Perez