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Re: [SG7] Reflection Usecases

From: Roland Bock <rbock_at_[hidden]>
Date: Wed, 12 Jan 2022 20:57:13 +0100
On 12.01.22 19:47, Jonathan O'Connor via SG7 wrote:
> Matus,
> As all the interesting work is in creating the select statement, it
> might be easier and quicker to just generate the string! Way easier to
> write tests too!

Actually, I am really looking forward to reflection and code generation
to rewrite https://github.com/rbock/sqlpp11 :-)

Having my SQL expressions validated by the compiler was life changing.

> From what I can see from the ruby world, people use the name of the
> class, function or parameter to generate strings and code. I would guess
> the tricky one is getting the name of the parameter, rather than an
> argument (I had to look up the difference. Parameter name is given in
> the declaration and argument name is given in the definition.)
> The examples you've shown so far seem to be writing a reflecting
> function thatreads, writes or calls members of a struct. My table proxy
> example would require you to generate a class and define member
> functions.Will that be possible with your code? I have to read more of
> the existing SG7 papers.
> Regards,
> Jonathan
> Sent with ProtonMail <https://protonmail.com/> Secure Email.
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Wednesday, January 12th, 2022 at 17:09, Matus Chochlik via SG7
> <sg7_at_[hidden]> wrote:
>> Hi Jonathan,
>> On Wed, Jan 12, 2022 at 4:58 PM Jonathan O'Connor via SG7
>> <sg7_at_[hidden] <mailto:sg7_at_[hidden]>> wrote:
>> Hi everyone,
>> I'm the new official Irish delegate on WG21, and I joined as I'm
>> specifically interested in SG7.
>> I promised Matus Chochlik on twitter the other day that I would
>> search for example reflection use cases in "Metaprogramming Ruby"
>> by Paolo Perrotta. An email is more appropriate than multiple
>> answers in twitter.
>> Sadly, and probably not surprisingly, I found few examples of
>> reflection that are appropriate to C++.
>> The one trick that is used a lot in the Ruby on Rails framework is
>> generating method code based on the name of the method. This trick
>> is often used to generate SQL finders:
>> For example:
>> ```ruby
>> User.findByName("Jonathan") # generates a SELECT * from users
>> where name = 'Jonathan'
>> ```
>> Converting this into C++ might require an interface class:
>> ```c++
>> struct IUserTable : public MagicDBTable {
>> std::vector<User> findByName(std::string_view name);
>> };
>> // The getTable function generates an implementation of the
>> IUserTable class that
>> // generates the appropriate select statement for each member
>> function.
>> // It uses the class name to figure out the DB table name. The
>> where clause is
>> // generated from the member function name.
>> auto userTable = MagicDB::getTableProxy<IUserTable>();
>> auto jonathans = userTable.findByName("Jonathan");
>> ```
>> Thanks for the suggestion, I'll set up a DB server and try to
>> implement this.
>> BTW. here is the current set of examples/use-cases:
>> https://github.com/matus-chochlik/mirror/tree/develop/example/mirror
>> <https://github.com/matus-chochlik/mirror/tree/develop/example/mirror>
>> Some might look somewhat silly, but I use this also to test various
>> parts of the compiler implementation.
>> Cheers,
>> --Matus

Received on 2022-01-12 19:57:15