How to use a functor to replace the lambda one?

[douyw]

Hello,
Thank you for this great sqlite wrapper! It's very convenient.
While using the wrapper is delightful, a problem about eliminating similar code occurs.

The functor:

struct ItemResultHandler {
    explicit ItemResultHandler(QVariantMap& map_) : map(map_) {}
    void operator() ( int id, std::string name, double price, int qty ) {
         map.insert("id", id);
         map.insert("name", QString::fromStdString(name));   
         map.insert("price", price);
         map.insert("qty", qty);
    }
    QVariantMap& map;
};

The code:

QVariantMap getItemById(int id) {
  QVariantMap map;
  database db(dbfile);
  db << "select id,name,price,qty from items where id=?"
     << id 
     >> ItemResultHandler(map);
  return map;
}

It has compiling errors:

../../myapp/src/utility/function_traits.h:12: error: type 'void (ItemResultHandler::*)(std::__1::basic_string<char>, std::__1::basic_string<char>, int, int)' cannot be used prior to '::' because it has no members
        decltype(&Function::operator())
                  ^

 ../../myapp/src/sqlite_modern_cpp.h:218: error: incomplete definition of type 'sqlite::utility::function_traits<ItemResultHandler>'
                        binder<traits::arity>::run(*this, func);
                               ~~~~~~^~

I have posted a question in SO. Could you please have a look at it? Thank you in advance.
http://stackoverflow.com/questions/30387896/how-to-eliminate-multiple-similar-sql-queries-with-one-resultset-hander

[Killili]

Hi, its a problem with mapping all those different result types we handle. So the Template got a bit complex and it looks like it does not handle this case. The new one has a more specific error but still does not handle this case.

Here is an example how you could do it with the type extensions.

    // The thing you want
    struct Item {
        int id;
        string name;
        double price;
        int qty;
    };
    // to get it back we need to give a deserialization method:
    namespace sqlite {
        template<> void get_col_from_db(database_binder& db, int inx, ::Item*& ret) {   // specify the type it will be used on, a pointer is necessary if you’re type has no default constructor. Otherwise a object ref does the job too.
            ret = new Item();
            get_col_from_db(db, inx++, ret->id);
            get_col_from_db(db, inx++, ret->name);
            get_col_from_db(db, inx++, ret->price);
            get_col_from_db(db, inx++, ret->qty);
        }
    }

    {// usage
        Item* item;
        db << "select val,name,price,qty from test" >> item;
        std::cout << item->name;
        delete item;
    }

But this is ugly and definitely needs to change, i will get rid of this pointer mess in the next Version.

[Killili]

There is however a shorter version, just convert youre functor to a lambda in this way:

auto ItemResultHandler = [](Item& item) -> std::function<void(int, std::string , double , int )> {
    return [&item](int id, std::string name, double price, int qty) mutable {
        item.id = id;
        item.name = name;
        item.price = price;
        item.qty = qty;
    };
};

and just use it like this:

Item item;
db << "select val,name,price,qty from test" >> ItemResultHandler(item);

[douyw]

I used the shorter version, and wrap the code in a factory function, something like MakeItemResultHandler.
You are a hero! Thank you very much!

[aminroosta]

@Killili please review https://github.com/aminroosta/sqlite_modern_cpp/pull/42
I don't have access to MSVC, could please also check on VS 2015.