C++ template meta sql

Question

I wrote a wrapper around boost-mpl and transformed it to an sql engine. It is a compile time sql thing, not related to normal databases. I have not finished it completely. I have some problem with the interface.

A template meta program run in compile time. So input data always present in compile time that is why I organized the inputs to tables.

Tables are simple:

#define DEF_COLUMN(name) struct Col##name {}; \
 template<typename T> struct name \
 { typedef typename mpl::pair<Col##name, T> type; };
struct TestTable1
{
 DEF_COLUMN(PrimaryKey)
 DEF_COLUMN(Storage)
 DEF_COLUMN(BaseType)
 typedef mpl::vector<ColPrimaryKey, ColStorage, ColBaseType> header;
 // sample table
 // PrimaryKey | Storage | BaseType
 // -----------+---------+-----------
 // CTypeA1 | int | CTypeABase
 // CTypeA2 | string | CTypeABase
 typedef mpl::vector<
 mpl::map3<PrimaryKey<CTypeA1>::type, Storage<int>::type, BaseType<CTypeABase>::type>,
 mpl::map3<PrimaryKey<CTypeA2>::type, Storage<std::string>::type, BaseType<CTypeABase>::type>
 > type;
};
struct TestTable2
{
 DEF_COLUMN(Tag)
 DEF_COLUMN(Attribute)
 typedef mpl::vector<
 mpl::map2<Tag<CTypeA1>::type, Attribute<CTypeB1>::type>,
 mpl::map2<Tag<CTypeA2>::type, Attribute<CTypeB1>::type>,
 mpl::map2<Tag<CTypeA2>::type, Attribute<CTypeB2>::type>
 > type;
};

And you can make queries on it:

void TestQuery1()
{
 typedef Select<
 Columns<MplFunction<Column<TestTable1::ColStorage>, mpl::sizeof_> >,
 From<TestTable1,
 Join<TestTable2, Equal<TestTable1::ColPrimaryKey, TestTable2::ColTag> >
 >
 > TResultSet;
 DEBUG_TYPE((TResultSet));
 print_table<TResultSet::type>();
}

I have type debug macro and table print template helper.

So the above prints:

TResultSet => Select<Columns<MplFunction<Column<TestTable1::ColStorage>, boost::mpl::sizeof_>, mpl_::na>, From<TestTable1, Join<TestTable2, Equal<TestTable1::ColPrimaryKey, TestTable2::ColTag> > >, Where<True> >
+------------------------+
| TestTable1::ColStorage |
+------------------------+
| mpl_::size_t<8ul> |
| mpl_::size_t<8ul> |
| mpl_::size_t<4ul> |
+------------------------+
3 rows in set

I have problem with the syntax, how can I improve this, to achive an easy to use meta query thing? I think I have to eliminate the Columns, Column thing from the

Select<Columns<MplFunction<Column< ... 

expression. And I need good Row fetch things and others. Any idea? And where can I upload the source?

So this gives back compile time types, not runtime values.

void TestQuery3()
{
 typedef Select<
 Columns<Column<TestTable1::ColStorage> >,
 From<TestTable1>,
 Where<
 Equal<TestTable1::ColPrimaryKey, CTypeA1>
 >
 > TResultSet;
 DEBUG_TYPE((TResultSet));
 print_table<TResultSet::type>();
 // fetch one row
 typedef mpl::back<TResultSet::type>::type TResultRow;
 // get column ColStorage which is an int, TRes will be an int
 typedef mpl::at<TResultRow, TestTable1::ColStorage>::type TRes;
 // use the "int" type
 TRes testInteger = 9;
 // prints test value
 std::cout << "testInteger:" << testInteger << std::endl;
}

this prints:

TResultSet => Select<Columns<Column<TestTable1::ColStorage>, mpl_::na>, From<TestTable1, mpl_::na>, Where<Equal<TestTable1::ColPrimaryKey, CTypeA1> > >
+------------------------+
| TestTable1::ColStorage |
+------------------------+
| int |
+------------------------+
1 row in set
testInteger:9

Answer 1

You my also want to have a look at how the Poco library achieves a similar thing using a Tuple class. From the docs:

typedef Poco::Tuple<std::string, std::string, int> Person;
typedef std::vector<Person> People;
People people;
people.push_back(Person("Bart Simpson", "Springfield", 12));
people.push_back(Person("Lisa Simpson", "Springfield", 10));
Statement insert(session);
insert << "INSERT INTO Person VALUES(:name, :address, :age)",
 use(people), now;
Statement select(session);
select << "SELECT Name, Address, Age FROM Person",
 into(people), now;
for (People::const_iterator it = people.begin(); it != people.end(); ++it)
{
 std::cout << "Name: " << it->get<0>() << 
 ", Address: " << it->get<1>() << 
 ", Age: " << it->get<2>() <<std::endl;
}

• Industrial-antidepressant: Thanks, but this is run time. My meta sql gives back types, not runtime values.
• Industrial-antidepressant: But I give you +1 because I did not know that library:)