How to code for SELECT * with SOCI?

Question

In most examples for SOCI, the caller must know the fields and types in a query and specifies them through a soci::rowset<> of boost::tuple<> or through some other means.

Is there a way to code without knowing the types and number of columns beforehand, as in a SELECT * FROM ... query?

If so, could you post a short example?

Answer 1

soci::rowset<soci::row> is exactly what you need — soci::row provides dynamic binding.

From the docs:

For certain applications it is desirable to be able to select data from arbitrarily structured tables (e.g. via "select * from ...") and format the resulting data based upon its type. SOCI supports this through the soci::row and soci::column_properties classes.

See:

http://soci.sourceforge.net/doc/3.2/exchange.html#dynamic
http://soci.sourceforge.net/doc/3.2/statements.html#rowset

for details.

Excerpt from the documentation:

For example, the code below creates an XML document from a selected row of data from an arbitrary table:

row r;
sql << "select * from some_table", into(r);

std::ostringstream doc;
doc << "<row>" << std::endl;
for(std::size_t i = 0; i != r.size(); ++i)
{
    const column_properties & props = r.get_properties(i);

    doc << '<' << props.get_name() << '>';

    switch(props.get_data_type())
    {
    case dt_string:
        doc << r.get<std::string>(i);
        break;
    case dt_double:
        doc << r.get<double>(i);
        break;
    case dt_integer:
        doc << r.get<int>(i);
        break;
    case dt_long_long:
        doc << r.get<long long>(i);
        break;
    case dt_unsigned_long_long:
        doc << r.get<unsigned long long>(i);
        break;
    case dt_date:
        std::tm when = r.get<std::tm>(i);
        doc << asctime(&when);
        break;
    }

    doc << "</" << props.get_name() << '>' << std::endl;
}
doc << "</row>";

The type T parameter that should be passed to row::get() depends on the SOCI data type that is returned from column_properties::get_data_type().

Answer 2

I don't believe there's a way. Likely because 'select *' is generally considered dangerous. Columns can be added, re-ordered, etc and your query is now broken. Save your future self some debugging and list the columns out.