Re: [Boost-users] [Tuple] Help getting started with Tuple-based meta-programming

20 Mar 2009


      AMDG

Dominique Devienne wrote:
...
// Context: retrieving all the rows (result set) from a given SQL query.
// pseudo code of boilerplate code I keep repeating right now
struct { int id; std::string name; } Row;
void doit() {
    ResultSet rset = sql_exec("select id, name from t");
    std::vector<Row> rows;
    while (rset.hasMoreRows()) {
        Row row;
        row.id = rset.get<int>(0);
        row.name = rset.get(1);
        rows.push_back(row);
    }
    // process rows
}
// Code I'd like to write instead
void doit() {
    typedef boost::tuple Row;
    std::vector<Row> rows;
    sql_get_rows("select id, name from t", rows); // optional arg to
get only first N elements
    // process rows
}
PS: returning the vector would be better, but because of the copy
involved I guess I must pass it as a non-const ref instead, to fill it
up. Will the new move semantic from the upcoming standard allow a
function to return the vector without the copy? Just curious
In this case, I think Boost.Fusion can help (warning untested).

#include 
#include 

struct get_t {
    ResultSet& r;
    int& n;
    template<class T>
    void operator()(T& t) const {
        t = r.get<T>(n++);
    }
};

template<Row>
void get_rows(const char* query, std::vector<Row>& out) {
    ResultSet rset = sql_exec(query);
    while(rset.hasMoreRows()) {
        Row row;
        int n = 0;
        get_t getter = { rset, n };
        boost::fusion::for_each(row, getter);
        out.push_back(row);
    }
}

In Christ,
Steven Watanabe