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	TABLE OF CONTENTS Supporting Custom C++ Types SOCI Implementation The Problem: Supporting Additional Types Traits & TypeConversion Pass-Through Version of TypeConversion Aggregate Types and Object-Relational Mapping

Traits are powerful techniques with many applications to generic programming in C++. In this article, I use examples from the SOCI database library (soci.sf.net) to show how a simple TypeConversion traits class can be used to create an extensible framework that supports arbitrary user-defined types. The complete source code that implements this framework is available at http://www.ddj.com/code/.

SOCI is a database library with a minimalist syntax that mimics that of embedded SQL. (For more information, see "A Simple Oracle Call Interface" by Maciej Sobczak; www.ddj.com/184405930.) Listing One is a complete program that connects to a database (Oracle in this case), creates a table, inserts a row, and queries the table.

#include <soci.h>
#include <iostream>
#include <string>

using namespace SOCI;
using std::string;

int main()
{
    try
    {
        Session sql("oracle", "service=gen1 user=scott " "password=tiger");
        sql << "create table Person(id number, name varchar2(50))";
        int id(100);
        string name("Bjarne");
        sql << "insert into Person values (:ID, :NAME)", use(id), use(name);
        int id2;
        string name2;
        sql << "select id, name from Person", into(id2), into(name2);
        assert(id2 == 100);
        assert(name2 == "Bjarne");
        std::cout << name2 << " has id "<< id2 << std::endl;
        sql << "drop table person";
    }
    catch(std::exception& e)
    {
        std::cout<<e.what()<<std::endl;
    }
}

To use SOCI to insert (or update) data, you first declare and initialize variables of a type appropriate to the column into which you will be inserting. Then use the free function use() to bind the variables to placeholders in your SQL statement.

Similarly, to use SOCI to select data, you define variables of an appropriate type and pass them by reference to the free function into() to have them populated with the values returned by your select statement.

1 Supporting Custom C++ Types | 2 SOCI Implementation | 3 The Problem: Supporting Additional Types | 4 Traits & TypeConversion<T> | 5 Pass-Through Version of TypeConversion | 6 Aggregate Types and Object-Relational Mapping Next Page

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