# Poco::Data

# template < class T >

# class TypeHandler

Library: Data
Package: DataCore
Header: Poco/Data/TypeHandler.h

## Description

Converts Rows to a Type and the other way around. Provide template specializations to support your own complex types.

Take as example the following (simplified) class:

    class Person
    {
    private:
        std::string _lastName;
        std::string _firstName;
        int         _age;
        [....] // public set/get methods, a default constructor, optional < operator (for set, multiset) or function operator (for map, multimap)
    };

The TypeHandler must provide a costum bind, size, prepare and extract method:

    template <>
    class TypeHandler<struct Person>
    {
    public:
        static std::size_t size()
        {
            return 3; // lastName + firstname + age occupy three columns
        }

        static void bind(std::size_t pos, const Person& obj, AbstractBinder* pBinder)
        {
            // the table is defined as Person (LastName VARCHAR(30), FirstName VARCHAR, Age INTEGER(3))
            // Note that we advance pos by the number of columns the datatype uses! For string/int this is one.
            poco_assert_dbg (pBinder != 0);
            TypeHandler<std::string>::bind(pos++, obj.getLastName(), pBinder);
            TypeHandler<std::string>::bind(pos++, obj.getFirstName(), pBinder);
            TypeHandler<int>::bind(pos++, obj.getAge(), pBinder);
        }

        static void prepare(std::size_t pos, const Person& obj, AbstractPreparation* pPrepare)
        {
            // the table is defined as Person (LastName VARCHAR(30), FirstName VARCHAR, Age INTEGER(3))
            poco_assert_dbg (pPrepare != 0);
            TypeHandler<std::string>::prepare(pos++, obj.getLastName(), pPrepare);
            TypeHandler<std::string>::prepare(pos++, obj.getFirstName(), pPrepare);
            TypeHandler<int>::prepare(pos++, obj.getAge(), pPrepare);
        }

        static void extract(std::size_t pos, Person& obj, const Person& defVal, AbstractExtractor* pExt)
        {
            // defVal is the default person we should use if we encunter NULL entries, so we take the individual fields
            // as defaults. You can do more complex checking, ie return defVal if only one single entry of the fields is null etc...
            poco_assert_dbg (pExt != 0);
            std::string lastName;
            std::string firstName;
            int age = 0;
            // the table is defined as Person (LastName VARCHAR(30), FirstName VARCHAR, Age INTEGER(3))
            TypeHandler<std::string>::extract(pos++, lastName, defVal.getLastName(), pExt);
            TypeHandler<std::string>::extract(pos++, firstName, defVal.getFirstName(), pExt);
            TypeHandler<int>::extract(pos++, age, defVal.getAge(), pExt);
            obj.setLastName(lastName);
            obj.setFirstName(firstName);
            obj.setAge(age);
        }
    };

Note that the TypeHandler template specialization must always be declared in the namespace Poco::Data. Apart from that no further work is needed. One can now use Person with into and use clauses.

## Member Summary

Member Functions: bind, extract, prepare, size

## Constructors

## Destructor

### ~TypeHandler

~TypeHandler();

## Member Functions

### bind

static void bind(
std::size_t pos,
const T & obj,
AbstractBinder * pBinder
);

### extract

static void extract(
std::size_t pos,
T & obj,
const T & defVal,
AbstractExtractor * pExt
);

### prepare

static void prepare(
std::size_t pos,
const T & obj,
AbstractPreparation * pPrepare
);

### size

static std::size_t size();