ReadData

dtl

Mapping a Table to a User Defined Object in Four Easy Steps:

1. Define an object to hold the rows from your query.

2. Define an association between fields in your query and fields in your object. This is what we call a 'BCA', which is short for Bind Column Addresses. In the example below, this is done via the functor "BCAExample". The job of the BCA is to equate SQL fields with object fields via the '==' operator which will then establish ODBC bindings to move data to or from a user query.

3. Create a view to select records from. This view is built from the template DBView and establishes which table(s) you want to access, what fields you want to look at (via the BCA), and an optional where clause to further limit the set of records that you are working with.

4. Use the DBView container to obtain an iterator to SELECT, INSERT, UPDATE or DELETE records from your view. These iterators may be used to either populate STL containers or apply algorithms from the Standard Template library.



In all the examples that follow we will assume that our database contains a table called DB_EXAMPLE of the form



SQL> desc db_example;

Name                            Type

------------------------------- --------

INT_VALUE                       INTEGER

STRING_VALUE                    VARCHAR

DOUBLE_VALUE                    FLOAT

EXAMPLE_LONG                    INTEGER

EXAMPLE_DATE                    DATE



// STEP 1 ////

// "Example" structure to hold rows from our database table

struct Example

{

                                        // tablename.columnname:

	int exampleInt;                 // DB_EXAMPLE.INT_VALUE

	string exampleStr;              // DB_EXAMPLE.STRING_VALUE

	double exampleDouble;           // DB_EXAMPLE.DOUBLE_VALUE

	long exampleLong;               // DB_EXAMPLE.EXAMPLE_LONG

	TIMESTAMP_STRUCT exampleDate;   // DB_EXAMPLE.EXAMPLE_DATE

};



// STEP 2 ////

// Create an association between table columns and fields in our object

template<> class dtl::DefaultBCA<Example>

{

public:

	void operator()(BoundIOs &cols, Example &rowbuf)

    	{

	   cols["INT_VALUE"] == rowbuf.exampleInt;

	   cols["STRING_VALUE"] == rowbuf.exampleStr;

	   cols["DOUBLE_VALUE"] == rowbuf.exampleDouble;

	   cols["EXAMPLE_LONG"] == rowbuf.exampleLong;

	   cols["EXAMPLE_DATE"] == rowbuf.exampleDate;

	}

};



// STEP 3 & 4

// Read the contents of the DB_EXAMPLE table and return a vector of the

// resulting rows

vector<Example> ReadData() {

	// Read the data

	vector<Example> results;

	DBView<Example> view("DB_EXAMPLE");



	DBView<Example>::select_iterator read_it = view.begin();

	for ( ; read_it != view.end();  ++read_it)

	{

		results.push_back(*read_it);

	}

	return results;

}

Permission to use, copy, modify, distribute and sell this software and its documentation for any purpose is hereby granted without fee, provided that the above copyright notice appears in all copies and that both that copyright notice and this permission notice appear in supporting documentation. Corwin Joy and Michael Gradman make no representations about the suitability of this software for any purpose. It is provided "as is" without express or implied warranty.

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