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		<title>XLL Containers</title>
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		<h1>XLL Containers</h1>
		<p><a href="#Introduction">Introduction</a><br>
			<a href="#common_requirements">Common Requirements</a><br>
			<A href="#BestPractices">Best Practices</A><br>
			<a href="#Examples">Examples</a><br>
		</p>
		<h2><a name="Introduction">Introduction</a></h2>
		<p>Implementation of Excel add-in functional interface often requires dealing with <strong>XLOPER</strong>
		structure created by Excel and a pointer to which passed to a user defined function. In addition,
		sometimes a user defined function should return a pointer to <strong>XLOPER</strong> back to Excel as a
		return value. <strong>XLOPER</strong> structure is actually a <em>variant</em> type, which can contain data of several
		different types, but the vast majority of all applicable treatments of <strong>XLOPER</strong> structure falls in the following three categories:
		<ul>
			<li>a single <em>cell</em>,</li>
			<li>a one-dimensional range, or a <em>vector</em> of elements</li>
			<li>a two-dimensional range, or a <em>matrix</em> of elements</li>
		</ul>
		The scope of the solution under consideration is limited to those three cases.</p>

		<p>To extract useful information from <strong>XLOPER</strong> structure application programmers usually implement
		various <em>converters</em> to transform <strong>XLOPER</strong> structure to and from base types: <pre>int, double</pre>
		or STL types: <pre>std::string, std::vector</pre>Such an approach although straightforward and easily
		comprehensible sometimes entails an undesirable overhead in terms of memory management and computational efficiency.<br/>
		In addition, <em>converters</em> usually support only limited set of input/output types making such a solution non-generic.<br/>
		Moreover, it is sometimes not quite easy to decide when memory allocated for <strong>XLOPER</strong> structure can be
		safely released, which often leads to memory leaks.</p>

		<p>The solution proposed here takes a different approach. It is based on classes-adaptors that wraps around <strong>XLOPER</strong>
		and allow to treat it as a single value of a specific base type, or as a container complying with requirements of STL.</p>
		<p>Thus, each class in the proposed solution can be viewed as a <em>dual</em> entity: for Excel such a class is seen as an <strong>XLOPER</strong> structure, but for application functions it can be treated as a C++ basis or container type.
		Such a <em>dualism</em> contributes a great deal to efficient memory management, code brevity and exception safety.</p>

		<p>Check out the <a href="#Examples">examples</a> of use of XLL containers below.</p>

		<p>The XLL containers template library provides six container class templates:</p>
		<div align="left">
			<table border="1" cellpadding="4" cellspacing="0">
				<tr>
					<td><a href="./XllCell.htm"><b>xll::Cell</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A simple wrapper around <strong>XLOPER</strong>. Non-constructable and non-copyable.</td>
				</tr>
				<tr>
					<td><a href="./XllCellAlloc.htm"><b>xll::CellAlloc</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A subclass of <b>xll::Cell</b> that allow for construction by a user. Copy-constructable.</td>
				</tr>
				<tr>
					<td><a href="./XllMatrix.htm"><b>xll::Matrix</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A 2-d Excel range that can be treated as a matrix. Non-constructable and non-copyable.</td>
				</tr>
				<tr>
					<td><a href="./XllMatrixAlloc.htm"><b>xll::MatrixAlloc</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A subclass of <b>xll::Matrix</b> that allow for construction by a user. Copy-constructable.</td>
				</tr>
				<tr>
					<td><a href="./XllVector.htm"><b>xll::Vector</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A 1-d Excel range that can be treated as a standard vector. Non-constructable and non-copyable.</td>
				</tr>
				<tr>
					<td><a href="./XllVectorAlloc.html"><b>xll::VectorAlloc</b></a></td>
					<td><a href="../Include/XllArrays.hpp">&lt;XllArrays.hpp&gt;</a></td>
					<td>A subclass of <b>xll::Vector</b> that allow for construction by a user. Copy-constructable.</td>
				</tr>
			</table>
		</div>

		<p>In addition, for each of <b>xll::Cell, xll::Matrix, xll::Vector</b> container classes the library provides
		implementation of five different pointer class concepts, which implement concepts of <a href="http://www.boost.org/libs/smart_ptr/scoped_ptr.htm">boost::scoped_pointer</a>,
		<a href="http://www.boost.org/libs/smart_ptr/shared_ptr.htm">boost::shared_pointer</a>, and <a href="http://www.boost.org/libs/smart_ptr/intrusive_ptr.html">boost::intrusive_pointer</a>:</p>
		<div align="left">
			<table border="1" cellpadding="4" cellspacing="0">
				<tr>
					<td><b>const-pointer</b></td>
					<td>Implements a const-pointer concept.	Read-only, no memory management is assumed.</td>
				</tr>
				<tr>
					<td><b>weak-pointer</b></td>
					<td>A pointer, which is not responsible for releasing allocated memory within definition scope.<br/>
					Memory release happens later, either in a user provided <code>xlAutoFree</code> function or Excel itself
					takes care of the memory.</td>
				</tr>
				<tr>
					<td><b>scoped-pointer</b></td>
					<td>A pointer for an object, which is supposed to be created and used only within a current scope.<br/>
					Implements a <b>boost::scoped_pointer</b> concept.</td>
				</tr>
				<tr>
					<td><b>shared-pointer</b></td>
					<td>A pointer to a shared object to be used beyond the current scope, e.g. in <a href="http://www.objecthandler.org/">ObjectHandler</a> framework.<br/>
					Implements a <b>boost::shared_pointer</b> concept.</td>
				</tr>
				<tr>
					<td><b>temp-pointer</b></td>
					<td>A pointer to a temporal object allocated in a static memory of XLL framework.</td>
				</tr>
			</table>
		</div>

		<p>When implementing an Excel add-in functional interface the following five essentially different situation
		usually should be addressed:</p>
		<ol>
			<li>Excel passes a pointer to <strong>XLOPER</strong> to a user defined function that should be treated as an input,
			read-only data. A <b>const-pointer</b> type is suitable in this case.</li>
			<li>A user defined function must construct an <strong>XLOPER</strong> structure and pass it as a result to Excel.
			In this case a <b>weak-pointer</b> is a perfect choice.</li>
			<li>A local <strong>XLOPER</strong> object should be created to pass it to <code>Excel4v</code> function as a
			parameter-result (e.g. <code>xlfGetName</code>). A <b>scoped-pointer</b> is a convenient choice in this case.</li>
			<li>An Excel object should be constructed and stored in some external container (e.g. <a href="http://www.objecthandler.org/">ObjectHandler</a>) for
			future use. A <b>shared-pointer</b> is the only solution in this case.</li>
			<li>A temporal <strong>XLOPER</strong> should be created to pass it as an input parameter to <code>Excel4v</code>
			function. In this case a <b>temp-pointer</b> can be used.
		</ol>

		<p>The table below summarizes the references to all XLL Containers library classes and binds them to the use cases listed
		above (see also <a href="#Examples">examples</a> below):</p>
		<div align="left">
			<table border="1" cellpadding="4" cellspacing="0">
				<tr>
					<td><b>namespace xll::</b></td>
					<td><b>const-pointer</b><br/>use case 1</td>
					<td><b>weak-pointer</b><br/>use case 2</td>
					<td><b>scoped-pointer</b><br/>use case 3</td>
					<td><b>shared-pointer</b><br/>use case 4</td>
					<td><b>temp-pointer</b><br/>use case 5</td>
				</tr>
				<tr>
					<td><a href="./XllCell.htm"><b>Cell</b></a></td>
					<td><a href="./XllCellPtr.htm#const"><b>CellPtrConst</b></a></td>
					<td><a href="./CellPtrWeak.htm"><b>CellPtrWeak</b></a></td>
					<td><a href="./CellPtrScoped.htm"><b>CellPtrScoped</b></a></td>
					<td><a href="./CellPtrShared.htm"><b>CellPtrShared</b></a></td>
					<td><a href="./CellPtrTemp.htm"><b>CellPtrTemp</b></a></td>
				</tr>
				<tr>
					<td><a href="./XllMatrix.htm"><b>Matrix</b></a></td>
					<td><a href="./MatrixPtrConst.htm"><b>MatrixPtrConst</b></a></td>
					<td><a href="./MatrixPtrWeak.htm"><b>MatrixPtrWeak</b></a></td>
					<td><a href="./MatrixPtrScoped.htm"><b>MatrixPtrScoped</b></a></td>
					<td><a href="./MatrixPtrShared.htm"><b>MatrixPtrShared</b></a></td>
					<td><a href="./MatrixPtrTemp.htm"><b>MatrixPtrTemp</b></a></td>
				</tr>
				<tr>
					<td><a href="./XllVector.htm"><b>Vector</b></a></td>
					<td><a href="./VectorPtrConst.htm"><b>VectorPtrConst</b></a></td>
					<td><a href="./VectorPtrWeak.htm"><b>VectorPtrWeak</b></a></td>
					<td><a href="./VectorPtrScoped.htm"><b>VectorPtrScoped</b></a></td>
					<td><a href="./VectorPtrShared.htm"><b>VectorPtrShared</b></a></td>
					<td><a href="./VectorPtrTemp.htm"><b>VectorPtrTemp</b></a></td>
				</tr>
			</table>
		</div>

		<h2><a name="common_requirements">Common Requirements</a></h2>
		<p>All XLL Containers library class templates have a template parameter, <b>T</b>, which
			specifies the type of the object stored in a container. The following types can be specified:
			<ul>
				<li>integer types: <code>int, long, short, bool</code>;
				<li>floating point types: <code>float, double</code>;
				<li><code>std::string</code>;
				<li><a href="http://boost.org/doc/html/any.html"><b>boost::any</b></a> (which makes it an abstract container);
				<li>an error type cell: <code>xll::ErrorCode</code>;
				<li>a special case int type: <code>xll::Int</code> (to handle <code>xltypeInt</code> XLL type).
			</ul>
		</p>

		<h2><a name="BestPractices">Best Practices</a></h2>
		<p>Although it is possible to create an object of type <b>CellAlloc</b>, <b>MatrixAlloc</b>, or <b>VectorAlloc</b>, it is recommended to use pointer classes instead.<br/>
		The reason for this is that unlike object-containers, pointer classes explicitly specify the intention and reason for creating an object: <b>const-pointer</b> means read only data,
		<b>weak-pointer</b> indicates that the object created will be passed to Excel, <b>scoped-pointer</b> indicates local "in-scope" use of object, and <b>shared-pointer</b> means creation of a global, reusable object.<br/>
		Construction of object-containers is not safe in this respect since it is easy to make a mistake choosing an inappropriate allocation method.</p>
		<p>It is also not advisable using <b>static</b> objects since use of pointer classes discussed here makes it senseless.</p>
		<p>An access to the features of underlying class-containers can be done via usual pointer dereferencing technique as demonstrated in <a href="#Examples">examples</a> below.</p>

		<h2><a name="Examples">Examples</a></h2>

		<p>The following example demonstrates the use of a <b>const-pointer</b>.</p>

<pre class="programlisting">
#include &lt;XLLArrays.hpp&gt;

// In the function below it is assumed that parameter x represents a vector of doubles
double Average (const XLOPER *x)
{
    try {
        <a href="">xll::VectorPtrConst</a> &lt;double&gt; px (x);
        double res = 0;
        for (size_t i = 0; i < px->size(); ++i)
            res += px->at(i); // or res += (*px)[i];
        return res / px->size();
    }
    catch (const std::bad_cast &)
    {
        cerr << "Parameter x either not an array or its elements cannot be converted to double" << endl;
    }
    return 0;
}
</pre>

<p><b><em>Notes.</em></b>
<ol>
	<li><b>VectorPtrConst</b> class checks the type of input parameter x. If it cannot be converted to array of doubles the constructor throws a <b>bad_cast</b> exception.</li>
	<li>The code above works fine even if a user passes a single numeric cell to this function. <b>VectorPtrConst</b> will treat it as an array of size 1 automatically in this case.
</ol></p>

		<p>The following example demonstrates the use of a <b>weak-pointer</b>.</p>

<pre class="programlisting">
#include &lt;XLLArrays.hpp&gt;
#include &lt;algorithm&gt;

// This function generates an array of random numbers and returns it to Excel
XLOPER * GenRandomVector (int how_many, int in_row)
{
    xll::VectorPtrWeakExcel &lt;int&gt; px (how_many, in_row != 0);
    std::generate (px->begin(), px->end(), rand);
    return px;
}
</pre>

Note. Memory allocated in the function above will be released by Excel.
If instead of VectorPtrWeakExcel one used VectorPtrWeak class, the <code>xlAutoFree</code> function would have to be provided
to release memory.

<p>The next example demonstrates the use of a <b>scoped-pointer</b> and a <b>temp-pointer</b>.<br/>

<pre class="programlisting">
#include &lt;XLLArrays.hpp&gt;

// A nifty implementation of xlAutoOpen interface function
int xlAutoOpen()
{
    using namespace xll;
    try
    {
        CellPtrScopedExcel &lt;std::string&gt; xDll;
        Excel(xlGetName, xDll, 0);

        Excel(xlfRegister, 0, 10, xDll,
            TempStr ("GenRandomVector"), //Function code name.
            TempStr ("PJJ"),		  //Parameter codes.
            TempStr ("GenRandomVector"), //Function display name.
            TempStr (""),
            TempStr ("1"), //Function type.
            TempStr ("My functions"), //Function category.
            TempStr (""), //shortcut text (command macros only).
            TempStr (""), //path to help file.
            TempStr ("Returns a vector of random numbers ")
        );
        return 1;
    }
    catch (const std::exception &ex)
    {
        Excel(xlcAlert, 0, 1, TempStr(ex.what()));
        return 0;
    }
    catch (...)
    {
        Excel(xlcAlert, 0, 1, TempStr("Unknown exception"));
        return 0;
    }
}
</pre>

Note use of CellPtrScopedExcel class in the function above. We do not have to worry about the call of <code>xlFree</code>
function to release memory allocated by Excel in <code>xlGetName</code> since implementation of CellPtrScopedExcel class takes care about it.
If, however, we use CellPtrScoped class instead (and it is admittedly very easy to confuse) the code above most likely end up in a crash
when CellPtrScoped destructor will try to release memory allocated by Excel in <code>xlGetName</code> call.

<p>The following example demonstrates the use of a <b>shared-pointer</b>.</p>

<pre class="programlisting">
#include &lt;XLLArrays.hpp&gt;
#include &lt;boost/shared_ptr.hpp&gt;
#include &lt;algorithm&gt;

// Calculates the sum of two arrays and returns the result to the outside world.
boost::shared_ptr&lt;XLOPER&gt; SumArrays(const XLOPER *x, const XLOPER *y)
{
    using namespace xll;
    VectorPtrConst &lt;double&gt; px(x);
    VectorPtrConst &lt;double&gt; py(y);
    VectorPtrShared &lt;double&gt; pz(px->size());

    std::transform (px->begin(), px->end(), py->begin(), pz->begin(), std::plus);

    return boost::static_pointer_cast &lt;XLOPER&gt; (pz);
}
</pre>

Note that a shared pointer returned by the function above can be stored in <a href="http://www.objecthandler.org/">ObjectHandler</a>.
Memory allocated by VectorPtrShared class is released in its destructor, which is called when reference counter for the instances of underlying object becomes zero.

		<h2><a name="History">History</a> and Acknowledgements</h2>
		<p>April 2007. Original version.</p>

		<h2><a name="References">References</a></h2>
		<p>[<a name="XLOPER">XLOPER</a>] MSDN, <a href="http://msdn.microsoft.com/archive/default.asp?url=/archive/en-us/office97/html/SF7EF.asp">
				The XLOPER Data Type</a></p>
		<p>Copyright 2007 ---</p>
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