Using a Multidimensional SAFEARRAY to pass data across from COM objects

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The SAFEARRAY is a standard way to pass arrays or collections between COM objects.
Using COM’s standard marshaller , we can pass a collection of OLE Automation compatible
data types using SAFEARRAYs.
Multidimensional SAFARRAYs help us pass various automation compatible data types through the same array

Assume , we own an icecream parlor and would like to give our customers a list of
all the icecream flavors and their prices.
Now wouldn’t it be nice to package an array with both the flavor (represented by a
BSTR) and the price (represented as a float data type).
Notice that we have two diferent data types one a float and another a BSTR and yet
we package them neatly in a SAFEARRAY and send them across using COM’s standard marshaller.

Our data structure should look something like this:

	 flavor 1     flavor 2		    flavor n

Flavors  (0,0)        (0,1)   ……….     (0,n)
	  price 1      price 2		    price n

Prices    (1,0)        (1,0)  ……….     (1,n)

You can extend this whole analogy to actually pack every record in a database
table into an N-dimensional SAFEARRAY where N represents the number of fields
in the table.
While wading through some of the SAFEARRAY documentation , you may happen to come across
the term array descriptor.
An array descriptor is actually a pointer to an allocated SAFEARRAY structure.

Time now to have a look at our Icecream parlor example.

//////////////////////////////////////////////////////////////////////////////

//Function : GetFlavorsWithPrices (Example for Multidimensional SAFEARRAY)

//Parameters: VARIANT (out parameter that contains a SAFEARRAY of VARIANTs

//			   helping us to pass BSTR and float in the same array)

//Return Type : HRESULT

//////////////////////////////////////////////////////////////////////////////
STDMETHODIMP CIceCreamOrder::GetFlavorsWithPrices(VARIANT *pVariant)

{

	// TODO: Add your implementation code here
	//Initialize the bounds for the array

	//Ours is a 2 dimensional array

	SAFEARRAYBOUND safeBound[2];
	//Set up the bounds for the first index

	//That’s the number of flavors that we have

	safeBound[0].cElements = m_vecIcecreamFlavors.size();

	safeBound[0].lLbound = 0;
	//Set up the bounds for the second index

	safeBound[1].cElements = m_vecIcecreamPrices.size();

	safeBound[1].lLbound = 0 ;
	///Initialize the VARIANT

	VariantInit(pVariant);

	//The array type is VARIANT

	//Storage will accomodate a BSTR and a float

	pVariant->vt = VT_VARIANT | VT_ARRAY;

	pVariant->parray = SafeArrayCreate(VT_VARIANT,2,safeBound);
	//Initialize the vector iterators

	std::vector::iterator iterFlavor;

	std::vector<float>::iterator iterPrices;
	//Used for indicating indexes in the Multidimensional array

	long lDimension[2];

	int iFlavorIndex = 0;
	//Start iteration

	iterPrices = m_vecIcecreamPrices.begin();

	iterFlavor = m_vecIcecreamFlavors.begin();
	//Iterate thru the list of flavors and prices

	while(iterFlavor != m_vecIcecreamFlavors.end())

	{
		//Put the Element at (0,0), (0,1)  , (0,2) ,………….(0,n)

		lDimension[1] = iFlavorIndex;

		lDimension[0] = 0;

		CComVariant variantFlavor(SysAllocString((*iterFlavor).m_str));

		SafeArrayPutElement(pVariant->parray,lDimension,&variantFlavor);
		//Put the Element at (1,0), (1,1)  , (1,2) ,………….(1,n)

		lDimension[1] = iFlavorIndex;

		lDimension[0] = 1;

		CComVariant variantPrices(*iterPrices);

		SafeArrayPutElement(pVariant->parray,lDimension,&variantPrices);
		iFlavorIndex++;

		iterPrices++;

		iterFlavor++;
	}
	return S_OK;
}

Download demo project (Server) – 32 KB

Download demo project (Client) – 18 KB