How to iterate through SAFEARRAY **

Question

how to iterate through C++ safearray pointer to pointer and access its elements.

I tried to replicate the solution posted by Lim Bio Liong http://social.msdn.microsoft.com/Forums/en-US/vcgeneral/thread/022dba14-9abf-4872-9f43-f4fc05bd2602 but the strangest thing is that the IDL method signature comes out to be

HRESULT __stdcall GetTestStructArray([out] SAFEARRAY ** test_struct_array);

instead of

HRESULT __stdcall GetTestStructArray([out] SAFEARRAY(TestStruct)* test_struct_array);

Any ideas?

thanks in advance

Answer 1

Safearrays are created with SafeArrayCreate or SafeArrayCreateVector, but as you ask about iterating over a SAFEARRAY, let's say you already have a SAFEARRAY returned by some other function. One way is to use SafeArrayGetElement API which is especially convenient if you have multidimensional SAFEARRAYs, as it allows, IMO, a bit easier specifying of the indices.

However, for vectors (unidimensional SAFEARRAY) it is faster to access data directly and iterate over the values. Here's an example:

Let's say it's a SAFEARRAY of longs, ie. VT_I4

// get them from somewhere. (I will assume that this is done 
// in a way that you are now responsible to free the memory)
SAFEARRAY* saValues = ... 
LONG* pVals;
HRESULT hr = SafeArrayAccessData(saValues, (void**)&pVals); // direct access to SA memory
if (SUCCEEDED(hr))
{
  long lowerBound, upperBound;  // get array bounds
  SafeArrayGetLBound(saValues, 1 , &lowerBound);
  SafeArrayGetUBound(saValues, 1, &upperBound);

  long cnt_elements = upperBound - lowerBound + 1; 
  for (int i = 0; i < cnt_elements; ++i)  // iterate through returned values
  {                              
    LONG lVal = pVals[i];   
    std::cout << "element " << i << ": value = " << lVal << std::endl;
  }       
  SafeArrayUnaccessData(saValues);
}
SafeArrayDestroy(saValues);

Answer 2

MSDN SafeArrayGetElement function gives you a code snippet on using SafeArrayGetElement to obtain individual object to array.

SAFEARRAY structure and SafeArray* functions explain the available API.

In ATL/MFC project you would want to use wrapper classes e.g. CComSafeArray to make things simpler and easier. See Simplifying SAFEARRAY programming with CComSafeArray on this.