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they are passed between COM objects in different threads and processes as parameters or function return values You can achieve fine-grained control of your arrays by learning what's in the SafeArray structure and how VB and the SafeArray API set respond to the different settings Here are the main SafeArray and SafeArrayBound structures as defined in VBoostTypes They are equivalent to the SAFEARRAY and SAFEARRAYBOUND structures defined by OLE Automation, but these structures are more VB friendly In practice, a SafeArray structure is always immediately followed in memory by one or more SafeArrayBound structures, depending on the cDims field of the SafeArray All the other flags and API functions used in this chapter are also included in the VBoostTypes type library Type SafeArray cDims As Integer fFeatures As Integer cbElements As Long cLocks As Long pvData As Long End Type Type SafeArrayBound cElements As Long lLbound As Long End Type You will usually work with a one-dimensional SAFEARRAY For convenience, VBoostTypes also defines a one-dimensional SafeArray structure with a single inline bound Type SafeArrayld cDims As Integer fFeatures As Integer cbElements As Long cLocks As Long pvData As Long cElements As Long lLbound As Long End Type
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Let's look at each of the SafeArray and SafeArrayBound elements individually, as well as the different flags we can apply to the fFeatures field Although tedious, this exercise is important because we will read these values from VB-created arrays as well as set them in descriptors that we create ourselves cDims is the number of dimensions in the array This must be set to 1 for a one-dimensional array cbElements is the number of bytes per element The memory address for the Index element in an array is calculated as pvData + Index * cbElements cLocks is a lock count for the array When an array is locked, you cannot ReDim, ReDim Preserve, or Erase the array You can change the value of elements in the array, but you cannot modify or destroy the array's structure VB locks an array when you pass an element of the array to a ByRef parameter or when you use a structure-type array element in a With statement Trying to Erase or ReDim [Preserve] an array with cLocks > 0 generates an error 10 ("This array is fixed or temporarily locked") pvData is the element in an array that points to the memory for the array's data pvData is the main player in the SafeArray structure: All other fields play supporting roles pvData is stored as a Long value, which has the same length as the void* pointer in the OLE Automation SAFEARRAY definition cElements specifies the number of elements allowed per bound ILbound is the lower bound of the array dimension VB's LBound function reads its value directly from this element, while UBound is calculated as ILbound + cElements- 1 fFeatures is a very important field The flags set in fFeatures enable the SafeArray* API set to correctly destroy an array and the standard COM marshaling engine to successfully move the array between threads and processes These flags fall into two categories: they specify either the type of elements in the array or the method used to allocate the memory for the descriptor and the array data itself
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The first group of flags includes three memory flags FADF_AUTO (&H1), FADF_STATIC (&H2), FADF_EMBEDDED (&H4) and FADF_FIXEDSIZE (&H10) These flags describe how the array was allocated, and they block ReDim [Preserve] If any of the memory flags are set, the Erase statement cleans up elements in the array, but it does not free the memory If FADF_STATIC is set, the memory block for any type is zeroed by Erase In VB variable-size arrays, none of these flags is set A fixed-size array has FADF_STATIC and FADF_ FIXEDSIZE set, and a fixed-size array in a structure adds the FADF_EMBEDDED flag as well VB doesn't generate arrays with the the FADF_AUTO flag set, but I'll use it later in the chapter to indicate that the array descriptor is allocated on the stack VB plays some nasty little games with the fFeatures flags and local fixed-size array variables (I had a wonderful time stepping through native assembly code to fully verify this) Although VB allocates the descriptor for a local fixed-size array variable on the stack, the data for the array is allocated on the heap with SafeArrayAllocData This method of allocation allows you to create arbitrarily large fixed-size local arrays, by passing the 64K upper limit of fixed-size arrays in structures However, this means that the array is neither fixed-size nor static in the sense that these flags were intended VB just uses the flags during the scope of the function or lifetime of the class to prevent other routines from modifying the array So when a local array variable goes out of scope, VB checks for arrays that are FIXEDSIZE and STATIC (but not EMBEDDED or AUTO) VB clears these flags before calling SafeArrayDestroyData, which then clears and releases the memory block that had been locked by the fFeatures settings The next group of flags indicates the type of element in the array One or none of these flags can be set for an array FADF_BSTR (&H100) indicates that the array contains Strings, so calling Erase on the array frees each of the string elements with the SysFreeString API, then destroys or zeroes the memory Similarly, FADF_VARIANT (&H800) forces a call to the VariantClear API for each element, and FADF_UNKNOWN (&H200) and FADF_DISPATCH (&H400) indicate that IUnknown-Release should be called on each nonzero element to release the reference on the object The array destruction routines are able to free memory associated with all simple types simply by looking at these flags
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