ThreeWave Is An Array Allocated?

This page describes a VBA procedure you can use to determine whether a dynamic array or an array in a Variant has been allocated.


There are two types of arrays: static arrays, in which the dimensions of the array are set in the Dim statement, and dynamic arrays, in which the array is allocated and dimensioned with the ReDim statement. An array is said to be allocated if it consumes memory, has valid lower and upper bounds, and contains data (even if that data is the default values for the data type of the array, such as empty strings of an array of String type variables). A static array is by definition always allocated. You can never resize, reallocate, deallocate, or release the memory consumed by a static array. With a dynamic array, however, you can resizse, reallocate, deallocate, and release the memory of the array. (A Varaint type variable that contains an array always contains a dynamic array.)

At times, you may need to test whether a dynamic array has been allocated or whether it has never been allocated or has been deallocated with the Erase statement. In nearly all cases, you can test whether a dynamic array has been allocated by simply attempting to read the LBound of the array. If an error occurs, then the array is not allocated. If no error occurs, the array has been allocated. For example,

     Dim Arr() As Long
    Dim N As Long
    On Error Resume Next
    N = LBound(Arr)
    If Err.Number = 0 Then 
        Debug.Print "Array is allocated."
        Debug.Print "Array is not allocated."
    End If

However, certain funcitons will set the LBound and UBound of an array even when the array is not allocated. For example, the Split function behaves this way. The test described above would treat this as an allocated array because retrieving the LBound of the array doesn't cause an error. This is wrong, though -- the array isn't properly allocated. For example,

     Dim S As String
    Dim V As Variant
    S = vbNullString
    V = Split(S,",")
    Debug.Print IsArray(V), LBound(V), UBound(V)

This illustrates a condition under which both the LBound and UBound contain values and retrieving them does not throw an error, but the array is not properly allocated.

With a normal array, attempting LBound(Arr) would cause a run time error. However, with Split and a handful of other functions, Lbound(Arr) and UBound(Arr) do not cause errors when the array is unallocated. This is a side-effect of the basic fact that, in VB and VBA, arrays are really structures of a type called SafeArray, which manages bounds and contains pointers to the actual data. You don't work with SafeArrays directly in VB/VBA. It is all handled behind the scenes. In the case of an array handled by the Split function, you need to test whether LBound is greater than UBound.

The function below, IsArrayAllocated will accurately return True or False indicating whether the array is allocated. This function will work for both static and dynamic arrays of any number of dimensions, and will correctly work for unallocated arrays with valid (non-error-causing) LBound values, such as those arrays set by the Split function.

    Function IsArrayAllocated(Arr As Variant) As Boolean
        On Error Resume Next
        IsArrayAllocated = IsArray(Arr) And _
                           Not IsError(LBound(Arr, 1)) And _
                           LBound(Arr, 1) <= UBound(Arr, 1)

This function will return True if Arr is a valid and allocted array. Under any other circumstance, it will return False. It works for array of any data type and with any number of dimensions.

LastUpdate This page last updated: 18-April-2009.