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'Definition of CTwoLongs in a class file Public Longl As Long Public Long2 As Long 'Calling code Sub Main() Dim TL As TwoLongs Dim CTL As New CTwoLongs DebugPrint "Structure Length: "; LenB(TL) 'Line 1 DebugPrint "Class Length: "; LenB(CTL) 'Line 2 End Sub As written, this is not a very satisfying exercise because VB won't compile Line 2 It knows up front that CTwoLongs is an object type and that the length of the memory block associated with an object type is undefined However, you can declare a second structure with a single CTwoLongs element and verify that the structure has a length of four bytes Although the LenB and Len functions work on all in-line types, they don't work on pointer types with the exception of String variables You'll learn why it is easy to get a string length in 14 When VB copies an in-line type, it simply copies the data However, when VB makes a copy of a pointer type, it needs to decide between simply copying the pointer value to the new location or duplicating all the pointed to data and getting a new pointer Copying just the pointer is known as a shallow copy; duplicating the actual data is known as a deep copy VB can always make a shallow copy with object parameters because of COM's reference counting mechanism (which I'll discuss in 3) However, there is no reference counting mechanism for String or array types, so VB always makes a deep copy when assigning a string or array variable Deep copies are very expensive in terms of performance and memory, but keep VB a safe language by not allowing you to access memory in locations that you don't own You'll learn about several techniques for strings and arrays that allow you to minimize deep copies by directly referencing memory that you don't own Of course, VB has no way of knowing that it doesn't own the memory in a normal variable, so I'll also show you how to clean up your self-managed reference before VB gets hold of it Passing a ByVal object type is a source of confusion for some VB developers Passing an object by value does not mean that you are making a deep copy of
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the object Instead, you are simply passing a reference, which means you pass the current pointer value and increase the reference count on the object The main difference between passing an object parameter ByVal and ByRef is that with a ByRef pass, you can modify the contents of the variable passed to the parameter However, you have no control over this variable with a ByVal pass This is the semantic difference between ByVal and ByRef in-line types There are also some more subtle differences between object types, which we'll discuss in the Parameters and IUnknown section in 3
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VB hides all its pointer manipulations behind pointer types and ByRef parameters, but it also gives you a built-in (albeit hidden) mechanism to access the actual pointer values of both in-line and pointer types Once you have a pointer value, you can manipulate it arithmetically just as you would in any other language Of course, because VB doesn't explicitly support pointer arithmetic and other operations, you step outside VB's guaranteed safe programming environment as soon as you start to use pointers You also don't get type checking on your pointer operations or compiler help with pointer arithmetic You are essentially on your own Pointers allow you to manipulate memory any way you like Basic pointer operations allow you to do many things that are otherwise impossible in VB For example, you can share variables between two class instances without copying them, treat a String as an array of Integers, and create an array from any block of memory In addition to direct manipulation, this book concentrates on leveraging the known layout of VB's arrays and object types Once you've constructed an object or array in memory, you can use direct pointer manipulation to assign the array or object to a VB variable Your goal should always be to get any custom objects and arrays into VB variables as soon as possible so you can use standard VB to make modifications to your data The key to memory manipulation in VB is the CopyMemory function You must provide a declaration for this function in either VB or in a type library To avoid ANSI/UNICODE string conversion issues (see 14), we recommend that you use a typelib declare for this function CopyMemory is a name that has been in common use since the 16-bit days of VB On 32-bit systems,
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