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CopyMemory is an alias for the RtlMoveMemory entry in the system's Kernel32DII CopyMemory moves an arbitrary number of bytes from one memory location to another For example, the following snippet moves 4 bytes from one Long variable to another Note that the VBoostTypes typelib described in the next section contains a CopyMemory declaration and that the VBoost object extensions provide optimized functions for many common operations that would otherwise require CopyMemory 'The VB declare statement for demonstration purposes Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _ (pDest As Any, pSource As Any, ByVal ByteLen As Long) 'Code to use it Dim Longl As Long Dim Long2 As Long Longl = 1 0 'Equivalent to Long1 = Long2 CopyMemory Long2, Long1, 4 'Long2 is now 10 There are three types of pointers in VB The first is the memory address of a variable The second is the address of a string, object, or array pointer type (Note that pointer-type variables have two pointers: one for the variable and one for the data) The third is a function pointer Each of these pointers is retrieved in different ways You use the VarPtr function to get the address of any nonarray variable VarPtr maps to a simple function in the VB runtime that takes a memory address and returns it Visual Basic's VarPtr function is declared in the VBA typelib and points to the VarPtr entrypoint in the VB runtime DLL By mapping different function declarations to this single function, you can get the address of different variable types For example, the Reading Array Variables section in 2 introduces the VarPtrArray and VarPtrStringArray functions Both are function declarations to call the VarPtr runtime function with different parameter input types Passing the address of a variable ByVal is equivalent to passing the variable itself ByRef, so the previous CopyMemory call can also be written with VarPtr CopyMemory ByVal V a r P t r ( L o n g 2 ) , ByVal V a r P t r ( L o n g l ) , 4
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There are two additional built-in VarPtr-based declarations in the VBA typelib ObjPtr gets the pointer to the object in any object-typed variable, and StrPtr gets the pointer to the string data in a string variable ObjPtr and StrPtr differ from VarPtr in that they retrieve the pointer to the actual data rather than the pointer to the variable itself Although you can also obtain these values with CopyMemory and VarPtr, it is much easier to use the StrPtr and ObjPtr functions directly
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'Copy the first seven characters from String1 into String2 Dim String1 As String Dim String2 As String Dim pString1 As Long String1 = "PowerVB" String2 = String$(7, 0) 'These four operations all do the same thing 'Try one: this requires a typelib CopyMemory declare CopyMemory ByVal String2, ByVal String1, 14 'Try two: Use StrPtr to guarantee no ANSI/UNICODE munging CopyMemory ByVal StrPtr(String2) , _ ByVal StrPtr(String1), 14 Try three: Get the string's pointer from VarPtr CopyMemory pString1, ByVal VarPtr(Stringl), 4 CopyMemory ByVal String2, ByVal pString1, 14 'Try four: Demonstrate VBoostAssign instead VBoostAssign pString1, ByVal VarPtr(Stringl) CopyMemory ByVal String2, ByVal pString1, 14
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The primary use of VarPtr and StrPtr in normal VB code is to enable calls to UNICODE API functions (see UNICODE Conversions in 14) ObjPtr is not needed for API functions, but it is very useful in providing a unique numeric identifier for any object This enables you to generate a unique key for an object without holding an actual reference to it (see Weak References and Collections in 6) You can also use ObjPtr to track the creation and destruction of your VB-created objects For example, the following code lets you see when a class instance is created and destroyed
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