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Now the problem of passing a POINT structure has been simplified. All you need to do is ask what kind of Visual Basic declaration can create this stack frame. The answer is |
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Private Declare Function PtInRect Lib
"user32" (lpRect As RECT, _
ByVal ptx As Long, ByVal pty As Long)
As Long |
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Figure T7-1:
Stack frame showing a POINT structure passed by value |
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Remember (from Tutorial 4) that parameters are pushed onto the stack in reverse order. Because we want the actual values of the X and Y fields on the stack, we pass the X and Y fields separately by value. The RECT structure is passed by reference. |
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The following sample code from the Struct.vbp project (on the CD that comes with this book) demonstrates use of the PtInRect function in Visual Basic: |
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Private Sub cmdPtInRect_Click()
Dim r As RECT
Dim pt As POINTAPI
r.Right = 100
r.Bottom = 100
pt.x = 50
pt.y = 50
Debug.Print PtInRect(r, pt.x, pt.y)
pt.x = 150
Debug.Print PtInRect(r, pt.x, pt.y)
End Sub |
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Visual Basic's inability to pass structures by value does require you to pass the fields of the structure individually, but at least the function can be called. |
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The good news is that the POINT and SIZE structures (which are identical except for different field names) are the only structures I know of that are passed by value to Win32 API functions. |
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