Sometimes a lot of "Old School" technology seems to pop up all at once. It's Tuesday and already I've worked on two old school issues. Here's what I told them and learned myself:
From a user's point of view, there are two implementations of ActiveX technology in Internet Explorer: ActiveX Controls (created with an <object> tag in an HTML page) and ActiveX Documents (created when IE sees a mime/type that is handled by an registered application.)
When you open up a Word Document or a Adobe PDF and it appears INSIDE Internet Explorer that is an "ActiveX Document." When you open up an HTML page that contains en <embed> or <object> tag that is an "ActiveX Control."
Both of these technologies, along with "Authenticode" (when a dialog pops up and warns you about downloading code, do you trust, etc.) are collectively, to the layman, "ActiveX."
When you turn off ActiveX in the IE Tools|Options|Security you are affecting ActiveX Documents as well. If you click on a PDF and try to open it in IE, you'll likely get a blank page or a "broken window" graphic. You'll be unable to View|Source because there is no HTML source to view!
In VB6, what is the difference SPECIFICIALLY between:
Dim oCalcutta As FooBar Set oCalcutta = New FooBar
and
Dim oCalcutta As New FooBar
Is there a speed difference? (there doesnt appear to be a major one) COM difference? (are internals handled differently?)
Turns out there is a difference. (Thanks to Stephen Forte, Francesco Balena, Ralf Westphal, and especially Henrik Nielsen)
When you write code like this:
Dim obj As New Class1 obj.Method
Then each reference to a member such as:
obj.Method
will be compiled to the following code (pseudo-code):
If obj Is Nothing Then Set obj = New Class1 End If obj.Method
This of course implies a certain performance hit but I wouldnt expect it to be in any way significant.
Much more important (in my opinion) is the difference in semantics (which follows from the above) between the two constructions:
Dim obj As Class1 Set obj = New Class1 Set obj = Nothing obj.Method
Dim obj As New Class1 Set obj = Nothing obj.Method
The first construction will result in a run-time error (Object variable or With block variable no set) and the second will not.
UPDATE: Some wisdom from Dan Appleman. "The reason [you're told] not to use [a Dim on one line] is that this can result in subtle bugs - especially during cleanup - when you are either accidentally recreating objects you think you cleaned up, or are referencing new objects when you accidentally cleared old ones."
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