Merge pull request #4708 from dforsyth/add_if
add if to borrowed ptr tutorial
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@ -50,7 +50,7 @@ Suppose we wanted to write a procedure that computed the distance between any
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two points, no matter where they were stored. For example, we might like to
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compute the distance between `on_the_stack` and `shared_box`, or between
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`shared_box` and `unique_box`. One option is to define a function that takes
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two arguments of type `Point`—that is, it takes the points by value. But we
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two arguments of type `Point`—that is, it takes the points by value. But if we
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define it this way, calling the function will cause the points to be
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copied. For points, this is probably not so bad, but often copies are
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expensive. Worse, if the data type contains mutable fields, copying can change
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