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Replaced explanation of assignment attempt with that of object test.

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Author:halw
Date:2012-04-06T16:23:19.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@1059 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
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halw
2012-04-06 16:23:19 +00:00
parent 8e824ebf18
commit d9a1b20bad
1 changed files with 8 additions and 21 deletions
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29
documentation/current/platform-specifics/microsoft-windows/net/eiffel-net-language/eiffel-net/inheritance.wiki
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@@ -304,7 +304,7 @@ Static typing tells us at compile time that it is safe to apply <code>perimeter<
Dynamic binding tells us that when we apply <code>perimeter</code>, we know that the most appropriate version of the feature will get applied at runtime.
===Assignment Attempt===
===Object Test===
Now let's add another situation. Consider the code below:
<code>
@@ -324,31 +324,18 @@ But in the case above, the guarantee cannot be made. <code>my_polygon</code> is
Does this mean that we can never do <code>RECTANGLE</code> things with this instance again, once we have attached it to <code>my_polygon</code>?
No. There is a language facility called the "assignment attempt" which will come to our rescue. The assignment attempt will allow us safely to attach our instance back to an entity typed as <code>RECTANGLE</code>. After doing so, we are free use <code>RECTANGLE</code> features.
No. There is a language facility called the '''object test''' which will come to our rescue. The object test will allow us safely to attach our instance back to an entity typed as <code>RECTANGLE</code>. After doing so, we are free use <code>RECTANGLE</code> features.
<code>
my_polygon := my_rectangle
print (my_polygon.perimeter)
another_rectangle ?= my_polygon
print (another_rectangle.width)
</code>
The assignment attempt uses the syntax "?=", versus the ":=" of assignment.
This is significant ... as is the name assignment attempt. The reason is that it is possible in some context that <code>my_polygon</code> will be attached to something other than a rectangle when we do this:
<code>
another_rectangle ?= my_polygon
</code>
If it were true that <code>my_polygon</code> were attached to an instance of say <code>TRIANGLE</code> when the assignment attempt above was executed, then <code>another_rectangle</code> would be left as a <code>Void</code> reference. As a consequence, it is prudent to check to see if the attachment has actually been made before trying to apply features. Applying a feature to a <code>Void</code> reference will cause an exception. Almost always, assignment attempt will be used in the following manner:
<code>
my_polygon := my_rectangle
print (my_polygon.perimeter)
another_rectangle ?= my_polygon
if another_rectangle /= Void then -- Make sure assignment happened
print (another_rectangle.width)
if attached {RECTANGLE} my_polygon as l_rect then
print (l_rect.width)
end
</code>
In this code, the entity <code>l_rect</code> is a fresh local entity produced during the object test. So, the code can be read: if at this point, <code>my_polygon</code> is attached to an instance of type <code>RECTANGLE</code>, then attach that instance to a fresh local entity named <code>l_rect</code>, then apply <code>{RECTANGLE}.width</code> to <code>l_rect</code> and print the result.
:'''Note:''' The object test replaces the functionality of an obsolete mechanism called assignment attempt. Assignment attempt used the syntax '''<code>?=</code>''' in the context of assignment versus the '''<code>:=</code>''' of normal assignment.