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Changed assignment attempt in `Retrieval' to object test. Other minor corrections.

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Author:halw
Date:2012-04-07T13:01:44.000000Z


git-svn-id: https://svn.eiffel.com/eiffel-org/trunk@1065 abb3cda0-5349-4a8f-a601-0c33ac3a8c38
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halw
2012-04-07 13:01:44 +00:00
parent a5e9c265c6
commit 211d9b7e48
1 changed files with 11 additions and 4 deletions
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documentation/current/solutions/basic-computing/eiffelbase/eiffelbase-tutorial/eiffelbase-kernel.wiki
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@@ -362,12 +362,19 @@ subsequent execution of the same system. If you need to store objects from a cer
==Retrieval==
You only need to be aware of the difference between basic and general store at storage time. The stored structure will always be available through feature retrieved; this feature will figure out, from the format of the stored structure, whether it was stored by <eiffel>basic_store</eiffel> or <eiffel>general_store</eiffel>, and will decode it accordingly. <br/>
Feature <eiffel>retrieved</eiffel> returns a result of type [[ref:/libraries/base/reference/storable_chart|STORABLE]] and is typically used through an assignment attempt of the form
<code>x ?= retrieved (my_descriptor)</code>
You only need to be aware of the difference between basic and general store at storage time. The stored structure will always be available through feature retrieved; this feature will figure out, from the format of the stored structure, whether it was stored by <eiffel>basic_store</eiffel>, <eiffel>general_store</eiffel>, or <eiffel>independent_store</eiffel> and will decode it accordingly. <br/>
Feature <eiffel>retrieved</eiffel> returns a result of type [[ref:/libraries/base/reference/any_chart|ANY]] and is typically used through an object test of the form
<code>
if attached {SOME_EXPECTED_TYPE} retrieved (my_descriptor) as l_temp then
-- Retrieved result is of expected type
-- Proceed with processing of result, typically with calls of the form `l_temp.some_feature'
else
-- Result was not of expected type
end
</code>
<br/>
The assignment attempt is necessary because <eiffel>retrieved</eiffel> returns a result of type [[ref:/libraries/base/reference/storable_chart|STORABLE]] whereas the type of <code>x</code> will be based on a proper descendant of [[ref:/libraries/base/reference/storable_chart|STORABLE]] . If the structure in the file has been corrupted and <eiffel>retrieved</eiffel> is unable to do its job, it will trigger an exception. The code for that exception in class [[ref:/libraries/base/reference/exceptions_chart|EXCEPTIONS]] (which inherits it from EXCEP_CONST and is discussed in the next section, together with the notion of exception code) is <eiffel>Retrieve_exception</eiffel>.
The object test is necessary because <eiffel>retrieved</eiffel> returns a result of type [[ref:/libraries/base/reference/any_chart|ANY]] whereas the type of <code>x</code> will be based on a proper descendant of [[ref:/libraries/base/reference/any_chart|ANY]] . If the structure in the file has been corrupted and <eiffel>retrieved</eiffel> is unable to do its job, it will trigger an exception. The code for that exception in class [[ref:/libraries/base/reference/exceptions_chart|EXCEPTIONS]] (which inherits it from EXCEP_CONST and is discussed in the next section, together with the notion of exception code) is <eiffel>Retrieve_exception</eiffel>.
==Recoverable storable==