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From: nagle@well.UUCP (John Nagle)
Newsgroups: comp.lang.c++
Subject: Handling trouble in C++
Keywords: exception error
Message-ID: <13262@well.UUCP>
Date: 22 Aug 89 06:39:59 GMT
Distribution: comp
Lines: 34


     In C++, unlike C, a considerable fraction of the execution can
take place implicitly, within constructors and destructors, rather
than in explicitly called functions.  But there is no way for a
constructor or a destructor to signal an error condition to the
invoker.  Declarations aren't allowed to fail.  But constructors
often do such things as open files, windows, devices, or databases.
Such operations can fail.  What then?

     At present, we have a few options:

     1.  Panic and abort the program.  This is the Pascal solution,
	 and is one of the reasons nobody does serious work in 
	 standard Pascal.

     2.  Force the caller to do the opening operation separately,
	 then pass a handle for the open object to the class, rather
	 than letting the class handle the open itself.  This is
         the solution chosen for the streams package.  One of the
         major advantages of class declarations, the automatic
	 execution of the destructor upon scope exit, is lost
	 with this approach.

     3.  If the open fails, put the class instance into a "bad"
	 state, so that later operations on it produce some error
	 condition.  This works, but provides no guarantee that
	 the proper check will be made later.  Even if it is,
	 special effort must be made to save the information that
	 describes what went wrong during construction and then
	 to retrieve and analyze it later.

Well, that's where we are today.  What should be done?

					John Nagle