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From: "Paul D. DeRocco" <pderocco@ix.netcom.com>
Subject: Re: a question about temporary exception object
Date: 1998/11/07
Message-ID: <3643DC0E.EB83C9CD@ix.netcom.com>#1/1
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Approved: Fergus Henderson <fjh@cs.mu.oz.au>
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Gilbert Chang wrote:
 > 
 > Consider the following code snippets:
 > 
 > //
 > // class Foo
 > // {
 > // public:
 > //     Foo();
 > //    ~Foo();
 > // };
 > //
 > 
 > try
 > {
 >     throw Foo();
 > }
 > catch(Foo & foo)
 > {
 > }
 > 
 > VS.
 > 
 > try
 > {
 >     throw Foo();
 > }
 > catch(Foo foo)
 > {
 > }
 > 
 > According to the standard, the constructor and the destructor of Foo
 > should be called exactly once for each in the first case, but they
 > could be called either once for each or twice for each.  Is my
 > understanding correct?

In the most literal sense, with no optimizations, the Foo() should cause
a temporary object to be constructed on the stack. Then the throw should
copy the Foo into some safe memory that is unaffected by the stack
unwinding. Then, in the second case, the catch should copy the object a
third time. However, the standard always allow copies of temporaries to
be optimized away, even if their constructors and destructors are
nontrivial, as long as no other semantic changes result. And in
practice, compilers are smart enough to know that the Foo can originally
be constructed into the safe memory where it will live while the stack
is unwound. In fact, the compiler may even construct the foo directly
into the stack space allocated for foo in the second catch clause.
However, I don't think this is typical.

-- 

Ciao,
Paul
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