Re: Is returning a const& valid?
Francis Glassborow wrote:
In article <cXTwh.14764$Um.485943@weber.videotron.net>, Carlos Moreno
<moreno_at_mochima_dot_com@mailinator.com> writes
doesn't work --- I mean, I'm not sure I understand what exact
rule from the standard makes it invalid; the first statement's
temporary has a lifetime that is extended since it is bound to
a reference-to-const; then, it is bound again to another
reference-to-const (the one that is returned); why isn't its
lifetime extended to match the lifetime of the returned reference,
and then again extended if the client code binds it again to a
reference-to-const?? Does the rule about extending the lifetime
specifically puts the restriction that it only works when you're
within the same scope? I can't find it anywhere in the standard.
I am not sure where it says it in the standard but the lifetime of a
temporary bound to a const reference is limited to the lifetime of that
reference. Note that it exists exactly for that lifetime so when you
attempt to return something by const reference its lifetime ends at the
point when the function returns. Trying to require anything else would
just result in a mess.
I think that there is a problem in the usual wording (and the
standard could be clearer about this): if I write something
like:
Foo const& f() { return Foo() ; }
Foo const& g = f() ;
the reference g is clearly bound to the temporary created in
f(), although both you and I know that this temporary will be
destructed on returning from f. IMHO, it would be much clearer
to say that if a temporary is used to initialize a reference,
its lifetime is extended to match the lifetime of the that
reference. This extension to the lifetime is not transitive.
Note too that the standard makes clear and explicit statements
concerning two cases ('12.2/5): "A temporary bound to a
reference member in a constructor s ctor-initializer (12.6.2)
persists until the constructor exits. [...] A temporary bound to
the returned value in a function return statement (6.6.3)
persists until the function exits." Off hand, no other cases
occur to me where transitivity could be used to extend the
lifetime of the temporary.
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