Re: lifetime extension of temoraries
On Mar 22, 11:47 pm, n.torrey.pi...@gmail.com wrote:
http://www.artima.com/cppsource/foreach.htmlsays
"temporary objects of derived type can have their lifetime extended by
binding them to a const reference to the base type."
====================================
#include <iostream>
struct base {};
struct derived : public base {
~derived() { std::cout << "derived is dead\n"; }
};
struct ref {
const base& ref_;
ref(const base& x) : ref_(x) {}
};
int main() {
ref r = derived();
std::cout << "ref is alive\n";}
====================================
If this is the case, how come the output of the above code (in VC++8
and G++ 3.4.4) is
derived is dead
ref is alive
What else should it be? You bound a temporary to the const
reference x in order to call the constructor of ref. So it's
lifetime would be extended to that of x, except that 1) it
already has a longer lifetime, and binding to a const reference
cannot shorten the lifetime, and 2) there is another special
rule here which also extends the lifetime---until we return from
the constructor. (Note that the "normal" lifetime is until the
end of the complete expression, and the call of the constructor
is, formally, not part of the expression.)
Curiously, if "ref r" is changed to "const base& r", the output
changes for both compilers,
If a temporary (rather than another reference) is used to
initialize the const reference r, then it's lifetime is extended
to match that of r (which here has a lifetime longer than that
which the temporary would normally have).
AND is now also compiler-dependent (two
temporaries in VC++)
Formally, the standard requires a copy, then authorizes the
compiler to eliminate it. (I think this rule is being reviewed,
and might change in the next version of the standard.)
Judging by this threadhttp://groups.google.com/group/comp.lang.c++/browse_frm/thread/e8c8fb...
there seems to be little consensus on how the standard should be
interpreted regarding lifetime extensions?
Actually, it's one point where the standard is very clear. The
thread you site seems to be more concerned with whether an
intermediate temporary is constructed or not---the standard is
also very clear there: it's unspecified (but it must be legal to
make the copy). Alos, the fact that some people don't know the
rules in the standard doesn't mean that there is no consensus
with regards to how to interpret them.
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