Re: Default param, is this legal
"Leigh Johnston" <leigh@i42.co.uk> wrote in message
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"Leigh Johnston" <leigh@i42.co.uk> wrote in message
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"Jonathan Lee" <jonathan.lee.975@gmail.com> wrote in message
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On Jul 14, 9:36 am, "Francesco S. Carta" <entul...@gmail.com> wrote:
According to what I see above, the object gets destroyed right after
the
end of the call to that static function, so I think you're NOT safe
using that reference afterwards.
I repeat, that's just an empirical test, I have no idea about what the
standard mandates about it, although I feel it's UB at least.
Mehhh... [class.temporary] says the temp should exist until the end of
the full expression. So you might be able to do something like
MyClass b = GetMyClass(MyClass());
and effectively assign to b from the temporary. But it's not at all
clear to me what a full-expression is. I mean, it's defined in
[intro.execution] but then it lists some exceptions to the rule,
which seem to muddy the issue for me.
It is obvious to me at least that the innards of GetMyClass is not part
of the expression used to invoke GetMyClass. Using a dangling reference
is UB and the reference *is* dangling as far as I am concerned.
/Leigh
Hmmm, maybe I am mistaken, both VC++ and g++ seem to have the temp's
lifetime extend beyond the construction of the copy target.
/Leigh
Obviously I am talking about the following case:
struct MyClass
{
MyClass() { std::cout << "ctor\n"; }
MyClass(const MyClass&) { std::cout << "cctor\n"; }
~MyClass() { std::cout << "dtor\n"; }
};
const MyClass& GetMyClass(const MyClass& t = MyClass()) { return t; }
int main()
{
MyClass b = GetMyClass();
}
which outputs:
ctor
cctor
dtor
dtor
/Leigh
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