Re: What happen with this 'B()'?
Hello,
James Kanze wrote:
On Apr 24, 1:13 am, Vols <volunte...@gmail.com> wrote:
class A{
public:
int x;
};
class B : public A{
public:
int y;
};
void foo()
{
A &a1 = B();
A *a2 = &B();
}
what happen with those 'B()'. I've never seen this before.
Me neither. Both lines in foo() are illegal in C++, and require
a diagnostic. (Much to my surprise, g++ only generates a
warning on the second, and Sun CC doesn't complain about
either! Which is IMHO simply not acceptable. Particularly the
second, which has never been legal C++.)
Can't follow that. All g++-3.3.6, 4.2.1, and 4.3.0 make the first line
in foo an error and the second a warning without any special settings.
The first line is the one that assigns reference to a temporary to a
non-const reference type, which cannot work. The second line seems to
have the same problem with const pointer and non-const pointer at
first.
liftim1.cc: In function ?void foo()?:
liftim1.cc:12: error: invalid initialization of non-const reference of
type ?A&? from a temporary of type ?B?
liftim1.cc:13: warning: taking address of temporary
A temporary object is not a const object by definition if there is no
const in the declaration. The standard mandates that assigning a
temorary to a non-const reference is not allowed, but this decision is
kind of deliberate, because it would produce hard to detect errors most
of the time. But taking the pointer is not the same es taking a
reference. Using that pointer could be problematic, especially
dereferencing, but the heuristical warning is the best one can hope for
when taking the address alone.
Bernd Strieder