Re: Binding temporaries to const references
On 16 Jan., 04:32, "Balog Pal" <p...@lib.hu> wrote:
"Jens Auer" <jens.muad...@googlemail.com>
I was playing around with const references and temporary objects to
make sure that I understood section 12.2. of the standard correctly.
struct RefHolder
{
RefHolder():
mReference(A(1)) // A iso c++ 12.2.4
...
A const& mReference;
};
That creates a dongling reference.
int main( int argc, char** argv )
{
{
A const& ref(A(2)); // C iso c++ 12.2.5
std::cerr << ref.mInt << std::endl;
}
This one is okay, temporary bound to ref lives till }.
Ok, reading the standard, the first output is produced because of the
assignment marked with C. The other two seem to be justified by
12.2.4:
"...when an expression appears as an initializer for a declarator
defining an
object. In that context, the temporary that holds the result of the
expression shall persist until the object s
initialization is complete.". What I don't get is why the temporary in
the initialization of case A has to be deleted so early and is not
bound to the member reference mReference.
It is bound alright. Then immediately after that the full expression is
over, the temporary is nuked, and you have mReference dongling. The
full-expression is
mReference(A(1))
Maybe I should rephrase my question. I know that the standard says
this, that is why I quoted it. But I don't get the intention why it is
ok to bind a temporary object to a non-const reference in
{
T const& constRef = returnTempObject();
}
but not when I initialize a member reference:
class ConstRefMember
{
public:
ConstRefMember(): mRef(returnTempObject()) {}
T const& mRef;
};
The compiler should be able to figure out the destructor and insert a
call to it in the class's destructor, so why is this case not
supported? It seems not quite consistent with the other, allowed case.
I think that the exception for binding a temporary object binding to a
const reference was added to make it possible to pass temporary
objects to functions having const reference parameters, but I do not
see why it has to be forbidden when assigning them to members.
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