Re: Temporary objects, l-values.
Erik Wikstr?m wrote:
[..]
While it would work to allow to bind (non-const) references to
temporary objects in the above code it would not work in many other
situations that would also be allowed. Consider the following:
class Foo {
int i;
public:
void print() {
std::cout << i << std::endl;
}
};
void bar() {
Foo& f = Foo();
f.print();
}
In this example f is referring to a non-existing object when pring()
is called (it is a so called dangling reference). Binding temporaries
to const references is a special rule in C++, what happens is that
the life of the temporary object is extended to be as long as the
life of the reference it is bound to.
What if the "special rule" would include refs to non-const? The
simple thing about it is that the temporary survives as long as
the reference to it. [The current rule allows only ref to const,
but the idea is the same, the temp survives as long as the ref]
Now, the problem is deeper than just the lifetime of the reference.
Binding is allowed with type conversion. If the conversion takes
place, there is another temporary created, to which the reference
is actually bound. As soon as you allow binding of a non-const
reference, you run into "what exactly am I changing" here (this is
the example Dr. Stroustrup uses in his D&E book):
void foo(int & i)
{
i = 42;
}
int main()
{
double d = 3.14159;
foo(d); // what's the value of 'd' after this?
}
At the time 'foo' is called, a temporary object of type 'int' is
created, and if the non-const reference is bound to it, the value
42 is assigned to that temporary object, but 'd' has nothing to
do with that. So, OOH 'foo' is supposed to change its argument's
value to 42, OTOH there is no way for it to accomplish that if
the factual argument is not of type 'int' but of type _convertible_
to 'int'.
V
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