Re: Return versus Side-Effect
Daniel Kr?gler ha scritto:
The upcoming C++0x will also provide one further
language extension that would programmers allow to
prevent the above mentioned current asymmetry, if
they don't like them. This is possible, because it
extends the concepts of rvalues and lvalues to the
object parameter of non-static member functions by
introducing so-called ref-qualifiers as described in:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2439.htm
So it would be possible to write
struct X {
void swap(X&&) &;
};
which nominates a member function that accepts
even rvalues as arguments, but where the invoking
*this must be an lvalue.
In this case we would no longer be allowed to write
X getX();
getX().swap(X());
because we would no longer support to invoke an &-function
given an rvalue as object parameter (The second rvalue X()
would be feasible of course).
Unfortunately, that would break a lot of existing valid code and
precisely all code using the so-called swap-trick, for example:
std::vector<int> v;
std::vector<int>().swap(v);
(notice that you can't write the trick the other way round, because
rvalues are currently not allowed as the function parameter)
The swap-trick has been advertised as the right way to deallocate a
vector's buffer (it's described even in Scott Meyers' EC++ Programming).
Making it illegal would be a suicide, IMHO.
On the other hand, the very recent paper n2590
(http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2008/n2590.pdf)
argues whether it's so wrong to allow rvalues on both sides of the swap.
The paper proposes to replace all the swap overloads for the too many
library classes with just one restricted template:
template <ClassType T>
auto concept MemberSwappable {
void T::swap(T &&); // notice: rvalues allowed on both sides
};
template <MemberSwappable T>
void swap(T&& a, T&& b) {
a.swap(b);
}
I don't know the reputation of the proposal and its chance to be
accepted, but the idea of removing 30+ groups of three identical
std::swap overloads from the library sounds a good thing to me, do you
agree?
HTH,
Ganesh
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