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Subject: Re: [boost] [move][container] Review Request (new versions ofBoost.Move and Boost.Container in sandbox and vault)
From: Howard Hinnant (howard.hinnant_at_[hidden])
Date: 2009-09-07 19:27:34

On Sep 7, 2009, at 6:50 PM, David Abrahams wrote:

> on Mon Sep 07 2009, Howard Hinnant <>
> wrote:
>>> Do we think swap(x,y) is also an error when x == y?
>> This is a decent argument to not assert in self-move-assignment.
> And not to call it undefined behavior. self-std:swapping was
> allowed in
> C++03 and I don't think we can break that.
>> But this is also almost certainly harmless:
>> #include <algorithm>
>> #include <iostream>
>> class A
>> {
>> int data_;
>> public:
>> explicit A(int data = 0) : data_(data) {}
>> A(A&& a) : data_(a.data_) {a.data_ = 0;}
>> A& operator=(A&& a) {data_ = a.data_; a.data_ = 0; return *this;}
>> friend std::ostream& operator<<(std::ostream& os, const A& a)
>> {
>> return os << a.data_;
>> }
>> };
>> int main()
>> {
>> A a(5);
>> std::cout << "Before swap a = " << a << '\n';
>> std::swap(a, a);
>> std::cout << "After swap a = " << a << '\n';
>> }
>> Before swap a = 5
>> After swap a = 5
> Yes, it's harmless, but I don't see how it's a "but." I must be
> missing
> your point, because that example seems to support my argument.
>> But if I caught my own code doing a self-swap, yeah, I would treat it
>> as a bug and correct it. For example, every time I've written
>> reverse
>> (which does nothing but swap x and y while x and y move closer to
>> each
>> other in the sequence), I'm careful to break out of the loop before
>> &x
>> == &y.
> Yes, but if it's reverse's job to avoid that, then it's also swap's
> job (both are algorithms, neh?) So should we put a self-swap test
> there?

No, I don't think so.

std::swap will only do a self-move-assignment using a moved-from
object. Of course vector will never use this definition. But if it
did, then:

template <class _Tp, class _Allocator>
vector<_Tp, _Allocator>&
vector<_Tp, _Allocator>::operator=(vector&& __x)
    return *this;

would work just fine. clear() is a no-op, and then you swap an empty
capacity vector with itself.

My point is that I don't want to be told I need to put a if(this !=
&x) in my move assignment operator. It is a useless performance
penalty. Everyone should not have to pay (in terms of lost
performance) for someone's bad code just to make sure self-move-
assignment does a no-op instead of crashes. If you want to cast an
lvalue to an rvalue, that's fine. Just make sure your code logic is
such that such a cast is safe. I believe std::swap's use of move
assignment is safe because by the time it move-assigns, if it is self
referencing, all of your resources have already been moved to a local


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