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Subject: Re: [boost] Improving the assignment operators of various Boosttypes
From: David Abrahams (dave_at_[hidden])
Date: 2008-09-11 10:07:50

on Thu Sep 11 2008, "Niels Dekker - mail address until 2008-12-31" <> wrote:

> David Abrahams wrote:
>>>> In generic code I strongly prefer
>>>> T& operator=(T arg)
>>>> {
>>>> swap(*this, arg);
>>>> return *this;
>>>> }
>> I'm just saying, I guess, that the form using the member
>> forces me to define a swap member even though it's not really needed
>> by generic code.
> I think we should stick to the guideline to always have a swap member function
> for a class, whenever the class has a custom free swap function. Don't you
> agree?

Sorry, but no, I just don't see the point. It feels like needless
clutter. Generic code needs a free function that can be found via ADL,
so I would normally make it a friend declared inside the class body.
There's nothing wrong with adding a swap member, but I'd be loathe to
make it a guideline.

> BTW the swap member function offers a C++03 way to "move assign" an rvalue:
> std::vector<int> generate_some_data(void);
> std::vector<int> my_data;
> // Move the result into my_data:
> generate_some_data().swap(my_data);

Well, that's true, but I much prefer:

      assign(my_data) = generate_some_data();

because it better reflects the logical operation.

> Peter Dimov wrote:
>> Incidentally:
>>> "It doesn't compile with Borland C++Builder 4 or BCC 5.5.
>> The next problem was associative_vector's assignment operator.
>> Borland didn't recognise it as one and generated a default.
>> Then it couldn't disambiguate the generated assignment operator
>> and the one supplied. [...]
>> My memory is correct on this one, too. I'm not sure why BCC 5.5.1
>> didn't exhibit the problem in my tests.
> Thanks Peter. Maybe it's a C++Builder 4/BCC 5.5 bug that was fixed by BCC 5.5.1.
> :-) Anyway, neither of them are still at the regression pages. Siliconman runs
> the tests on BCC 5.6.4, 5.8.2, and 5.9.3, while JustSoftwareSolutions does BCC
> 5.8.2.
> Thanks Joaquín, for already applying the patch from ticket #2313 to
> multi_index_container::operator= Things still look fine at the regression page
> <> but I guess
> we have to wait a few days longer...

Niels: are you planning to pursue tickets for the classes with implicit
assignment operators and the others I mentioned in response to your
original post?

Dave Abrahams
BoostPro Computing

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