Re: [boost] [move] interest: the pass-by-value and swap idiom, and explicit copy constructors

2014-02-15 14:58 GMT+01:00 Krzysztof Czainski <1czajnik@gmail.com>:

2014-02-15 14:11 GMT+01:00 Adam Wulkiewicz :

Hi,

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Krzysztof Czainski wrote:

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2014-02-15 1:13 GMT+01:00 Adam Wulkiewicz :

b) here I have a copy as well, probably because Matrix has a non-static

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data member without a move assignment operator and that is is not trivially copyable c = boost::move(b); after declaring "proper" move assignment in the member class (instead of copy and swap) or using copy and swap + move ctor in Matrix everything works as expected.

Do you get a copy in C++11 here? I get a move() in C++11, and a copy in C++98. And that's what I expect, because C++11 auto-generates copy- and move-assignment operators, and both use the pass-by-value assignment of CopyMoveDemo correctly.

Yes, in GCC4.7, Clang3.2, MinGW 4.7, VS2010, VS2013.

However VS isn't good for testing the standard. For instance it doesn't implicitly delete the copy ctor if move ctor is defined and therefore permits copying.

Assuming that my understanding is correct, with copy and swap the move

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assignment operator can't be implicitly declared.

Why?

I asked about this on comp.lang.c++.moderated: https://groups.google.com/forum/#!topic/comp.lang.c++.moderated/6KvOCkFdcR8

In short, MinGW-4.8 seems to follow the proposed resolution to this defect: http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1402 In any case, don't let this issue hide the main idea in my proposal, which is: - adding to Boost.move a possibility for the user to define the assignment operator in terms of pass-by-value and swap; - making copy constructors explicit, to be used together with `copy()` -- a complimentary function to `move()`. Cheers, Kris