copy constructor and move constructor

Sui Huang

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Sep 20, 2015, 9:15:51 AM9/20/15

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Hi everyone, I got trouble with copy or move operations.

I wrote a class HasPtr when learning chapter 13 of <C++ Primer>.
/* header file */
class HasPtr{
public:
HasPtr(const std::string &str = std::string()) :
ps(new std::string(str)), val(0) {std::cout << "default constructor";}
HasPtr(const HasPtr &); //output "copy constructor"
HasPtr(HasPtr &&) noexcept; //output "move constructor"
HasPtr &operator=(const HasPtr &); //output "copy assignment"
HasPtr &operator=(HasPtr &&) noexcept; //output "move assignment"
~HasPtr();
private:
std::string *ps;
int val;
};

I have implemented HasPtr correctly, in each member function, print the function name. And I cann't understand these outputs that :
/* test code */
#1. HasPtr ptr = HasPtr(); //call "Default Constructor"
#2. HasPtr ptr2(HasPtr()); //outpus nothing
#3. vector<HasPtr> items = {HasPtr("abc"), HasPtr("def")}; //first call default constructor, then copy constructor

#1. I think right side object is rvalue, why not first call default then move constructor?
#2. I think this should work like #1.
#3. When copy objects to vector, why not call move constructor?

By the way, my compile environment is gcc 4.9.2 in Ubuntu 15.04

Öö Tiib

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Sep 20, 2015, 11:13:46 AM9/20/15

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On Sunday, 20 September 2015 16:15:51 UTC+3, Sui Huang wrote:
> Hi everyone, I got trouble with copy or move operations.

No. You got trouble with most vexing parse.
https://en.wikipedia.org/wiki/Most_vexing_parse

> #2. HasPtr ptr2(HasPtr()); //outpus nothing

There you declare a function.

mark

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Sep 20, 2015, 1:37:17 PM9/20/15

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On 2015-09-20 15:15, Sui Huang wrote:
> #3. vector<HasPtr> items = {HasPtr("abc"), HasPtr("def")}; //first call default constructor, then copy constructor

> [...]

> #3. When copy objects to vector, why not call move constructor?

Initializer lists don't support move, the elements can only be accessed
as const and thus must be copied.

There is a proposal to support move, so this may be possible in C++17.

Paavo Helde

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Sep 20, 2015, 3:34:53 PM9/20/15

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Sui Huang <hailiy...@gmail.com> wrote in
news:2eda2e20-0bad-4cde...@googlegroups.com:

> #1. HasPtr ptr = HasPtr(); //call "Default Constructor"
>

> #1. I think right side object is rvalue, why not first call default
> then move constructor?

Most probably the move constructor has been optimized away and the object
is constructed directly in the right place.

Others have already answered other questions.

Sui Huang

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Sep 21, 2015, 3:54:14 AM9/21/15

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在 2015年9月20日星期日 UTC+8下午11:13:46，Öö Tiib写道：

thanks, that is really a vexing parse.
It is really confusing that HasPtr() is parsed as a function returning type HasPtr.
whatever, I got it. thank you.

Sui Huang

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Sep 21, 2015, 3:56:35 AM9/21/15

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在 2015年9月21日星期一 UTC+8上午3:34:53，Paavo Helde写道：

So excepts copy elision, RVO as far as I know, are there many other compiler optimizations in C++?

Sui Huang

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Sep 21, 2015, 3:59:42 AM9/21/15

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在 2015年9月21日星期一 UTC+8上午1:37:17，mark写道：

thank you, I don't know much about initializer list. I just learn that it will excutes copy-initialization. Does copy-initialization either call copy constructor or move constructor?

Paavo Helde

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Sep 21, 2015, 4:21:10 AM9/21/15

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Sui Huang <hailiy...@gmail.com> wrote in

news:01090f77-d0ca-44c8...@googlegroups.com:

There are many optimizations, but AFAIK elision of copy and move
constructors are the only ones which are allowed to change observable
behavior in a valid portable C++ program. Basically this means the
program should not rely on if and when the copy and move constructors are
called.

If the program uses undefined, unspecified or implementation-specified
behavior, then of course one may see changes in the observable behavior
also elsewhere. A good example is optimizing away some code which is
breaking strict aliasing rules
(http://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-
rule)

Cheers
Paavo

Öö Tiib

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Sep 21, 2015, 11:21:18 AM9/21/15

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AFAIK only clang does hint [-Wvexing-parse] that your code seems to
contain it. It is one of most obscure features of C++. Most programmers
keep function declarations in header files and static function declarations
in start of cpp file. Therefore it is surprising that most compilers tell nothing
about a function declaration in function body. It is allowed, but unusual so
worth a warning IMHO.

Richard

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Sep 22, 2015, 1:21:05 PM9/22/15

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[Please do not mail me a copy of your followup]

Sui Huang <hailiy...@gmail.com> spake the secret code
<d836ff05-1efe-4982...@googlegroups.com> thusly:

>在 2015年9月20日星期日 UTC+8下午11:13:46，Öö Tiib写道：
>> On Sunday, 20 September 2015 16:15:51 UTC+3, Sui Huang wrote:
>> > Hi everyone, I got trouble with copy or move operations.
>>
>> No. You got trouble with most vexing parse.
>> https://en.wikipedia.org/wiki/Most_vexing_parse
>>
>> > #2. HasPtr ptr2(HasPtr()); //outpus nothing
>>
>> There you declare a function.
>
>thanks, that is really a vexing parse.

C++11 brace initialization gives you what you want, so another reason
to prefer that for initializing a variable.
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