Re: [boost] [type_traits] optimized std::copy example wrong?

Subject: Re: [boost] [type_traits] optimized std::copy example wrong?
From: Thorsten Ottosen (nesotto_at_[hidden])
Date: 2010-04-24 16:08:21

Steven Watanabe skrev:
> AMDG
> 
> strasser_at_[hidden] wrote:
>> Zitat von Thorsten Ottosen <nesotto_at_[hidden]>:
>>
>>> Hi John & co,
>>>
>>> IIRC, std::copy() cannot use memcpy(), since the memory is allowed to 
>>> overlap. Therefore it can only use memmove().
>>
>> std::copy() doesn't allow overlapping ranges. -> std::copy_backward()
> 
> Both copy and copy_backwards allow some overlapping ranges.
> 
> int array[10] = {};
> std::copy(&array[0], &array[7], &array[1]); // illegal
> std::copy(&array[1], &array[8], &array[0]); // okay
> std::copy_backwards(&array[0], &array[7], &array[8]); // okay
> std::copy_backwards(&array[1], &array[8], &array[7]); // illegal

Ok, so what is the conclusion? From the C standard:

   void *memcpy(void * restrict s1,
                        const void * restrict s2,
                        size_t n);

        Description

        [#2] The memcpy function copies n characters from the object
        pointed  to  by  s2  into  the  object pointed to by s1.  If
        copying  takes  place  between  objects  that  overlap,  the
        behavior is undefined.

I recall that STLPorts std::copy() used memmove(). How do we interpret
"objects that overlap"?

-Thorsten