Encountered piece of code using memcpy

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Hi, I encountered such code:

C++

for(int i=0;i<count;i++)
{
...
CustomStruct obj;                memcpy(&obj,Variable[index].get_blob().data(sizeof(CustomStruct )*i),sizeof(CustomStruct ));

}

as I understood memcpy part here, is basically doing a binary copy of CustomStruct data structures (starting at index sizeof(CustomStruct )*i) to the obj variable right?

Posted 2-Jul-13 2:58am

Dan page

Updated 2-Jul-13 3:01am

v2

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CPallini · Answer 1 · 2013-07-02T03:06:00

Solution 1

Yes, it is just doing such a binary copy.

Posted 2-Jul-13 3:06am

CPallini

v2

Comments

Dan page 2-Jul-13 9:20am

hi, thanks also actually there is another trick in the code. like: `memcpy(&obj,Variable[index].get_blob().data(sizeof(OtherCustomStruct )*i),sizeof(OtherCustomStruct ));` obj is again of type CustomStruct, however OtherCustomStruct is basically similar as CustomStruct just they differ on one element, OtherCustomStruct seems to be kind of subset of CustomStruct. And the copy is still working, seems it should be like that right?

JackDingler 2-Jul-13 12:31pm

memcpy() doesn't care what the data means.

It simply copies a range of bytes from one location to another.

CPallini 2-Jul-13 14:24pm

It looks a (an ugly) trick, ~~basically a copy of the CustomStruct part of OtherCustomStruct. It works, provided OtherCustomStruct 'extends' (that is contains as first memory block) CustomStruct~~.
[update]It is the other way around, as correctly pointed out by the OP[/update]

Dan page 2-Jul-13 17:04pm

Hi CPallini, thanks, just as I remember it was other way around, meaning in my example, say CustomStruct contains four elements first three int-s and one char. Then, OtherCustomStruct would contain just those first three int-s.

CPallini 2-Jul-13 17:15pm

You are right (and my previous remark was wrong: I overlooked the third parameter of memcpy (I wrongly assumed it was sizeof(CustomStruct)). It should work, setting just the first three ints of CustomStruct.