Re: c-style arrays used in setbranch for std::vector branches

From: Erkcan Ozcan <eo_at_hep.ucl.ac.uk>
Date: Mon, 17 Jan 2011 19:57:10 +0200

Hi John,

Thanks. When I said cast, I indeed phrased wrongly: I had meant &v[0] as you suggested. However, I think it is still not the best thing to do: C++98 standard does not guarantee that the memory allocated by std::vector will be contiguous, that requirement was added in TR1. Luckily, I think all compilers do it contiguously these days, that is why people can use &v[0].

But please note that if the size of the std::vector changes, std::vector might carry the elements elsewhere in memory and the memory address for v.begin() might change. Furthermore, my question might not be simply a std::vector to c-array dereferencing question, it is related to how the ROOT authors implemented std::vectors in TTree and whether TTree->SetBranchAddress() works safely when I give a pointer to a c-array instead of a pointer to an std::vector to it.

Thanks for the input though.

Cheers,
e.

On 17 Jan 2011, at 01:42, John Idarraga wrote:

> Hello Erkcan,
> 
> This is fine because there is not really a cast taking place.  If you create a std::vector of floats for instance:
> 
> vector<float> v;
> 
> you can always safely reference the array of floats inside by doing &v[0] (say for example in the constructor of a TGraph ... that kind of situations).
> 
> So it is safe as long as you deal correctly with the size of the array.  You probably have a variable for that.  In case you don't there is always the clever trick of sizeof(array)/sizeof(float) ;)
> 
> cheers !
> 
> John Idarraga
> 
> On 01/16/2011 05:59 PM, Erkcan Ozcan wrote:

>> Dear ROOT experts,
>>
>> We have an ntuple that contains std::vectors of floats and ints. When we do MakeClass() we get branches like this:
>>
>> vector<float> *pvx;
>>
>> We also have an old piece of code written before the ntuple format was modified and it used to use c-style arrays. The same branch used to be:
>>
>> float pvx[10];
>>
>> And we were doing the following setbranch step:
>>
>> anltree->SetBranchAddress("pvx" ,&pvx);
>>
>> We have found out that the code seems to work on the new ntuples without any modifications, ie. we still define c-style arrays and setbranch to a branch which is really a pointer to an std::vector.
>>
>> Is this safe to do? I know normally it is not ok to cast an std::vector to a c-style array, but given that in this scenario we will only be reading from a ROOT file, perhaps what we do is completely fine. Could some expert confirm please?
>>
>> Thanks a lot,
>> e.
>>
>> PS: I have had some problems when I sent this email the first time. Apologies to all the mailing list members if we end up getting it twice.
>> PS2: We are using ROOT 5.26/00b.
>>
>>

>

-- 

In case they are not written explicitly, please be aware that my greetings and farewell are inherently implied in this email.

V. Erkcan Ozcan
Research Fellow
University College London
Dept. of Physics & Astronomy


-- 

In case they are not written explicitly, please be aware that my greetings and farewell are inherently implied in this email.

V. Erkcan Ozcan
Research Fellow
University College London
Dept. of Physics & Astronomy

Received on Mon Jan 17 2011 - 18:57:30 CET