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

Hi,

> Given a branch which is found to be a *vector<POD> by MakeClass(), can I instead call SetBranchAddress with a c-style array?

No this unfortunately does not work. The address passed must be for an actual std::vector object.

Philippe.

On 1/18/11 5:06 AM, Erkcan Ozcan wrote:
> Hi Axel,
>
> That is good to know. Actually my question was the reverse. Given a branch which is found to be a *vector<POD> by MakeClass(), can I instead call SetBranchAddress with a c-style array?
>
> Thanks a lot,
> e.
>
> On 18 Jan 2011, at 10:24, Axel Naumann wrote:
>
>> Hi,
>>
>> to clarify my reply:
>>
>> Given a branch containing a c-style array (POD* where POD is float,
>> double,...), you can call SetBranchAddress() passing a vector<POD>'s
>> first element's address (&v[0]), as long as the vector has sufficient
>> capacity and doesn't resize. I believe that's what the question really
>> was in the end.
>>
>> Cheers, Axel.
>>
>> Axel Naumann wrote on 01/17/2011 09:14 PM:
>>> Hi,
>>>
>>> That should work as long as the vector doesn't resize, as you already pointed out. And make sure you reserve enough capacity before calling SetBranchAddress().
>>>
>>> Cheers, Axel
>>>
>>> "Erkcan Ozcan"<eo_at_hep.ucl.ac.uk> wrote:
>>>
>>>> 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 Tue Jan 18 2011 - 19:09:58 CET