STL UnaryPredicates with TCollection

Newbie
1

Does ROOT not support STL’s UnaryPredicates, or am I doing something wrong? Thanks for your help!

root [0] TObjArray *arr = new TObjArray()
root [1] all_of(arr->begin(), arr->end(), [](TObject *obj) { return true; })
In module 'std' imported from input_line_1:1:
/usr/include/c++/9/bits/stl_algo.h:173:8: error: no matching function for call to '__iterator_category'
                            std::__iterator_category(__first));
                            ^~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/9/bits/stl_algo.h:566:19: note: in instantiation of function template specialization 'std::__find_if_not<TIter, __gnu_cxx::__ops::_Iter_pred<(lambda at
      ROOT_prompt_1:1:34)> >' requested here
      return std::__find_if_not(__first, __last,
                  ^
/usr/include/c++/9/bits/stl_algo.h:509:29: note: in instantiation of function template specialization 'std::find_if_not<TIter, (lambda at ROOT_prompt_1:1:34)>' requested here
    { return __last == std::find_if_not(__first, __last, __pred); }
                            ^
ROOT_prompt_1:1:1: note: in instantiation of function template specialization 'std::all_of<TIter, (lambda at ROOT_prompt_1:1:34)>' requested here
all_of(arr->begin(), arr->end(), [](TObject *obj) { return true; })
^
/usr/include/c++/9/bits/stl_iterator_base_types.h:205:5: note: candidate template ignored: substitution failure [with _Iter = TIter]: no type named 'iterator_category' in
      'std::iterator_traits<TIter>'
    __iterator_category(const _Iter&)
    ^
2

It does work on some platforms and not others. It looks like the C++ std libraries you are using here as “additional/new” requirements for containers that we are not yet added (it seems to be that we are missing std::iterator_traits<TIter>)

If you can not switch to std::vector. You can use this work around for TObjArray:

(arr->GetEntries()>0) && all_of(&((*arr)[0]), &((*arr)[arr->GetEntries()-1]), [](TObject *obj) { return true; })
3

Thanks, but I guess that will not check for the last element. There are definitely cleaner ways to test this but this is the first thing that came to my mind:

root [0] TObjArray *arr = new TObjArray()
root [1] (*arr)[0] = new TH1F("h", "", 1, 0, 1)
root [2] (*arr)[1] = new TH1F("g", "", 1, 0, 1)
root [3] (arr->GetEntries()>0) && all_of(&((*arr)[0]), &((*arr)[arr->GetEntries()-1]), [](TObject *obj) { return obj->GetName() == TString("h"); })
(bool) true
4

This sounds like a good candidate for a bug report at Issues · root-project/root · GitHub – however as Philippe mentioned we typically suggest to switch to STL containers if possible.

Cheers,
Enrico

5

In my specific case it is not my code, so it’s not possible to use STL containers for me.

I can file a bug report, but probably I don’t have the necessary knowledge to understand precisely what is the bug there…

In the meantime as a workaround I wrote my own function:

#include <functional>

bool all_of(TCollection *collection, const std::function<bool(TObject *)> &predicate)
{
  bool result = true;
  for (auto object : *collection)
    if (!predicate(object)) {
      result = false;
      break;
    }
  return result;
}
6

Oups :innocent:, simple off by one error:

(arr->GetEntries()>0) && all_of(&((*arr)[0]), &((*arr)[arr->GetEntries()]), [](TObject *obj) { return obj->GetName() == TString("h"); })
(bool) false