Std::unordered_map with TString

ROOT
1

Hello All,

I was wondering if it were possible to use std::unordered_map with TString.

It seems that statements like

 std::unordered_map <Int_t,int> aMap_1;

are acceptable, while something like:

 std::unordered_map <TString,int> aMap_2;

can not be compiled.

The error message I get is:

clang++ -O2 -pipe -Wall -W -Woverloaded-virtual -pthread -stdlib=libc++ -std=c++11 -m64 -I/Users/sadeh/work/root/root/include -std=c++0x   -c -o myMain.o myMain.cpp
In file included from myMain.cpp:1:
In file included from ./myMain.hpp:1:
In file included from /Users/sadeh/work/root/root/include/TROOT.h:29:
In file included from /Users/sadeh/work/root/root/include/TDirectory.h:25:
In file included from /Users/sadeh/work/root/root/include/TNamed.h:26:
In file included from /Users/sadeh/work/root/root/include/TObject.h:31:
In file included from /Users/sadeh/work/root/root/include/Rtypes.h:30:
In file included from /Users/sadeh/work/root/root/include/Rtypeinfo.h:32:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/typeinfo:61:
In file included from /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/exception:81:
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/type_traits:948:38: error: implicit instantiation of undefined template
      'std::__1::hash<TString>'
    : public integral_constant<bool, __is_empty(_Tp)> {};
                                     ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_map:362:54: note: in instantiation of template class
      'std::__1::is_empty<std::__1::hash<TString> >' requested here
template <class _Key, class _Cp, class _Hash, bool = is_empty<_Hash>::value
                                                     ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/unordered_map:731:13: note: in instantiation of default argument for
      '__unordered_map_hasher<TString, std::__1::__hash_value_type<TString, int>, std::__1::hash<TString> >' required here
    typedef __unordered_map_hasher<key_type, __value_type, hasher>   __hasher;
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
myMain.cpp:20:36: note: in instantiation of template class 'std::__1::unordered_map<TString, int, std::__1::hash<TString>, std::__1::equal_to<TString>,
      std::__1::allocator<std::__1::pair<const TString, int> > >' requested here
  std::unordered_map <TString,int> aMap_2;
                                   ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:3065:29: note: template is declared here
template <class _Tp> struct hash;
                            ^
1 error generated.
make: *** [myMain.o] Error 1

I am working on Mac OSX (Yosemite 10.10.4), using ROOT v6-04-02
(similar problems also occurred on a SL Linux machine with ROOT v5.34/14.)

A minimal example is attached (uncomment line 16 in myMain.cpp to compile w/o errors).

Any advice would be appreciated.
Thanks,

Iftach Sadeh.
minExample.tgz (1.25 KB)

2

Hi,

have a look at the definition of std::unordered_map:

en.cppreference.com/w/cpp/contai … rdered_map

template<
class Key,
class T,
class Hash = std::hash,
class KeyEqual = std::equal_to,
class Allocator = std::allocator< std::pair<const Key, T> >

class unordered_map;

If you use a non-standard type like TString as a key, you have to provide hashing and equal functions to the map.

Alternatively, you could look at TMap of Root, which should work with TObjString.

Cheers,
Jochen

3

Thank you very much, Jochen!

For those who are interested, adding the following to the .hpp file did the trick:

#include <unordered_map>

namespace std {

  template <>
  struct hash<TString> {
    std::size_t operator()(const TString & k) const {
      return std::hash<std::string>()(k.Data());
    };
  };
  
}

Best,
Iftach.

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