Save a std::map<int,TH1D> to a TFile

Newbie
1

I am writing a program in C++ and have a std::map<int,TH1D> that I would like to write to my TFile. I understand that it is possible to write “simple” STL containers such as std::vector<int>; however, I think the TH1D is complicating things. Thanks in advance for the help!

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ROOT Version: Not Provided
Platform: Not Provided
Compiler: Not Provided

2

For further information, the runtime error given by my program (compiles just fine) is "Error in <TFile::WriteObjectAny>: The class requested (map<int,TH1D>) for the key name "name" is an instance of an stl collection and does not have a compiled CollectionProxy. Please generate the dictionary for this collection (map<int,TH1D>). No data will be written."

I have tried adding a line that says

gInterpreter->GenerateDictionary("map<int,TH1D>", "map");

but that gives me several compile time errors

3

Hi,

as a work around, histograms can be saved to a TTree :slight_smile:

I can reproduce the error. Wrapping the map inside a class like the following do not help. Following the example in root/tutorial/tree/dictionary do not work either.

#include <map>
#include <memory>
#include <TH1D.h>

class myclass{
  public: 
    std::map<int, std::unique_ptr<TH1D> > m; 
};

After a quick look in the forum, there are recent posts regarding the function Generate Dictionary, and it seems to be intended for testing purposes.

Best,
Alvaro

4

Hi Alvaro! Thanks for the reply and the time you spent checking this out!

I thought about a TTree as well; it’s a good idea. However, I was curious how to generate the dictionary if I wanted to avoid the TTree for whatever reason (or some other alternative which would allow me to save a container [like a map] directly).

5

Hi,

I tried to generate dictionary for the following class but it does work only if the data is saved into a TTree. If it is saved as object into a TFile, it can not be retrieved.

class myclass{
public: 
  int i; 
  TH1D* h; 
};

I hope you find a way :slight_smile:

Best,
Alvaro

6

I appreciate the time you put into this. If I ever manage to figure it out, I’ll try to post a solution here.

7

gInterpreter->GenerateDictionary("map<int,TH1D>", "map"); seems to work just fine. What’s the error you see?

8

I’m not sure what the issue was before. I just got it to compile with no errors. However, I still get runtime errors. Essentially the code is:

#include<map>
#include "TFile.h"
#include "TH1D.h"
#include "TInterpreter.h"

int main()
{
  TFile f("output.root");
  map<int,TH1D> m;
  /* fill the map */
  gInterpreter->GenerateDictionary("map<int,TH1D>", "map");
  f.WriteObject(&m, "name");
  return 0;
}

And the error I am getting is:

Warning in cling::IncrementalParser::CheckABICompatibility():
  Possible C++ standard library mismatch, compiled with __GLIBCXX__ '20150623'
  Extraction of runtime standard library version was: '20200408'
Error in <TFile::WriteObjectAny>: The class requested (map<int,TH1D>) for the key name "name" is an instance of an stl collection and does not have a compiled CollectionProxy. Please generate the dictionary for this collection (map<int,TH1D>). No data will be written.

The program finishes running after the error message and the exit code is 0

Edit:
I originally did not include this in the post, but since it may now be relevant:
ROOT Version: 6.24/08
Platform: Linux x86_64
Compiler: c++ (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2)

9

This indicates that there is a mismatch between the way your verison of ROOT was build and the available compiler.

10

These are the relevant lines in my makefile:

EXE = ./name
SRC = ./name.cpp

CXX = $(shell root-config --cxx)
CXXFLAG = $(shell root-config --cflags --libs | sed -e 's.-std=c++11.-std=c++14.')

$(EXE): $(SRC)
$(CXX) $(SRC) $(CXXFLAG) -o $(EXE)

Is the issue that I am using c++14 instead of c++11? My program uses features from the c++14 standard and I don’t have permissions to reinstall ROOT on the machine I am compiling and running the code on. Does this mean I am out of luck?

11

As a general rule, you need to compile your C++ programs using the same C++ standard version that was used to build the ROOT installation on your system (root-config --cflags | grep std).