String to const char* for FindObject

ROOT
1

Dear root experts,

I am trying to convert a string (or TString) to const char* in order to use it as argument for the FindObject function.

I have tried both c_str() and TString.Data() but in both cases FindObject returns a null pointer (cannot find the relevant histogram)

Any idea about it?

Thank you,
Panagiotis

2

Try with “FindObjectAny”. If it doesn’t succeed, try: gDirectory->ls();

3

Hi @Wile_E_Coyote ,

it didnt succeed… I can get the histogram by setting its name directly in FindObject() (or FindObjectAny), but not with the converted string…

Any other idea?

thank you,
Panagiotis

4

Make sure that your std::string (or TString) contains what you expect.

5

I guess that by directly you are referring to specifying a string literal as in FindObject("name"). Both, c_str() member function of std::string and TString::Data() should return a C string (NUL terminated), which shouldn’t have any difference w.r.t. a string literal.

Therefore, as @Wile_E_Coyote suggested, make sure that the original std::string or TString contains what you expect.

Cheers,
J.

6

Hi @Wile_E_Coyote @jalopezg ,

thank you for you replies!

So, the problem may be a bit different. I tried to get the histogram with Get instead of FindObject. It worked for case 1 and 2, but not for case 3 which is the one that i want to use. It looks like coming from the addition of 2 strings. This is the relevant part of the code

ifstream SysTXT("Sys.txt");
string str;

int I=-1;
while (std::getline(SysTXT, str))
{

string s="Systematics/hhttbbVBFl1cvv0p5cv1_SysTAUS_TRUEHADTAU_EFF_RNNID_HIGHPT__1down";
const char * c = s.c_str();

string s1="Systematics/hhttbbVBFl1cvv0p5cv1_";
string s12=s1+str;
const char * c2 = s12.c_str();
cout<<"Sys : "<<c2<<endl;

TH1F* H1    = (TH1F*)file->Get("Systematics/hhttbbVBFl1cvv0p5cv1_SysTAUS_TRUEHADTAU_EFF_RNNID_HIGHPT__1down");
TH1F* H2    = (TH1F*)file->Get(c);
TH1F* H3    = (TH1F*)file->Get(c2);

cout<<H1->GetName()<<endl;
cout<<H2->GetName()<<endl;
cout<<H3->GetName()<<endl;

This is the terminal output :

Sys : Systematics/hhttbbVBFl1cvv0p5cv1_SysTAUS_TRUEHADTAU_EFF_RNNID_HIGHPT__1down
hhttbbVBFl1cvv0p5cv1_SysTAUS_TRUEHADTAU_EFF_RNNID_HIGHPT__1down
hhttbbVBFl1cvv0p5cv1_SysTAUS_TRUEHADTAU_EFF_RNNID_HIGHPT__1down
Error in : Trying to dereference null pointer or trying to call routine taking non-null arguments.
Execution of your code was aborted.
In file included from input_line_8:1:
/afs/cern.ch/work/p/pbellos/HH_WS/WSMaker_HH_bbtautau/inputs/Rew.cpp:63:11: warning: null passed to a callee that requires a non-null argument [-Wnonnull]
cout<< H3->GetName()<<endl;

I also checked for aditional space character at the end of the “str” string in the txt file. Any other suggestion?

Thank you,
Panagiotis

7

It could be that you have some “invisible” character somewhere in your input text file.
Try:

std::cout << strlen(c) << std::endl;
std::cout << strlen(c2) << std::endl;
std::cout << strcmp(c, c2) << std::endl;
8

Hi @Wile_E_Coyote,

yes, that was the problem. It is running now!

thank you,
Panagiotis

9

@panagiotis_bellos Also, for future reference (and similar issues), if a string is coming from a text file, you might also need to ensure that the file was not written using a multi-byte encoding.

That might be especially confusing, as your terminal emulator might still render the “expected” character sequence, while the null-terminated C string that you are passing contains extra bytes.

1 Like
10

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