Reading out std::string from a TTree/TFile

ROOT
1

Hi,

This is related to an earlier topic: “Read TTree from TFile, Fill it again and save to new TFile” ([url]Read TTree from TFile, Fill it again and save to new TFile
One of the branches of the TTree, is actually of type “std::string”.
For simpleness sake, suppose a code like this:

	TTree* tree = new TTree( "mytree", "title" );
	std::string m_chipIDstr;
	tree->Branch("chipIDstr", &m_chipIDstr);
	m_chipIDstr = "A6GH100";
	tree->Fill();

This works fine. I can write the TTree to a file, and look at it with ROOT again and it looks good.
But, when I read in the TFile, apparantly it’s not possible to have a variable of type std::string. If I have a code like this:

	std::string m_chipIDstr;
	TTree* oldtree = (TTree*) hfile_OLD->Get(  "mytree" );
	oldtree->SetBranchAddress("chipIDstr", &m_chipIDstr);

ROOT gives a segmentation fault and tells me:

I can solve this by working with pointers to std::string:

	std::string* m_chipIDstr;
	TTree* oldtree = (TTree*) hfile_OLD->Get(  "mytree" );
	oldtree->SetBranchAddress("chipIDstr", &m_chipIDstr);

and then everything works fine, but I would like to know why this is.
Cheers,

Machiel

2

When reading a TTree, you MUST use:
std::string *m_chipIDstr = 0;
or:
std::string *m_chipIDstr = new std::string;
After you close your ROOT files, you should:
delete m_chipIDstr;