Bug in rootcling?

[peter.skensved]

---

_ROOT Version:6.20 / 6.22
_Platform:CentOS8.2
_Compiler:gcc8.3.1

---

Very simple bit of code creating a class

#include "TObject.h"
#include "TVector3.h"

class testData : public TObject  {

 public:
	testData() { }; 
	virtual ~testData() { };

 private :
	std::vector<TVector3> fDirection[ 255 ];

	ClassDef( testData, 1 );

};

If I try to compile the dictionary created by rootcling from the above with either 6.20 or 6.22 with gcc8 I get loads of errors like :

Dict.C: In member function ‘virtual void testData::Streamer(TBuffer&)’:
Dict.C:125:27: error: expected identifier before numeric constant
          vector<TVector3>[255] &R__stl =  fDirection[R__l];
                           ^~~
Dict.C:125:27: error: expected ‘]’ before numeric constant
          vector<TVector3>[255] &R__stl =  fDirection[R__l];
                           ^~~
                           ]
Dict.C:125:26: warning: structured bindings only available with -std=c++17 or -std=gnu++17
          vector<TVector3>[255] &R__stl =  fDirection[R__l];
                          ^
Dict.C:125:26: error: structured binding declaration cannot have type ‘std::vector<TVector3>’
Dict.C:125:26: note: type must be cv-qualified ‘auto’ or reference to cv-qualified ‘auto’
Dict.C:125:26: error: empty structured binding declaration
Dict.C:125:32: error: expected initializer before ‘&’ token
          vector<TVector3>[255] &R__stl =  fDirection[R__l];
                                ^
.....

The same code compiles fine with ROOT5 on say CentOS6.10 ( gcc4.4.7 ) . The Dict.C file is attached below ( I hope … )

Dict.C (10.0 KB)

[pcanal]

Do you really mean to create an array of 255 distinct std::vector objects (each containing a variable number of TVector3 objects) ?

If this is the case, consider using:

std::vector<std::vector<TVector3>> { 255 };  // Initialization via the curly bracket to reserve slot for 255 vector of TVector3

[peter.skensved]

We want a vector with 255 individual TVector3’s in it - yes . gcc accepts the syntax and does the right thing ( confirmed in gdb ) . ROOT5 creates a dictionary that compiles . ROOT6 also makes a dictionary without errors but gcc won’t compile it. So to me it looks like cling is mangling / misinterpreting the construct ( but cint doesn’t )

                      peter

[peter.skensved]

Two additional comments :

1. If I compare the code produced by cling ( ROOT 6 ) to that made by cint ( ROOT5 ) the only difference is the “extra” [255]" in say

vector<TVector3>[255] &R__stl =  fDirection[R__l];

2. Removing any “[255]” by hand from the cling output makes things work again with ROOT6

So - this looks like a bug in cling

[pcanal]

@peter.skensved

We want a vector with 255 individual TVector3’s in it - yes .

This is not what the code provided does. It creates 255 vectors which an unspecified number of TVector3 inside each of those vectors. You probably meant to use:

std::vector<TVector3> { 255 };  

which create a single vector with 255 slots for TVector3 objects.

ROOT5 creates a dictionary that compiles .

It does compiles but it does not “work”. The result is that only the first of the 255 vectors is stored.

More importantly I noticed that your dictionary is using the “very old” I/O streaming technique (that generate a Streamer function). That 2 main weakness of this old technique is poor/broken support for STL container and no support for splitting or I/O customization rules.

Switching to the new StreamerInfo based I/O streaming technique will remove that code from the dictionary entirely. To do so, in your LinkDef file add a trailing + to the dictionary request:

#pragma link C++ class testData+;

Cheers,
Philippe.