Signal/Slot ; a error message when using gPad firstime


Root Version: 5.01
OS: Linux RHEL 4

I am trying to adapt “exec3.C”, which is an example application of Signal/Slot found in ROOT tutorials.

The problem is that when I call “gPad” for the first time, an error/warning message is given:

“error: non class, struct, union, object …”

The simplest example that shows the symtomp has only 1 extra line (gPad->GetUniqueID():wink: added to 'exec3.C". The example is given below.

I thought gPad is a global variable and it should be known inside the Slot method. Strangely, “gPad->…” seems to return valid results (I think); my adapted application seems to run alright.

Please advise what I can do to avoid the error message. Thank you in advance.


void exec4()
// Example of using signal/slot in TCanvas/TPad to get feedback
// about processed events. Note that slots can be either functions
// or class methods. Compare this with tutorials exec1.C and exec2.C.

TH1F h = new TH1F(“h”,“h”,100,-3,3);
TCanvas c1=new TCanvas(“c1”);
)", 0, “”,

void exec3event(Int_t event, Int_t x, Int_t y, TObject *selected)
TCanvas *c = (TCanvas *) gTQSender;
printf(“Canvas %s: event=%d, x=%d, y=%d, selected=%s\n”, c->GetName(),
event, x, y, selected->IsA()->GetName());
gPad->GetUniqueID(); // This call causes an error/warning

Hi Tim,

It is a CINT problem…
A solution (?) is to add gPad->GetUniqueID(); after
c1->Connect(“ProcessedEvent(Int_t,Int_t,Int_t,TObject*)”, 0, “”, “exec3event(Int_t,Int_t,Int_t,TObject*)”);
to make gPad known by CINT.


Thanks Bertrand,

Your sugesstion works.

Tim :smiley: