Why using pointer of classes

Newbie
1

Dear Experts

I’m new to c++, I saw many people using pointer to define ROOT class member like:

TTree* t = //some Get function
TH2D* h = new TH2D();
TCanvas* c = new Canvas();

instead of

TTree t = //some Get function
TH2D h;

What’s the difference and what advantage will I have if I use pointer to define them?

2

file canvas1.C

void canvas1() {
   TCanvas c;
   TText t (.5,.5,"Hello");
   t.Draw();
}

file canvas2.C

void canvas2() {
   auto c = new TCanvas();
   auto t = new TText(.5,.5,"Hello");
   t->Draw();
}

Then try:

root[0] .x canvas1.C

and

root[0] .x canvas2.C

and you will see the difference.

3

The first one doesn’t give anything on the screen, where the 2nd one gives canvas and text on the canvas…Why?

4

Because with the 1st one, the object is local: created in the macro and deleted when the macro exits. In the 2nd one, the pointer stays alive.

5

The same question was asked 10 years ago: Why do so many use pointers to ROOT objects?

6

Thank you for the explanation, understood that pointer will stay alive after macro exits. What if I write a c++ program and compile with root? Will it be any difference between pointer and object? Like:

int main(){
   TTree* t =
}

and

int main(){
   TTree t;
}
7

Then that’s a C++ question. Normal objects vanish automatically when they are out of scope (basically when exiting { } ) and pointers must be deleted otherwise you get a memory leak.