When should you declare variables as pointers, and when shouldn't you?

ROOT
1

I’m trying to understand under what circumstances a variable should be declared as a pointer. For example, what would guide a programmer to choose between the following two ways of declaring a histogram?

TH1D *h = new TH1D("h","h",10,0,10);
TH1D h("h","h",10,0,10);

I understand the difference in how to use them, at least at a basic level (such as using -> vs .), but I don’t understand why I would choose one method of initialization over another.

Thanks!

2
3

Well, sadly with ROOT there’s a bit more to it:

Many ROOT 6 objects prefer heap allocation: new and pointers, due to ROOT’s traditional memory management (e.g. the file owning the histograms). We are moving away from that; all of ROOT’s new and future classes (RDataFrame, RHist, RCanvas, RNtuple etc) happily live on the stack!

Cheers, Axel.

4

Thank you, this is the context that I was looking for! Will ROOT’s old classes eventually be updated, or will they remain as they are?

5

This is great information, thank you.

6

We will not change the existing ROOT classes that fundamentally - most existing code would be broken if we change e.g. the ownership scheme for TH1.

7

@eggsAndCoffee given that this is relevant in general, would you be okay with moving this into the #root section?

8

Hi @Axel, yes I would be okay with moving this topic. Is that something that I can do, or do you need to do it?

9

Thanks, it’s now moved!

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