Another neophyte question

[Harry]

What does an asterisk after a class name mean? (see the two lines of code)

TFile trigfile = new TFile(“trigger.root”,“recreate”);
TTree trig = new TTree(“trig”,“trig”);

thanks,
harry

[brun]

Harry,

I suggest to read some C++ tutorials. For example
www-pnp.physics.ox.ac.uk/~west/w … s/cpp.html
cplusplus.com/doc/tutorial/
cplusplus.com/doc/tutorial/tut3-3.html

Rene

[Harry]

i read such things very carefully and they don’t have an asterisk AFTER a class name, I suspect that it is a ROOT nomenclature but it is not explained in the manual, I do know what a pointer is.

[brun]

This has nothing todo with the ROOT nomanclature. It is just the way
a C++ pointer is declared.

Rene

[tpochep]

Hello.

@Harry

i read such things very carefully and they don’t have an asterisk AFTER a class name, I suspect that it is a ROOT nomenclature but it is not explained in the manual, I do know what a pointer is.

VERY CAREFULLY ???
Link number 3 from Rene’s list contains:

Section 3.3
Pointers

cplusplus.com/doc/tutorial/tut3-3.html
and for example:

Declaring variables of type pointer
Due to the ability of a pointer to directly reference the value that it point to, it becomes necessary to specify which data type a pointer points to when declaring it. It is not the same to point to a char as it is to point to an int or a float type.
Therefore, the declaration of pointers follows this form:

type * pointer_name;
where type is the type of data pointed, not the type of the pointer itself. For example:

int * number;
char * character;
float * greatnumber;

they are three declarations of pointers. Each one points to a different data type, but the three are pointers and in fact the three occupy the same amount of space in memory (the size of a pointer depends on the operating system), but the data to which they point do not occupy the same amount of space nor are of the same type, one is int, another one is char and the other one float.
I emphasize that the asterisk () that we use when declaring a pointer means only that it is a pointer, and should not be confused with the reference operator that we have seen a bit earlier which is also written with an asterisk (). They are simply two different tasks represented with the same sign.

[tpochep]

cplusplus.com/doc/tutorial/tut4-1.html

Pointers to classes
It is perfectly valid to create pointers pointing to classes, in order to do that we must simply consider that once declared, the class becomes a valid type, so use the class name as the type for the pointer. For example:
CRectangle * prect;
is a pointer to an object of class CRectangle.
As it happens with data structures, to refer directly to a member of an object pointed by a pointer you should use operator ->. Here is an example with some possible combinations:

// pointer to classes example
#include <iostream.h>

class CRectangle {
int width, height;
public:
void set_values (int, int);
int area (void) {return (width * height);}
};

void CRectangle::set_values (int a, int b) {
width = a;
height = b;
}

int main () {
CRectangle a, *b, *c;
CRectangle * d = new CRectangle[2];
b= new CRectangle;
c= &a;
a.set_values (1,2);
b->set_values (3,4);
d->set_values (5,6);
d[1].set_values (7,8);
cout << "a area: " << a.area() << endl;
cout << "*b area: " << b->area() << endl;
cout << "*c area: " << c->area() << endl;
cout << "d[0] area: " << d[0].area() << endl;
cout << "d[1] area: " << d[1].area() << endl;
return 0;
}

[Harry]

rene, we shouldn’t be upset with each other, look at the lines i copied carefully. it has “TTree*” with no space after TTree, this is new to me, I was worried that it was something special to root, the examples you have quoted do not have such an instance, i have looked very carefully at various resources and have not seen a “*” right after a class name without a space. Maybe my fault is to scrutinize too carefully, I want to make sure that I use Root properly as I think it’s very nice of all of you to create it. I used PAW extensively and was one of the first ntuple users in my group at bnl physics. I have been away from the field and am bootstrapping myself so little trivialities trip me up.

harry

[Harry]

sorry, tpochep quoted code, not Rene. and thank you, rene, simply telling me that it is a C++ artifact and not ROOT is a helpful answer.
harry

[brun]

Harry,

There are two styles to declare pointers

Myclass* obj; Myclass *obj;

Most of the time in C++ code, you find the first style.
In ROOT code, you mostsly find the second style. The reason is that we see many mistakes of the type

where the user in fact was intending to declare

Rene

[Harry]

now I understand the problem, believe it or not, all of the code I’ve seen has the second style,

Myclass *obj;

If you are accustomed to the first style (which I agree is better) then my question appears as if I did no homework, which I always do so as to not waste any one’s time. I only ask when I can’t find something.

thsnk you
harry

[tpochep]

Hello, Harry.

Yesterday, after I quoted this text and before your answer I realized your problem From compiler’s POV there are tokens:

class_name
*
ptr_name

new
class_name
(
etc…

So it does not matter, there is *. You can declare

Type ptr
Type ptr
even
Type*ptr