Why should I pass a pointer to a function?

Question

I decided to retry to learn C++ by my own and now I have a question regarding pointers. I understand what pointers do, but can somebody explain the difference between these two pieces if code?

I have a structure student:

struct student {
    char* name;
    int age;
    int grade;
};

and two ways of outputting it's data:

void printData(student p) {
    cout << p.name << endl;
    cout << p.age << endl;
    cout << p.grade << endl;
}

int main() {
    student ST = { "Dave",20,81 };
    printData(ST);
    return 0;
}

and:

void printData(student* p) {
    cout << p->name << endl;
    cout << p->age << endl;
    cout << p->grade << endl;
}

int main() {
    student ST = { "Dave",20,81 };
    printData(&ST);
    return 0;
}

The question is: why should I stick with pointers if I could just pass a student object to the printData function?

For the various cases where you want the function to access the actual `ST` variable, and not a copy of it. — Quentin, Aug 31 '15 at 11:29
First, you should use references, not pointers. Second, the `(student p)` syntax copies the original object and passes the copy to the function resulting in a lot of waste — Panagiotis Kanavos, Aug 31 '15 at 11:30
"so I decided to retry to learn c++ by my own" - I suggest following a [good book](http://stackoverflow.com/q/388242/1782465) during such endeavours. — Angew is no longer proud of SO, Aug 31 '15 at 11:31
You really really should be careful about copying things with pointers. Not to mention as you don't alter the parameter in printData, you should make it a const pointer — Tom Tanner, Aug 31 '15 at 11:31
@PYPL I haven't actually read Stroustrup's book, so I cannot comment, but the guy's pretty smart and actually invented the language, so I would assume his book(s) to be good. I just posted the comment as a general "here's a list of good books maintained by the C++ SO community, you might want to pick up one from there." — Angew is no longer proud of SO, Aug 31 '15 at 11:48

score 0 · Answer 1 · edited May 23 '17 at 12:22

Because it is cheaper. Passing by value creates a new copy. Passing a pointer does not (passing by ref would be a bit cleaner I think). It might not matter in this case because your student has few fields. However, if you have larger objects or a lot of them, then you will start noticing quite a difference in performance. For more information see: What's the difference between passing by reference vs. passing by value?

Why should I pass a pointer to a function?

1 Answers1