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I have written C++ code for some time now using both pointers and
references.

I was asked what the point was in using pointers and could not give a
short an explanatory example.

I would say that you use a pointer when data are updated across function
calls.

But does anyone have an idea to a minimal example where its impossible
to avoid pointer?

[8.6] When should I use references, and when should I use pointers?
http://www.parashift.com/c++-faq-lit...s.html#faq-8.6

wrote:
> [8.6] When should I use references, and when should I use pointers?
> http://www.parashift.com/c++-faq-lit...s.html#faq-8.6

Thanks but I was looking for some code sample that illustrates the
differences. In the below code 'test' takes a pointer to a list. But I
never use 'new' to make it work. Why would it ever be necessary to use
'new' when I can just pass the address to the list to 'test' using '&'?


#include<iostream>
#include<list>
void test(std::list<int>* l) {
l->push_back(555);
}

int main() {

std::list<int> l;
std::list<int>::iterator list_it;
l.push_back(1);
l.push_back(2);
l.push_back(3);

for (list_it = l.begin(); list_it != l.end(); list_it++) {
printf("%d\n",*list_it);
}
std::cout<<std::endl;
test(&l);
for (list_it = l.begin(); list_it != l.end(); list_it++) {
printf("%d\n",*list_it);
}
return 0;
}

On Feb 13, 10:57*pm, saneman <y...@dd.com> wrote:

> But does anyone have an idea to a minimal example where its impossible
> to avoid pointer?

Think about polymorphism in C++.

Best Regards,
Szabolcs

On 2008-02-13 22:57, saneman wrote:
> I have written C++ code for some time now using both pointers and
> references.
>
> I was asked what the point was in using pointers and could not give a
> short an explanatory example.
>
> I would say that you use a pointer when data are updated across function
> calls.
>
> But does anyone have an idea to a minimal example where its impossible
> to avoid pointer?

In any situation where you need to be able to "reseat" the reference. On
example would be walking through the nodes in a linked list without
using recursion, something like:

node* n;
while (n != 0)
{
// do something with n
n = n->next;
}

--
Erik Wikström

saneman <> wrote:
> wrote:

> > [8.6] When should I use references, and when should I use pointers?
> > http://www.parashift.com/c++-faq-lit...s.html#faq-8.6
>
> Thanks but I was looking for some code sample that illustrates the
> differences. In the below code 'test' takes a pointer to a list. But I
> never use 'new' to make it work. Why would it ever be necessary to use
> 'new' when I can just pass the address to the list to 'test' using '&'?

That's a change of subject. First you ask "why pointers?", now you are
asking "why new?".

What is it that you really want to ask?

saneman wrote:
> I have written C++ code for some time now using both pointers and
> references.
>
> I was asked what the point was in using pointers and could not give a
> short an explanatory example.
>
> I would say that you use a pointer when data are updated across function
> calls.
>
> But does anyone have an idea to a minimal example where its impossible
> to avoid pointer?

You can have a vector of pointers, but not a vector of references.

As a particular case of the "reseating" issue already noted by Erik,
consider a class that may (according to your design philosophy) have a
member either a reference member or a pointer member. If the target
object of the pointer or reference (the referee?) is not yet available
when the referring (pointy?) object is constructed, then a reference is
out of the question. Code like the following even flags a compile-time
error from g++:

struct S {
S() { }
void set_i(int*);
private:
int& i_;
};

g++ -ansi -pedantic -Wall -c -o main.o main.cc
main.cc: In constructor 'S::S()':
main.cc:2: error: uninitialized reference member 'S::i_'
make: *** [main.o] Error 1

A pointer, by contrast, can be temporarily initialized to a reasonable
default value, and later assigned a more permanent value.

A pointer is an object in its own right, with its very own address in
memory. A reference is just a way of referring to an existing object,
and may or may not require run-time support from pointer-like entities.
You can have a reference to a pointer, but not a reference to a reference.

On Feb 13, 5:11 pm, saneman <y...@dd.com> wrote:
> past...@gmail.com wrote:
> > [8.6] When should I use references, and when should I use pointers?
> >http://www.parashift.com/c++-faq-lit...s.html#faq-8.6
>
> Thanks but I was looking for some code sample that illustrates the
> differences. In the below code 'test' takes a pointer to a list. But I
> never use 'new' to make it work. Why would it ever be necessary to use
> 'new' when I can just pass the address to the list to 'test' using '&'?

What makes you think that pointers and new are related in any way?
To get back on topic, isn't the code below an example where a
reference is preferred over a pointer?

>
> #include<iostream>
> #include<list>
> void test(std::list<int>* l) {
> l->push_back(555);
>
> }

Note the difference between:

std::list<int>* l; // pointer can be reseated
const std::list<int>* c_l; // same, any const list will do
std::list<int>* const c_l; // const pointer, can't be reseated, list
is mutable
const std::list<int>* const c_l_c;

Thats is, the above function should really have the following
signature if you absolutely must use a pointer:

void test(std::list<int>* const l) { ... } // pointer can't be
reseated

Which then makes the following better. Its intentions are clear to the
user:

void test(std::list<int>& l) { ... }

>
> int main() {
>
> std::list<int> l;
> std::list<int>::iterator list_it;
> l.push_back(1);
> l.push_back(2);
> l.push_back(3);
>
> for (list_it = l.begin(); list_it != l.end(); list_it++) {
> printf("%d\n",*list_it);
> }
> std::cout<<std::endl;
> test(&l);
> for (list_it = l.begin(); list_it != l.end(); list_it++) {
> printf("%d\n",*list_it);
> }
> return 0;
>
> }

On Feb 13, 5:24 pm, Szabolcs Ferenczi <szabolcs.feren...@gmail.com>
wrote:
> On Feb 13, 10:57 pm, saneman <y...@dd.com> wrote:
>
> > But does anyone have an idea to a minimal example where its impossible
> > to avoid pointer?
>
> Think about polymorphism in C++.
>
> Best Regards,
> Szabolcs

Polymorphism works quite nicely with references too.

Daniel T. wrote:
> saneman <> wrote:
>> wrote:
>
>>> [8.6] When should I use references, and when should I use pointers?
>>> http://www.parashift.com/c++-faq-lit...s.html#faq-8.6
>> Thanks but I was looking for some code sample that illustrates the
>> differences. In the below code 'test' takes a pointer to a list. But I
>> never use 'new' to make it work. Why would it ever be necessary to use
>> 'new' when I can just pass the address to the list to 'test' using '&'?
>
> That's a change of subject. First you ask "why pointers?", now you are
> asking "why new?".
>
> What is it that you really want to ask?

Sorry for the confusion. I would like an example where its impossible to
avoid use 'new' (dynamic allocation).

The reason is that I often use pointers and references but seldom the
keyword 'new'.