STL question

[Rusty]

Is there any way to use the STL algorithm for_each if my container
contains pointers? For example,

class A
{
int a;
virtual void print (void) {cout << a << endl;}
};
class B: public A
{
int b;
virtual void print (void) {cout << a << "," << b << endl;}
}

I want to have a list that I can call print() on for each member, so I
figured vector <A*>. Is there anyway to get the following with
for_each?

vector <A*>::iterato r i;
for (i=list.begin() ; i<list.end(); ++i) (*i)->print();

Russ

[Jonathan Turkanis]

Re: STL question

"Rusty" <russR-E-M-O-V-Eford@shaw.ca> wrote in message
news:b7tl00l2po j67i28cmesau5da c7ce17ulk@4ax.c om...[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers? For example,
>
> class A
> {
> int a;
> virtual void print (void) {cout << a << endl;}
> };
> class B: public A
> {
> int b;
> virtual void print (void) {cout << a << "," << b << endl;}
> }
>
> I want to have a list that I can call print() on for each member, so[/color]
I[color=blue]
> figured vector <A*>. Is there anyway to get the following with
> for_each?
>[/color]

struct Printer : std::unary_func tion<A*, void> {
void operator() (A* a) const { /* write to cout */ }
}

std::for_each(l ist.begin(), list.end(), Printer());

HTH.

Jonathan

[Jacques Labuschagne]

Re: STL question

Rusty wrote:[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers? For example,
>
> class A
> {
> int a;
> virtual void print (void) {cout << a << endl;}
> };
> class B: public A
> {
> int b;
> virtual void print (void) {cout << a << "," << b << endl;}
> }
>
> I want to have a list that I can call print() on for each member, so I
> figured vector <A*>. Is there anyway to get the following with
> for_each?[/color]

Sure..

struct call_print{
void operator()(A* a){ a->print(); }
};

and then

for_each(v.begi n(), v.end(), call_print());
[color=blue]
>
> vector <A*>::iterato r i;
> for (i=list.begin() ; i<list.end(); ++i) (*i)->print();[/color]

Don't call your vector 'list', there's already a std::list class.
[color=blue]
>
> Russ[/color]

[Daniel T.]

Re: STL question

Rusty <russR-E-M-O-V-Eford@shaw.ca> wrote:
[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers? For example,
>
> class A
> {
> int a;
> virtual void print (void) {cout << a << endl;}
> };
> class B: public A
> {
> int b;
> virtual void print (void) {cout << a << "," << b << endl;}
> }
>
> I want to have a list that I can call print() on for each member, so I
> figured vector <A*>. Is there anyway to get the following with
> for_each?
>
> vector <A*>::iterato r i;
> for (i=list.begin() ; i<list.end(); ++i) (*i)->print();[/color]

for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );

[Jacques Labuschagne]

Re: STL question

Daniel T. wrote:[color=blue][color=green]
>>vector <A*>::iterato r i;
>>for (i=list.begin() ; i<list.end(); ++i) (*i)->print();[/color]
>
>
> for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );[/color]

Hang on, it's a vector of pointers... Surely there's no way of getting
around the extra functor to dereference the pointer.

Jacques

[Jerry Coffin]

Re: STL question

In article <5VCOb.17855$ws .2199935@news02 .tsnz.net>,
jacques@clawshr imp.com says...[color=blue]
> Rusty wrote:[color=green]
> > Is there any way to use the STL algorithm for_each if my container
> > contains pointers? For example,
> >
> > class A
> > {
> > int a;
> > virtual void print (void) {cout << a << endl;}
> > };
> > class B: public A
> > {
> > int b;
> > virtual void print (void) {cout << a << "," << b << endl;}
> > }
> >
> > I want to have a list that I can call print() on for each member, so I
> > figured vector <A*>. Is there anyway to get the following with
> > for_each?[/color]
>
> Sure..
>
> struct call_print{
> void operator()(A* a){ a->print(); }
> };[/color]

Using operator<< to do formatted printing of an object is a widely known
idiom, so it's probably better (at least IMO) to use it to do this job.
I'd change the definitions a little bit so the member functions take the
stream to write to as a parameter instead of always writing to
std::cout, and return the stream they're writing to.

class A
{
// I've defined it to take the ostream as a parameter instead of
// always writing to cout.
virtual std::ostream &print (std::ostream &os) const {return os<<a;}

// since we've defined print to be private, make operator<< a friend
// so it can call print.
friend std::ostream &operator<<(std ::ostream &, A const *);
protected:
// a can't be private if you're going to access it directly in B
int a;
public:
A(int init) : a(init) {}
};

class B: public A
{
int b;
std::ostream &print(std::ost ream &os) const {
return os << a << "," << b;
}
public:
B(int inita, int initb) : A(inita), b(initb) {}
};

and then add the definition of operator<<:

std::ostream &operator<<(std ::ostream &os, A const *a) {
return a->print(os);
}

IMO, std::for_each isn't really the best tool for the job at hand
either. We're copying the contents of the vector to a stream, so
std::copy is really the algorithm for the job at hand:

std::ostream_it erator<A*> out(std::cout, "\n");
std::vector<A*> a;

// code to populate 'a' elided.

std::copy(a.beg in(), a.end(), out);

--
Later,
Jerry.

The universe is a figment of its own imagination.

[Jonathan Turkanis]

Re: STL question


"Jacques Labuschagne" <jacques@clawsh rimp.com> wrote in message
news:0cFOb.1788 2$ws.2203713@ne ws02.tsnz.net.. .[color=blue]
> Daniel T. wrote:[color=green][color=darkred]
> >>vector <A*>::iterato r i;
> >>for (i=list.begin() ; i<list.end(); ++i) (*i)->print();[/color]
> >
> >
> > for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );[/color]
>
> Hang on, it's a vector of pointers... Surely there's no way of[/color]
getting[color=blue]
> around the extra functor to dereference the pointer.
>
> Jacques
>[/color]

mem_fun returns a function object taking a pointer argument, so this
should work. Of course, it only addresses the example, not the general
problem.

Jonathan

[Jacques Labuschagne]

Re: STL question

Jonathan Turkanis wrote:[color=blue]
> mem_fun returns a function object taking a pointer argument, so this
> should work. Of course, it only addresses the example, not the general
> problem.[/color]

Right you are...

Thanks,
Jacques

[Jumbo]

Re: STL question


"Rusty" <russR-E-M-O-V-Eford@shaw.ca> wrote in message
news:b7tl00l2po j67i28cmesau5da c7ce17ulk@4ax.c om...[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers? For example,
>
> class A
> {
> int a;
> virtual void print (void) {cout << a << endl;}
> };
> class B: public A
> {
> int b;
> virtual void print (void) {cout << a << "," << b << endl;}
> }
>
> I want to have a list that I can call print() on for each member, so I
> figured vector <A*>. Is there anyway to get the following with
> for_each?
>
> vector <A*>::iterato r i;
> for (i=list.begin() ; i<list.end(); ++i) (*i)->print();
>
> Russ[/color]

Yes see code below:

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

class Print{
public:
void operator()(cons t int* i)
{
cout<< *i << '\n';
}
};

int main(){
Print print;
vector<int*,all ocator<int> > intArr(3);
int x=1;
int y=2;
int z=3;
intArr[0] = &x;
intArr[1] = &y;
intArr[2] = &z;

for_each( intArr.begin(), intArr.end(), print);

return 0;
}


HTH.

[Daniel T.]

Re: STL question

Jacques Labuschagne <jacques@clawsh rimp.com> wrote:
[color=blue]
> Daniel T. wrote:[color=green][color=darkred]
> >>vector <A*>::iterato r i;
> >>for (i=list.begin() ; i<list.end(); ++i) (*i)->print();[/color]
> >
> >
> > for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );[/color]
>
> Hang on, it's a vector of pointers... Surely there's no way of getting
> around the extra functor to dereference the pointer.[/color]

mem_fun returns a functor that dereferences the pointer.

[Daniel T.]

Re: STL question

"Jonathan Turkanis" <technews@kanga roologic.com> wrote:[color=blue]
> "Jacques Labuschagne" <jacques@clawsh rimp.com> wrote:[color=green]
> > Daniel T. wrote:[color=darkred]
> > >>vector <A*>::iterato r i;
> > >>for (i=list.begin() ; i<list.end(); ++i) (*i)->print();
> > >
> > >
> > > for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );[/color]
> >
> > Hang on, it's a vector of pointers... Surely there's no way of[/color]
> getting[color=green]
> > around the extra functor to dereference the pointer.
> >
> > Jacques
> >[/color]
>
> mem_fun returns a function object taking a pointer argument, so this
> should work.[/color]

Of course it should work, this is exactly the type of problem that
mem_fun was designed to solve.

[color=blue]
> Of course, it only addresses the example, not the general
> problem.[/color]

It does address the general problem which was (from the OP):[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers?[/color]

The answer is, "Yes, one way is to use mem_fun."

[Jonathan Turkanis]

Re: STL question


"Daniel T." <postmaster@eat hlink.net> wrote in message
news:postmaster-D6484F.23331518 012004@news02.e ast.earthlink.n et...[color=blue]
> "Jonathan Turkanis" <technews@kanga roologic.com> wrote:[color=green]
> > "Jacques Labuschagne" <jacques@clawsh rimp.com> wrote:[color=darkred]
> > > Daniel T. wrote:
> > > >>vector <A*>::iterato r i;
> > > >>for (i=list.begin() ; i<list.end(); ++i) (*i)->print();
> > > >
> > > >
> > > > for_each( vec.begin(), vec.end(), mem_fun( &A::print ) );
> > >
> > > Hang on, it's a vector of pointers... Surely there's no way of[/color]
> > getting[color=darkred]
> > > around the extra functor to dereference the pointer.
> > >
> > > Jacques
> > >[/color]
> >
> > mem_fun returns a function object taking a pointer argument, so[/color][/color]
this[color=blue][color=green]
> > should work.[/color]
>
> Of course it should work, this is exactly the type of problem that
> mem_fun was designed to solve.
>[/color]

Would you feel better if I said 'does' instead of 'should'? I know it
works; it was a normative 'should'.
[color=blue]
>[color=green]
> > Of course, it only addresses the example, not the general
> > problem.[/color]
>
> It does address the general problem which was (from the OP):[color=green]
> > Is there any way to use the STL algorithm for_each if my container
> > contains pointers?[/color]
>[/color]

I don't we disagree about anything important. The more general problem
is how to iterate over a container of pointers when the operation you
wish to perform doesn't happen to be a member function.

Jonathan

[Martijn Lievaart]

Re: STL question

On Sun, 18 Jan 2004 21:08:56 +0000, Rusty wrote:
[color=blue]
> Is there any way to use the STL algorithm for_each if my container
> contains pointers? For example,
>
> class A
> {
> int a;
> virtual void print (void) {cout << a << endl;}
> };
> class B: public A
> {
> int b;
> virtual void print (void) {cout << a << "," << b << endl;}
> }
>
> I want to have a list that I can call print() on for each member, so I
> figured vector <A*>. Is there anyway to get the following with
> for_each?
>
> vector <A*>::iterato r i;
> for (i=list.begin() ; i<list.end(); ++i) (*i)->print();
>
> Russ[/color]

vector <A*>::iterato r i;
for_each (list.begin(), list.end(), mem_fun(&A::pri nt));

HTH,
M4

[Daniel T.]

Re: STL question

"Jonathan Turkanis" <technews@kanga roologic.com> wrote:
[color=blue]
> The more general problem
> is how to iterate over a container of pointers when the operation you
> wish to perform doesn't happen to be a member function.[/color]

That's easy too.

for_each( vec.begin(), vec.end(), myFunc );

for_each works fine with function pointers.

If the function happens to be in a different object. IE

class A { };
class B {
public:
void func( A* a );
};

vector<A*> vec;
B b;

Then one simply does this:

for_each( vec.begin(), vec.end(), bind1st( mem_fun(&B::fun c), &b ) );

[Jonathan Turkanis]

Re: STL question

"Daniel T." <postmaster@eat hlink.net> wrote in message
news:postmaster-741053.08062119 012004@news02.e ast.earthlink.n et...[color=blue]
> "Jonathan Turkanis" <technews@kanga roologic.com> wrote:
>[color=green]
> > The more general problem
> > is how to iterate over a container of pointers when the operation[/color][/color]
you[color=blue][color=green]
> > wish to perform doesn't happen to be a member function.[/color]
>
> That's easy too.
>
> for_each( vec.begin(), vec.end(), myFunc );
>
> for_each works fine with function pointers.
>[/color]

This is what I mentioned in my original post.

I know how to use the STL. What is your point?

Jonathan