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Hello,

I would like to ask how come the design of C++ includes
std:air. First of all I don't think many programmers
would use it. For starters, what the first and second
members are depends on what you are using the pair
for. For instance if I am using coordinates in two
dimensional space then I like to use x and y. So
I might as well define my own struct with x and
y members in it and create a constructor so
that I can easily instantiate pairs.

I wonder if there is a way to create a pair class
using std:air but typedef its first and second
to x and y using C++. The only way I can think
of is to subclass std:air<,>.

Suggestions and reccomendation on the best
practices and conventions for using std:air<,>
are most welcome.

Thanks,

Neil

Here is my way of making use of std:air<class T1, class T2>:

#include <iostream>
#include <utility>

template<class T1, class T2>
class Coordinate: public std:air<T1, T2> {
public:
Coordinate(T1 x, T2 y): std:air<T1, T2>(x, y), x(first), y(second) { }
T1 &x;
T2 &y;
};

int main() {
Coordinate<int, int> foo(1, 2);
std::cout << "x = " << foo.x << "\ny = " << foo.y << std::endl;
}

> Suggestions and reccomendation on the best
> practices and conventions for using std:air<,>
> are most welcome.
>
> Thanks,
>
> Neil
>
>

> I would like to ask how come the design of C++ includes
> std:air.

I think primarly for std::map and the map-like containers.

>First of all I don't think many programmers
> would use it.

Depends on the programmer, I think. I use it quite
often.

>For starters, what the first and second
> members are depends on what you are using the pair
> for. For instance if I am using coordinates in two
> dimensional space then I like to use x and y. So
> I might as well define my own struct with x and
> y members in it and create a constructor so
> that I can easily instantiate pairs.

Yes. std:air<> was not created specifically to replace
every ud structs representing two entities.

> I wonder if there is a way to create a pair class
> using std:air but typedef its first and second
> to x and y using C++. The only way I can think
> of is to subclass std:air<,>.

That would be one way if you want your struct to be
compatible with std:air<>. If not, make it
a private member or, as you said, create a new
struct.

> Suggestions and reccomendation on the best
> practices and conventions for using std:air<,>
> are most welcome.

There are none, imho.


Jonathan

Neil Zanella wrote:
>
> Hello,
>
> I would like to ask how come the design of C++ includes
> std:air. First of all I don't think many programmers
> would use it.


I generally use std::make_pair().




Brian Rodenborn

Neil Zanella wrote:
>
> Hello,
>
> I would like to ask how come the design of C++ includes
> std:air. First of all I don't think many programmers
> would use it. For starters, what the first and second
> members are depends on what you are using the pair
> for. For instance if I am using coordinates in two
> dimensional space then I like to use x and y. So
> I might as well define my own struct with x and
> y members in it and create a constructor so
> that I can easily instantiate pairs.
>
> I wonder if there is a way to create a pair class
> using std:air but typedef its first and second
> to x and y using C++. The only way I can think
> of is to subclass std:air<,>.
>
> Suggestions and reccomendation on the best
> practices and conventions for using std:air<,>
> are most welcome.

I think you can start by doing something like

template <typename T>
class Point : public std:air<T,T>
{
public:
T& x,y;
Point(const T& a, const T& b) : std:air<T,T>(a,b), x(first),
y(second) {}
/* ... */
};

This gives you more of a pointy interface while also modeling a
container.

/david

--
Andre, a simple peasant, had only one thing on his mind as he crept
along the East wall: 'Andre, creep... Andre, creep... Andre, creep.'
-- unknown

Neil Zanella wrote:
> Here is my way of making use of std:air<class T1, class T2>:
>
> #include <iostream>
> #include <utility>
>
> template<class T1, class T2>
> class Coordinate: public std:air<T1, T2> {
> public:
> Coordinate(T1 x, T2 y): std:air<T1, T2>(x, y), x(first),
> y(second) { } T1 &x;
> T2 &y;
> };

The above class cannot be assigned, or copy constructed. Plus: do not use
inheritance when composition suffices:

template<class T1, class T2>
struct Coordinate {
Coordinate():
store(T1(),T2()), x(store.first), y(store.second) { }
Coordinate(T1 px, T2 py):
store(px,py), x(store.first), y(store.second) { }
Coordinate(Coordinate const &o):
store(o.store), x(store.first), y(store.second) { }
Coordinate operator=(Coordinate const &o)
{ store = o.store; }
T1 &x;
T2 &y;
private:
std:air<T1, T2> store;
};

Also note that the above class will be bigger than the pair. In case of
int, on a usual architecture it will be twice as big. So IMHO you are
better off having accessor functions - if you must access the elements from
the outside.

template<class T1, class T2>
struct Coordinate {
Coordinate(): store(T1(),T2()) { }
Coordinate(T1 px, T2 py): store(px,py) { }
Coordinate(Coordinate const &o): store(o.store) { }
Coordinate operator=(Coordinate const &o) { store = o.store; }
T1 &x() {return store.first; }
T2 &y() {return store.second; }
private:
std:air<T1, T2> store;
};

But to be honest I would just leave std:air out of this completely and
make a little template struct of my own, with the right names.

--
WW aka Attila

"Neil Zanella" <> wrote in message
newsine.LNX.4.44.0310031537040.23774-...
>
> Hello,
>
> I would like to ask how come the design of C++ includes
> std:air.

It's useful for some things. If you don't find it
useful, don't use it.

Also note that the standard container types 'std::map'
and 'std::multimap' use type 'std:air' objects to represent
their elements. If you use a map or multimap, you're
(at least indirectly) using 'std:air' objects.
Also e.g. one of the overloads of 'std::map::insert()'
returns a 'std:air' object (not an element value,
but a pair of values, the first of which is an iterator
object pointing to the inserted element (if one was inserted),
and the other a type 'bool' indicating whether or not
the insertion occurred.)

'std:air' can also be used as a 'general purpose'
set of two values.

> First of all I don't think many programmers
> would use it.

I do.

>For starters, what the first and second
> members are depends on what you are using the pair
> for.

Well of course.

>For instance if I am using coordinates in two
> dimensional space then I like to use x and y.

But what are their types? Are the types the same?
The already existing template 'std:air' will
handle them whatever they are.

> So
> I might as well define my own struct with x and
> y members in it and create a constructor so
> that I can easily instantiate pairs.

This can already be done with 'std:air', which
has three constructors defined (one of them a
template). "Don't reinvent the wheel" and all that.

>
> I wonder if there is a way to create a pair class
> using std:air but typedef its first and second
> to x and y using C++.

Are 'x' and 'y' types or values? In either case,
std:air can handle it.


>The only way I can think
> of is to subclass std:air<,>.

IMO no reason to.

std:air<T,T>(x,y);

Done.

Also see 'std::make_pair()'


>
> Suggestions and reccomendation on the best
> practices and conventions for using std:air<,>
> are most welcome.

Use it if useful, don't use it if not.

-Mike

Mike Wahler wrote:

> > I wonder if there is a way to create a pair class
> > using std:air but typedef its first and second
> > to x and y using C++.
[snip]
> >The only way I can think
> > of is to subclass std:air<,>.
>
> IMO no reason to.
>
> std:air<T,T>(x,y);

The question is, is a Point a pair? From the description of pair, it
seems like the answer is yes. So, if you want a Point with members x and
y, why not subclass?

/david

--
Andre, a simple peasant, had only one thing on his mind as he crept
along the East wall: 'Andre, creep... Andre, creep... Andre, creep.'
-- unknown

David Rubin wrote:

[snip]
> This gives you more of a pointy interface while also modeling a
> container.

Not true; std:air does not model a Container.

/david

--
Andre, a simple peasant, had only one thing on his mind as he crept
along the East wall: 'Andre, creep... Andre, creep... Andre, creep.'
-- unknown

"David Rubin" <> wrote in message
news:...
> Mike Wahler wrote:
>
> > > I wonder if there is a way to create a pair class
> > > using std:air but typedef its first and second
> > > to x and y using C++.
> [snip]
> > >The only way I can think
> > > of is to subclass std:air<,>.
> >
> > IMO no reason to.
> >
> > std:air<T,T>(x,y);
>
> The question is, is a Point a pair? From the description of pair, it
> seems like the answer is yes.

Agreed.

> So, if you want a Point with members x and
> y, why not subclass?

Why not simply:

std:air<int,int> coord(x,y);

or if you like:

typedef std:air<int,int> Point;

Point coord(x,y);

??

-Mike