std::pair<,>

[Neil Zanella]

Hello,

I would like to ask how come the design of C++ includes
std::pair. 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::pair but typedef its first and second
to x and y using C++. The only way I can think
of is to subclass std::pair<,>.

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

Thanks,

Neil

[Neil Zanella]

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

#include <iostream>
#include <utility>

template<class T1, class T2>
class Coordinate: public std::pair<T1, T2> {
public:
Coordinate(T1 x, T2 y): std::pair<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::pair<,>
are most welcome.

Thanks,

Neil

[Jonathan Mcdougall]

> I would like to ask how come the design of C++ includes

std::pair.
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::pair<> 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::pair but typedef its first and second
to x and y using C++. The only way I can think
of is to subclass std::pair<,>.
That would be one way if you want your struct to be
compatible with std::pair<>. 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::pair<,>
are most welcome.

There are none, imho.
Jonathan

[Default User]

Neil Zanella wrote:

Hello,

I would like to ask how come the design of C++ includes
std::pair. First of all I don't think many programmers
would use it.

I generally use std::make_pair( ).


Brian Rodenborn

[David Rubin]

Neil Zanella wrote:

Hello,

I would like to ask how come the design of C++ includes
std::pair. 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::pair but typedef its first and second
to x and y using C++. The only way I can think
of is to subclass std::pair<,>.

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

I think you can start by doing something like

template <typename T>
class Point : public std::pair<T,T>
{
public:
T& x,y;
Point(const T& a, const T& b) : std::pair<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

[WW]

Neil Zanella wrote:

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

#include <iostream>
#include <utility>

template<class T1, class T2>
class Coordinate: public std::pair<T1, T2> {
public:
Coordinate(T1 x, T2 y): std::pair<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(Coor dinate const &o):
store(o.store), x(store.first), y(store.second) { }
Coordinate operator=(Coord inate const &o)
{ store = o.store; }
T1 &x;
T2 &y;
private:
std::pair<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(Coor dinate const &o): store(o.store) { }
Coordinate operator=(Coord inate const &o) { store = o.store; }
T1 &x() {return store.first; }
T2 &y() {return store.second; }
private:
std::pair<T1, T2> store;
};

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

--
WW aka Attila

[Mike Wahler]

"Neil Zanella" <nz******@cs.mu n.ca> wrote in message
news:Pi******** *************** *************** @garfield.cs.mu n.ca...

Hello,

I would like to ask how come the design of C++ includes
std::pair.
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::pair' objects to represent
their elements. If you use a map or multimap, you're
(at least indirectly) using 'std::pair' objects.
Also e.g. one of the overloads of 'std::map::inse rt()'
returns a 'std::pair' 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::pair' 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::pair' 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::pair', 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::pair but typedef its first and second
to x and y using C++.
Are 'x' and 'y' types or values? In either case,
std::pair can handle it.

The only way I can think
of is to subclass std::pair<,>.
IMO no reason to.

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

Done. :-)

Also see 'std::make_pair ()'


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

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

-Mike

[David Rubin]

Mike Wahler wrote:

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

IMO no reason to.

std::pair<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]

David Rubin wrote:

[snip]

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

Not true; std::pair 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

[Mike Wahler]

"David Rubin" <bo***********@ nomail.com> wrote in message
news:3F******** *******@nomail. com...

Mike Wahler wrote:

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

std::pair<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::pair<int,i nt> coord(x,y);

or if you like:

typedef std::pair<int,i nt> Point;

Point coord(x,y);

??

-Mike