overloading [][]

[Andre]

Hi,

I have a class which contains a two-dimensional array. I need to
overload something like the [][] operator so that I'm able to access the
internal array elements via the class object. For example:

Matrix a = new Matrix(10,10);

a[1][1] = 6;

Is this possible? How can I simulate a two dimensional array using a
class in c++? Thanks

-Andre

[John Harrison]

Re: overloading [][]


"Andre" <food_crazy@hot mail.com> wrote in message
news:3f4900eb@c larion.carno.ne t.au...[color=blue]
> Hi,
>
> I have a class which contains a two-dimensional array. I need to
> overload something like the [][] operator so that I'm able to access the
> internal array elements via the class object. For example:
>
> Matrix a = new Matrix(10,10);
>
> a[1][1] = 6;
>
> Is this possible? How can I simulate a two dimensional array using a
> class in c++? Thanks
>
> -Andre
>[/color]

There is no operator[][], there is only the operator[]. You can do what you
want with a proxy class, something like this

class Proxy
{
public:
double operator[](int j) const;
double& operator[](int j);
};

class Matrix
{
public:
const Proxy operator[](int i) const;
Proxy operator[](int i);
};

I hope you get the idea, you overload operator[] on your Matrix object to
return another class (the Proxy class), you then overload operator[] on the
proxy class to return a Matrix element.

john

[Andre]

Re: overloading [][]

Neat :) thatnks, I get the idea.

cheers

-Andre

John Harrison wrote:
[color=blue]
> "Andre" <food_crazy@hot mail.com> wrote in message
> news:3f4900eb@c larion.carno.ne t.au...
>[color=green]
>>Hi,
>>
>>I have a class which contains a two-dimensional array. I need to
>>overload something like the [][] operator so that I'm able to access the
>>internal array elements via the class object. For example:
>>
>>Matrix a = new Matrix(10,10);
>>
>>a[1][1] = 6;
>>
>>Is this possible? How can I simulate a two dimensional array using a
>>class in c++? Thanks
>>
>>-Andre
>>[/color]
>
>
> There is no operator[][], there is only the operator[]. You can do what you
> want with a proxy class, something like this
>
> class Proxy
> {
> public:
> double operator[](int j) const;
> double& operator[](int j);
> };
>
> class Matrix
> {
> public:
> const Proxy operator[](int i) const;
> Proxy operator[](int i);
> };
>
> I hope you get the idea, you overload operator[] on your Matrix object to
> return another class (the Proxy class), you then overload operator[] on the
> proxy class to return a Matrix element.
>
> john
>
>[/color]

[Sim]

Re: overloading [][]

Just wondering.. what does they "const" in front of a function do:

double operator[](int j) const;

Thanks

Sim


John Harrison wrote:
[color=blue]
> "Andre" <food_crazy@hot mail.com> wrote in message
> news:3f4900eb@c larion.carno.ne t.au...
>[color=green]
>>Hi,
>>
>>I have a class which contains a two-dimensional array. I need to
>>overload something like the [][] operator so that I'm able to access the
>>internal array elements via the class object. For example:
>>
>>Matrix a = new Matrix(10,10);
>>
>>a[1][1] = 6;
>>
>>Is this possible? How can I simulate a two dimensional array using a
>>class in c++? Thanks
>>
>>-Andre
>>[/color]
>
>
> There is no operator[][], there is only the operator[]. You can do what you
> want with a proxy class, something like this
>
> class Proxy
> {
> public:
> double operator[](int j) const;
> double& operator[](int j);
> };
>
> class Matrix
> {
> public:
> const Proxy operator[](int i) const;
> Proxy operator[](int i);
> };
>
> I hope you get the idea, you overload operator[] on your Matrix object to
> return another class (the Proxy class), you then overload operator[] on the
> proxy class to return a Matrix element.
>
> john
>
>[/color]

[John Harrison]

Re: overloading [][]


"Sim" <simba@no.com > wrote in message
news:3f49163f$1 @clarion.carno. net.au...[color=blue]
> Just wondering.. what does they "const" in front of a function do:
>
> double operator[](int j) const;
>
> Thanks
>
> Sim
>[/color]

It means that the function does not modify the object and therefore that the
function can be called on a const object.

E.g.

struct X
{
void f();
void g() const;
};

const X x;
x.f(); // error
x.g(); // ok

Functions which do not modify objects should almost always be declared
const. const objects don't get used much, but const references are very
common.

john

[Immanuel Albrecht]

Re: overloading [][]

"John Harrison" <john_andronicu s@hotmail.com> wrote in
news:bib1jv$798 kd$1@ID-196037.news.uni-berlin.de:
[color=blue]
>
> "Andre" <food_crazy@hot mail.com> wrote in message
> news:3f4900eb@c larion.carno.ne t.au...[color=green]
>> Hi,
>>
>> I have a class which contains a two-dimensional array. I need to
>> overload something like the [][] operator so that I'm able to access
>> the internal array elements via the class object. For example:
>>
>> Matrix a = new Matrix(10,10);
>>
>> a[1][1] = 6;
>>
>> Is this possible? How can I simulate a two dimensional array using a
>> class in c++? Thanks
>>
>> -Andre
>>[/color]
>
> There is no operator[][], there is only the operator[]. You can do
> what you want with a proxy class, something like this
>
> class Proxy
> {
> public:
> double operator[](int j) const;[/color]
const double&[color=blue]
> double& operator[](int j);
> };
>
> class Matrix
> {
> public:
> const Proxy operator[](int i) const;[/color]
^^^^^
const Proxy & and[color=blue]
> Proxy operator[](int i);[/color]
Proxy& will have better performance, plus, you can have the
proxyclass save a reference to your data matrix-row and I would make this
class inside the Matrix namespace to avoid name collisions with other
Proxy classes for different multi-dimensional arrays.[color=blue]
> };[/color]

References combined with inline functions will have improved performance,
since there will be no copy-constructor and destructor calls, less
overhead.

[Sim]

Re: overloading [][]

Thanks. That clears it :-)

Sim

Gianni Mariani wrote:[color=blue]
> Sim wrote:
>[color=green]
>> Just wondering.. what does they "const" in front of a function do:
>>
>> double operator[](int j) const;[/color]
>
>
> It declares that the method does not alter any "non mutable" object state.
>
> e.g.
>
> struct A {
> int a;
> mutable int b;
> void doer_a() const
> {
> a = 3; // illegal;
> }
> void doer_b() const
> {
> b = 3; // legal;
> }
> };
>[/color]

[Sim]

Re: overloading [][]

The topic seemed interesting so I gave it a try myself. I can't get my
proxy class running. I get a number of errors when I do something like:

matrixA[0][0] = 5.0;

or std::cout << matrixA[0][0];

I simply did a:

const Proxy operator[](int i) const {
return this->proxy[i];
}

Proxy operator[](int i) {
return this->proxy[i];
}

and in proxy I did:

double operator[](int j) const {
return matrix->[j];
}
double& operator[](int j) {
return matrix->[j];
}

and I had my two-dim array in the proxy class:

double **matrix;


What am I doing wrong? Andre if you got it working could you help me out?

thanks

Sim


Immanuel Albrecht wrote:
[color=blue]
> "John Harrison" <john_andronicu s@hotmail.com> wrote in
> news:bib1jv$798 kd$1@ID-196037.news.uni-berlin.de:
>
>[color=green]
>>"Andre" <food_crazy@hot mail.com> wrote in message
>>news:3f4900eb @clarion.carno. net.au...
>>[color=darkred]
>>>Hi,
>>>
>>>I have a class which contains a two-dimensional array. I need to
>>>overload something like the [][] operator so that I'm able to access
>>>the internal array elements via the class object. For example:
>>>
>>>Matrix a = new Matrix(10,10);
>>>
>>>a[1][1] = 6;
>>>
>>>Is this possible? How can I simulate a two dimensional array using a
>>>class in c++? Thanks
>>>
>>>-Andre
>>>[/color]
>>
>>There is no operator[][], there is only the operator[]. You can do
>>what you want with a proxy class, something like this
>>
>>class Proxy
>>{
>>public:
>> double operator[](int j) const;[/color]
>
> const double&
>[color=green]
>> double& operator[](int j);
>>};
>>
>>class Matrix
>>{
>>public:
>> const Proxy operator[](int i) const;[/color]
>
> ^^^^^
> const Proxy & and
>[color=green]
>> Proxy operator[](int i);[/color]
>
> Proxy& will have better performance, plus, you can have the
> proxyclass save a reference to your data matrix-row and I would make this
> class inside the Matrix namespace to avoid name collisions with other
> Proxy classes for different multi-dimensional arrays.
>[color=green]
>>};[/color]
>
>
> References combined with inline functions will have improved performance,
> since there will be no copy-constructor and destructor calls, less
> overhead.[/color]

[John Harrison]

Re: overloading [][]


"Immanuel Albrecht" <xrxixpx@gmx.de > wrote in message
news:bib6g1$l5c $03$2@news.t-online.com...[color=blue]
> "John Harrison" <john_andronicu s@hotmail.com> wrote in
> news:bib1jv$798 kd$1@ID-196037.news.uni-berlin.de:
>[color=green]
> >
> > "Andre" <food_crazy@hot mail.com> wrote in message
> > news:3f4900eb@c larion.carno.ne t.au...[color=darkred]
> >> Hi,
> >>
> >> I have a class which contains a two-dimensional array. I need to
> >> overload something like the [][] operator so that I'm able to access
> >> the internal array elements via the class object. For example:
> >>
> >> Matrix a = new Matrix(10,10);
> >>
> >> a[1][1] = 6;
> >>
> >> Is this possible? How can I simulate a two dimensional array using a
> >> class in c++? Thanks
> >>
> >> -Andre
> >>[/color]
> >
> > There is no operator[][], there is only the operator[]. You can do
> > what you want with a proxy class, something like this
> >
> > class Proxy
> > {
> > public:
> > double operator[](int j) const;[/color]
> const double&[color=green]
> > double& operator[](int j);
> > };
> >
> > class Matrix
> > {
> > public:
> > const Proxy operator[](int i) const;[/color]
> ^^^^^
> const Proxy & and[color=green]
> > Proxy operator[](int i);[/color]
> Proxy& will have better performance,[/color]

If you return a reference, then you have to have an object to refer to.
Where do you propose to store that object? I think its well known that this
scheme is impossible to implement correctly and safely. See for instance
Effective C++ by Scott Meyer who gives all the various flawed possibilities
a good going over.

Unless of course you have some code that shows differently ...

john

[John Harrison]

Re: overloading [][]


"Sim" <simba@no.com > wrote in message
news:3f49202f$1 @clarion.carno. net.au...[color=blue]
> The topic seemed interesting so I gave it a try myself. I can't get my
> proxy class running. I get a number of errors when I do something like:
>
> matrixA[0][0] = 5.0;
>
> or std::cout << matrixA[0][0];
>
> I simply did a:
>
> const Proxy operator[](int i) const {
> return this->proxy[i];
> }
>
> Proxy operator[](int i) {
> return this->proxy[i];
> }
>
> and in proxy I did:
>
> double operator[](int j) const {
> return matrix->[j];
> }
> double& operator[](int j) {
> return matrix->[j];
> }
>
> and I had my two-dim array in the proxy class:
>
> double **matrix;
>
>
> What am I doing wrong? Andre if you got it working could you help me out?
>[/color]

Well the two dim array is supposed to be in the Matrix class. The proxy
class just hold a reference to the matrix and the first index.

Something like this (untested code)

class Matrix;

class Proxy
{
friend class Matrix;
double& operator[](int j) { return m.matrix[i][j]; }
private:
Proxy(Matrix& mm, int ii) : m(mm), i(ii) {}
Matrix& m;
int i;
};

class Matrix
{
friend class Proxy;
public:
Proxy operator[](int i) { return Proxy(*this, i); }
private:
double** matrix;
};

Basically the Proxy class just holds the parameters used in the first
operator[] call, so that when the second operator[] call happens they are
available to get the element from the Matrix.

john
[color=blue]
> thanks
>
> Sim
>
>
> Immanuel Albrecht wrote:
>[color=green]
> > "John Harrison" <john_andronicu s@hotmail.com> wrote in
> > news:bib1jv$798 kd$1@ID-196037.news.uni-berlin.de:
> >
> >[color=darkred]
> >>"Andre" <food_crazy@hot mail.com> wrote in message
> >>news:3f4900eb @clarion.carno. net.au...
> >>
> >>>Hi,
> >>>
> >>>I have a class which contains a two-dimensional array. I need to
> >>>overload something like the [][] operator so that I'm able to access
> >>>the internal array elements via the class object. For example:
> >>>
> >>>Matrix a = new Matrix(10,10);
> >>>
> >>>a[1][1] = 6;
> >>>
> >>>Is this possible? How can I simulate a two dimensional array using a
> >>>class in c++? Thanks
> >>>
> >>>-Andre
> >>>
> >>
> >>There is no operator[][], there is only the operator[]. You can do
> >>what you want with a proxy class, something like this
> >>
> >>class Proxy
> >>{
> >>public:
> >> double operator[](int j) const;[/color]
> >
> > const double&
> >[color=darkred]
> >> double& operator[](int j);
> >>};
> >>
> >>class Matrix
> >>{
> >>public:
> >> const Proxy operator[](int i) const;[/color]
> >
> > ^^^^^
> > const Proxy & and
> >[color=darkred]
> >> Proxy operator[](int i);[/color]
> >
> > Proxy& will have better performance, plus, you can have the
> > proxyclass save a reference to your data matrix-row and I would make[/color][/color]
this[color=blue][color=green]
> > class inside the Matrix namespace to avoid name collisions with other
> > Proxy classes for different multi-dimensional arrays.
> >[color=darkred]
> >>};[/color]
> >
> >
> > References combined with inline functions will have improved[/color][/color]
performance,[color=blue][color=green]
> > since there will be no copy-constructor and destructor calls, less
> > overhead.[/color]
>[/color]

[Immanuel Albrecht]

Re: overloading [][]

"John Harrison" <john_andronicu s@hotmail.com> wrote in
news:bib7vu$7j6 lp$1@ID-196037.news.uni-berlin.de:

First of all, sorry for being late, I was at vacation...
[color=blue]
> If you return a reference, then you have to have an object to refer
> to. Where do you propose to store that object?[/color]
Of course, in your Matrix class your object should be stored.[color=blue]
> I think its well known
> that this scheme is impossible to implement correctly and safely. See
> for instance Effective C++ by Scott Meyer who gives all the various
> flawed possibilities a good going over.[/color]

I'm sorry, but I haven't read this book.[color=blue]
>
> Unless of course you have some code that shows differently ...[/color]

So what's wrong with e.g. this:


#include <iostream>
#include <ostream>

#include <vector>

template <int rows,int cols> class Matrix
{
private:
double mat[rows][cols];
public:
class Proxy {
private:
Matrix* m;
int row;

friend class Matrix;

void SetRow(int r)
{ row = r;}

void SetMat(Matrix* matrix)
{ m = matrix;}

public:
Proxy() {}

const double& operator [](int i) const;
double& operator [](int i);

};

friend class Proxy;
private:
std::vector<Pro xy> prox;

public:

Matrix() : prox(rows)
{
for (unsigned int r=0;r<rows;r++)
{
prox[r].SetRow(r);
prox[r].SetMat(this);
}
}

const Proxy& operator[] (int i) const
{
return prox[i];
}

Proxy& operator[] (int i)
{
return prox[i];
}

};

template <int rows, int cols>
const double& Matrix<rows,col s>::Proxy::oper ator[](int i) const
{
return m->mat[row][i];
}

template <int rows, int cols>
double& Matrix<rows,col s>::Proxy::oper ator[](int i)
{
return m->mat[row][i];
}



int main()
{
Matrix<4,4> m;

for (unsigned int r=0;r<4;r++)
for (unsigned int c=0;c<4;c++)
{
std::cout << r << " " << c <<std::endl;
m[r][c] = r+c;
}

for (unsigned int r=0;r<4;r++)
{
for (unsigned int c=0;c<4;c++)
std::cout << m[r][c] << " ";

std::cout << std::endl;
}


return 0;
}

[John Harrison]

Re: overloading [][]


"Immanuel Albrecht" <xrxixpx@gmx.de > wrote in message
news:biq3ak$4pn $07$1@news.t-online.com...[color=blue]
> "John Harrison" <john_andronicu s@hotmail.com> wrote in
> news:bib7vu$7j6 lp$1@ID-196037.news.uni-berlin.de:
>
> First of all, sorry for being late, I was at vacation...
>[color=green]
> > If you return a reference, then you have to have an object to refer
> > to. Where do you propose to store that object?[/color]
> Of course, in your Matrix class your object should be stored.[color=green]
> > I think its well known
> > that this scheme is impossible to implement correctly and safely. See
> > for instance Effective C++ by Scott Meyer who gives all the various
> > flawed possibilities a good going over.[/color]
>
> I'm sorry, but I haven't read this book.[color=green]
> >
> > Unless of course you have some code that shows differently ...[/color]
>
> So what's wrong with e.g. this:
>
>
> #include <iostream>
> #include <ostream>
>
> #include <vector>
>
> template <int rows,int cols> class Matrix
> {
> private:
> double mat[rows][cols];
> public:
> class Proxy {
> private:
> Matrix* m;
> int row;
>
> friend class Matrix;
>
> void SetRow(int r)
> { row = r;}
>
> void SetMat(Matrix* matrix)
> { m = matrix;}
>
> public:
> Proxy() {}
>
> const double& operator [](int i) const;
> double& operator [](int i);
>
> };
>
> friend class Proxy;
> private:
> std::vector<Pro xy> prox;
>
> public:
>
> Matrix() : prox(rows)
> {
> for (unsigned int r=0;r<rows;r++)
> {
> prox[r].SetRow(r);
> prox[r].SetMat(this);
> }
> }
>
> const Proxy& operator[] (int i) const
> {
> return prox[i];
> }
>
> Proxy& operator[] (int i)
> {
> return prox[i];
> }
>
> };
>
> template <int rows, int cols>
> const double& Matrix<rows,col s>::Proxy::oper ator[](int i) const
> {
> return m->mat[row][i];
> }
>
> template <int rows, int cols>
> double& Matrix<rows,col s>::Proxy::oper ator[](int i)
> {
> return m->mat[row][i];
> }
>
>
>
> int main()
> {
> Matrix<4,4> m;
>
> for (unsigned int r=0;r<4;r++)
> for (unsigned int c=0;c<4;c++)
> {
> std::cout << r << " " << c <<std::endl;
> m[r][c] = r+c;
> }
>
> for (unsigned int r=0;r<4;r++)
> {
> for (unsigned int c=0;c<4;c++)
> std::cout << m[r][c] << " ";
>
> std::cout << std::endl;
> }
>
>
> return 0;
> }[/color]

Well you've answered my point
[color=blue][color=green]
> > If you return a reference, then you have to have an object to refer
> > to. Where do you propose to store that object?[/color][/color]

by creating a vector of proxys. But that's an overhead for every matrix to
carry around. Arguably its a small amount of space compared to the matrix as
a whole. Whether your method is better than copying the Proxy class as I
suggested would depend on the application I think. In any case, as John
Carson pointed out, this time the proxy class need only be a pointer, which
is better than both our methods.

john