overloading [][]

A

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
 
J

John Harrison

Andre said:
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;
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
 
A

Andre

Neat :) thatnks, I get the idea.

cheers

-Andre

John said:
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;
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
 
S

Sim

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

double operator[](int j) const;

Thanks

Sim


John said:
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;
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
 
J

John Harrison

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

double operator[](int j) const;

Thanks

Sim

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
 
I

Immanuel Albrecht

Andre said:
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; const double&
double& operator[](int j);
};

class Matrix
{
public:
const Proxy operator[](int i) const;
^^^^^
const Proxy & and
Proxy operator[](int i);
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.

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

Sim

Thanks. That clears it :)

Sim

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

double operator[](int j) const;


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;
}
};
 
S

Sim

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;
}

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

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 said:
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;

const double&
double& operator[](int j);
};

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

^^^^^
const Proxy & and
Proxy operator[](int i);

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.


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

John Harrison

Immanuel Albrecht said:
Andre said:
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; const double&
double& operator[](int j);
};

class Matrix
{
public:
const Proxy operator[](int i) const;
^^^^^
const Proxy & and
Proxy operator[](int i);
Proxy& will have better performance,

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
 
J

John Harrison

Sim said:
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;
}

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

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?


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[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
thanks

Sim


Immanuel said:
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;

const double&
double& operator[](int j);
};

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

^^^^^
const Proxy & and
Proxy operator[](int i);

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.


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

John Harrison

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[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;
};


Actually that could be better, in this case all that is needed is for Proxy
to hold a double*.

class Matrix;

class Proxy
{
friend class Matrix;
double& operator[](int j) { return ptr[j]; }
private:
Proxy(double* p) : ptr(p) {}
double* ptr;
};

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

Which I think was what Sim was getting at.

john
 
J

Josh Sebastian

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

If you don't mind the syntax (some people even like it better), you can
instead provide an operator() that takes two indexes.

a(1, 1) = 6;

It would look something like

class Matrix {
// ...
int& operator()(int row, int col);
int operator()(int row, int col) const;
};

Josh
 
J

John Carson

John Harrison said:
Andre said:
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;
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


There may be good reasons for using a proxy class, but if the matrix is
stored internally as

double **matrix;

as per your later examples, then you can implement a double subscript
operator by simply having the following in the Matrix class:

public:
double * operator[](int i)
{ return matrix; }
const double * operator[](int i) const
{ return matrix; }

The built in [] operator will do the rest.
 
I

Immanuel Albrecht

First of all, sorry for being late, I was at vacation...
If you return a reference, then you have to have an object to refer
to. Where do you propose to store that object?
Of course, in your Matrix class your object should be stored.
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.

I'm sorry, but I haven't read this book.
Unless of course you have some code that shows differently ...

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<Proxy> 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;
}

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

};

template <int rows, int cols>
const double& Matrix<rows,cols>::proxy::eek:perator[](int i) const
{
return m->mat[row];
}

template <int rows, int cols>
double& Matrix<rows,cols>::proxy::eek:perator[](int i)
{
return m->mat[row];
}



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;
}
 
J

John Harrison

Immanuel Albrecht said:
First of all, sorry for being late, I was at vacation...
If you return a reference, then you have to have an object to refer
to. Where do you propose to store that object?
Of course, in your Matrix class your object should be stored.
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.

I'm sorry, but I haven't read this book.
Unless of course you have some code that shows differently ...

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<Proxy> 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;
}

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

};

template <int rows, int cols>
const double& Matrix<rows,cols>::proxy::eek:perator[](int i) const
{
return m->mat[row];
}

template <int rows, int cols>
double& Matrix<rows,cols>::proxy::eek:perator[](int i)
{
return m->mat[row];
}



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;
}


Well you've answered my point

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
 

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