Class array ? (2 dimensional)

[eb]

Hello,

This *should* be simple, but I'm not knowledgeable enough so as to get it by
myself.

Suppose I want a double array of a custom class (say MyTile, to tile a
square).

How would I declare it in MyTiledSquare.h ?

I did not succeed with "MyTile **myTiles ;"
The trick is : I want the tiling size (the number of tiles per row / lines)
to depend on a variable. Therefore, I don't want to fix the array size
there.
Then, how would I initialise it in MyTiledSquare.c pp ?
The array size is dependent on a variable which is in the MyTiledSquare
class.

In the end, I would like to access an element with MyTile[i][j]

Thanks for any clues.

[Alan Johnson]

eb wrote:

Hello,

This *should* be simple, but I'm not knowledgeable enough so as to get it by
myself.

Suppose I want a double array of a custom class (say MyTile, to tile a
square).

How would I declare it in MyTiledSquare.h ?

I did not succeed with "MyTile **myTiles ;"
The trick is : I want the tiling size (the number of tiles per row / lines)
to depend on a variable. Therefore, I don't want to fix the array size
there.
Then, how would I initialise it in MyTiledSquare.c pp ?
The array size is dependent on a variable which is in the MyTiledSquare
class.

In the end, I would like to access an element with MyTile[i][j]

Thanks for any clues.

You'll save yourself a lot of headache if you use std::vector. Example:

unsigned rows = 25, cols = 80 ;
std::vector< std::vector<T> > v(rows, std::vector<T>( cols)) ;

--
Alan Johnson

[Axter]

eb wrote:

Hello,

This *should* be simple, but I'm not knowledgeable enough so as to get it by
myself.

Suppose I want a double array of a custom class (say MyTile, to tile a
square).

How would I declare it in MyTiledSquare.h ?

I did not succeed with "MyTile **myTiles ;"
The trick is : I want the tiling size (the number of tiles per row / lines)
to depend on a variable. Therefore, I don't want to fix the array size
there.
Then, how would I initialise it in MyTiledSquare.c pp ?
The array size is dependent on a variable which is in the MyTiledSquare
class.

In the end, I would like to access an element with MyTile[i][j]

Thanks for any clues.

I agree with Alan Johnson, in that using vector<vector<T > > would be
the better approach.
However, if you need a **T type, or if you need contiguous memory for
the 2D array, you can consider using one of the following methods:

(Allocate2DArra y function that returns type **T with contiguous 2D
memory)
http://code.axter.com/allocate2darray_t.h

(Generic wrapper class that creates contiguous 2D memory)
http://code.axter.com/dynamic_2d_array.h

[eb]

Thx Alan,

Not being a programmer, I was not aware of the existence of Vector<>
I'll try that

Alan Johnson wrote:

eb wrote:

Hello,

This *should* be simple, but I'm not knowledgeable enough so as to get it
by myself.

Suppose I want a double array of a custom class (say MyTile, to tile a
square).

How would I declare it in MyTiledSquare.h ?

I did not succeed with "MyTile **myTiles ;"
The trick is : I want the tiling size (the number of tiles per row /
lines) to depend on a variable. Therefore, I don't want to fix the array
size there.
Then, how would I initialise it in MyTiledSquare.c pp ?
The array size is dependent on a variable which is in the MyTiledSquare
class.

In the end, I would like to access an element with MyTile[i][j]

Thanks for any clues.

You'll save yourself a lot of headache if you use std::vector. Example:

unsigned rows = 25, cols = 80 ;
std::vector< std::vector<T> > v(rows, std::vector<T>( cols)) ;

[eb]

Alan Johnson wrote:

You'll save yourself a lot of headache if you use std::vector. Example:

unsigned rows = 25, cols = 80 ;
std::vector< std::vector<T> > v(rows, std::vector<T>( cols)) ;

Well, it's not that simple to me ...
I'm using a QT class as T : QCanvasLine

I started with a simple version :
unsigned rows = 25
std::vector< QCanvasLine > v(rows) ;

Here is the error message.

/home/eb/Packages/qgo.new/src/board.cpp: In member function `void
Board::drawGatt er2()':

/home/eb/Packages/qgo.new/src/board.cpp:344: warning: unused variable 'cols'
/usr/lib/gcc/i686-pc-linux-gnu/3.4.6/include/g++-v3/bits/stl_vector.h: In
constructor `std::vector<_T p, _Alloc>::vector (size_t) [with _Tp =
QCanvasLine, _Alloc = std::allocator< QCanvasLine>]':

/home/eb/Packages/qgo.new/src/board.cpp:345: instantiated from here

/usr/lib/gcc/i686-pc-linux-gnu/3.4.6/include/g++-v3/bits/stl_vector.h:20 7:
error: no matching function for call to `QCanvasLine::Q CanvasLine()'

/usr/qt/3/include/qcanvas.h:685: note: candidates are:
QCanvasLine::QC anvasLine(const QCanvasLine&)
/usr/qt/3/include/qcanvas.h:687: note: QCanvasLine::QC anvasLine(QCanv as*)
However, when try to use a 'candidate'
(say QCanvasLine::QC anvasLine(canva s), where canvas is already declared ),
things don't get better :

/home/eb/Packages/qgo.new/src/board.cpp: In member function `void
Board::drawGatt er2()':
/home/eb/Packages/qgo.new/src/board.cpp:341: error: conversion from
`QCanvasLine*' to non-scalar type `QCanvasLine' requested
/home/eb/Packages/qgo.new/src/board.cpp:345: error: `Board::canvas' cannot
appear in a constant-expression
/home/eb/Packages/qgo.new/src/board.cpp:345: error: template argument 1 is
invalid
/home/eb/Packages/qgo.new/src/board.cpp:345: error: template argument 2 is
invalid
/home/eb/Packages/qgo.new/src/board.cpp:345: error: invalid type in
declaration before '(' token

Aby clue ?

[Alan Johnson]

eb wrote:

Alan Johnson wrote:

You'll save yourself a lot of headache if you use std::vector. Example:

unsigned rows = 25, cols = 80 ;
std::vector< std::vector<T> > v(rows, std::vector<T>( cols)) ;

Well, it's not that simple to me ...
I'm using a QT class as T : QCanvasLine

I started with a simple version :
unsigned rows = 25
std::vector< QCanvasLine > v(rows) ;

There are two different forms of vector's constructor that are used in
the line of code I proposed. The first is:
vector<T> v(num) ;

This makes a vector containing num objects of type T, and uses T's
default constructor to initialize them. An alternative is:
vector<T> v(num, object) ;

This creates a vector with num copies of object. Looking at the
documentation for QCanvasLine (disclaimer: I don't know Qt), it doesn't
have a default constructor, so the first form won't work. However,
something like the following should (assuming canvas has been properly
defined):

size_t cols = 80 ;
std::vector<QCa nvasLine> vcols(cols, QCanvasLine(can vas)) ;

That will make 80 copies of the temporary object created by
QCanvasLine(can vas).

Now, you want two dimensions, so what you want is several copies of the
vector we just defined:

size_t rows = 25 ;
std::vector< std::vector<QCa nvasLine> > lines(rows, vcols) ;

Or, if you want to do it all together:

size_t rows = 25, cols = 80 ;
std::vector< std::vector<QCa nvasLine> > lines(rows,
std::vector<QCa nvasLine>(cols, QCanvasLine(can vas))) ;

--
Alan Johnson

[eb]

Alan Johnson wrote:

Or, if you want to do it all together:

size_t rows = 25, cols = 80 ;
std::vector< std::vector<QCa nvasLine> > lines(rows,
std::vector<QCa nvasLine>(cols, QCanvasLine(can vas))) ;

Close ... What dod the trick was :

std::vector< std::vector<QCa nvasLine> > HGatter (board_size,
std::vector<QCa nvasLine>(board _size, QCanvasLine::QC anvasLine(canva s))) ;

Now I can compile.

Unfortunately, the code does not work yet
(I have then the initialisation of each HGatter line, but they would not
show :-( )

int i,j;

for (i=0; i<board_size; i++)
for (j=0; j<board_size; j++)
{
HGatter[i][j].setPoints(offs etX + square_size * i, offsetY,
offsetX + square_size * i, offsetY + board_pixel_siz e);
HGatter[i][j].show();
}

Thanks anyway.