C++Talk.NET

Martin Vorbrodt · Guest

I'm designing a Matrix class of 4x4 matrices used in computer graphics. Both
OpenGL and Direct3D can take a pointer to array of 16 floats which represent
the values in the matrix. OGL takes it in column order, D3D in row order.
Therefore row = 2, col = 3 will result in different offset into the float
array depending on the API. Here is my basic design test program:

#include <iostream>
using std::cout;
using std::endl;

class OGLOffset {
public:
inline int GetOffset(int row, int col) const {
cout << "OGL offsets (" << row << "," << col << ") = ";
return row + col * 4;
}
};

class D3DOffset {
public:
inline int GetOffset(int row, int col) const {
cout << "D3D offsets (" << row << "," << col << ") = ";
return col + row * 4;
}
};

template class Matrix {
public:
inline int GetOffset(int row, int col) const
{ return Offset.GetOffset(row, col); }

private:
OffsetPolicy Offset;
};

int main() {
Matrix<int, OGLOffset> m1;
Matrix<int, D3DOffset> m2;

cout << m1.GetOffset(2, 3) << endl;
cout << m2.GetOffset(2, 3) << endl;
}

Now my question is this:

In this example, i use inline functions, and the matrix class has-a object
of a given offseting policy type. What i could do instead, is to have a
static member function instead, and NOT keep a copy of the policy class
inside the matrix. Which approach would be better? I can see the basic
tradeoff: inline member is probably "faster", but there is storage overhead
for having OffsetPolicy member (no class can have a size of 0, according to
the standard right?) On the other hand, static costs me 0 space overhead.
But is it slower? Does it require an additional function call (even though
the body of the static is visible)?

Please advise. I'm new to the policy based deisgn. I know STL makes
extensive use of such approaches (allocators, traits, etc). Please point me
to information (i'm in the process of reading Modern C++ Design, by
Alexandrescu), i would like to get as much info as possible on this kind
ofdesign approaches.

Thanks in advance!

Martin

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hydrajak@yahoo.com · Guest

Why not inheret from the policy?

template<typename T, class OffsetPolicy>
class Matrix : public OffsetPolicy
{

};

Then write your code and call your functions. It won't compile if you
put a policy class in there that doesn't support the GetOffset
function.

A static member function sort of defeats the purpose of policy based
design.

Also, when using policy, I like to state in the docs of the templated
class what the EXPECTED interface of the policy is. Helps the poor dumb
sap that comes along later.

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Yuri Khan · Guest

Martin Vorbrodt wrote:

mlimber · Guest

[cross-posting deleted]

Yuri Khan wrote:
[snip]

mdlinux7@yahoo.co.in · Guest

mdlinux7@yahoo.co.in · Guest

Sorry I forgot to send link :

http://www.gamedev.net/reference/articles/article2054.asp

Regards
Dinesh

[email]mdlinux7 (AT) yahoo (DOT) co.in[/email] wrote: