trying to use default template arguments

[marko]

I have a matrix class that's a template, it used to be
set up like this:

//matrix.h

template <class T> Matrix
{
  public:
    Matrix() : m_(0), n_(0), mn_(0) { }
    ~Matrix();
   void size(int rows, int cols)
   {
    // sets size
   }
......
};

then in some other class's h file that wants to use a matrix it's just:

#include "matrix.h"

class Foo
{

  private:
   Matrix <double> foo;
   Matrix <float> bar;
};


int main()
{
     Foo mymatrix;
     mymatrix.size(6,1);  // old code sets the size this way
    ....
}

the old design I got with this template was setup so the
"T" array that holds the data is a fixed size and it wastes the
unused part of the array if the user uses less than that number
of elements. I was reading about using template overloading and was
wondering if that would allow creating specific sizes of Matrix's:

class Foo
{
   ....
  private:
   Matrix <double, 6,1> foo;  // use the size 6x1
   Matrix <float> bar;       // use the default storage size
};

by changing Matrix to :

template <class T, int rows = -1, int cols = -1> Matrix
{

};

so that if the optional rows and cols are missing, they're -1.
The default constructor uses the rows*cols to set the
T array size if those optional args are != -1. But the problem
is the compiler is throwing errors when I use something like
Matrix<double,6,1> foo; inside a class and then use a
member function to return that:

Matrix<double> getPositionMatrix() { return foo; }

cartesian_.h: In member function 'Matrix<double, -0x000000001, -0x000000001> Cartesian::getPositionMatrix()':
cartesian_.h:226: error: conversion from 'Matrix<double, 6, 1>' to non-scalar type 'Matrix<double, -0x000000001, -0x000000001>' requested

But, since the 2nd and 3rd arg are optional, how can we
use default template arguments then? Do all the types I have
that are Matrix<T> have to be changed to Matrix<T,int,int> ?

When I try that

Matrix<double, int, int> getPositionMatrix(void)
                { return positionMatrix; }

I get a different error:

error: type/value mismatch at argument 2 in template parameter list for 'template<class T, int rows, int cols> class Matrix'
cartesian_.h:225: error: expected a constant of type 'int', got 'int'

And another error for the third int optional arg.

Huh? It expected "int" and got "int" so what's the problem?


UPDATE I found a syntax error in the templates, now they compile
ok.

Mark