NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part2/15206

Text File | 1996-08-05 | 3.7 KB | 132 lines

Path: hobbes.cc.uga.edu!news
From: Andy Dustman <[e-mail]>
Newsgroups: comp.lang.c++
Subject: Template troubles with partial solution (?)
Date: Wed, 03 Apr 1996 23:49:55 -0500
Organization: University of Georgia, Athens
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I'm trying to modify an existing library of matrix routines into a template
and I'm having lots of trouble. I'm using G++ 2.7.0, but 2.7.2 seems to have
the same problems.

The original looks something like this:

matrix.h:

class matrix {
    double* mat;
    ...
    matrix& operator+= (const matrix& m) ;
    ...
} ;
...
matrix operator+ (const matrix& m, const matrix& n);

*********
matrix.c:

#include "matrix.h"

matrix& matrix::operator+= (const matrix& m)
{ ... }

matrix operator + (const matrix& m, const matrix& n)
{
        matrix p = m;
        p += n;
        return p;
}


This compiles and works well. However, I would like to turn it into a
template so I could use matrix<double> or matrix<complex>. So: I did
s/double/T/ and s/matrix/matrix<T>/ throughout the source tree, plus some
other editing and come up with this:

matrix.h:

template<class T>
class matrix {
    T* mat;
    ...
    matrix<T>& operator+= (const matrix<T>& m);
    ...
} ;
template<class T> matrix<T> operator+ (const matrix<T>& m, const matrix<T>& n); \\ A

*********
matrix.c:

#include "matrix.h"

template<class T> matrix<T> matrix<T>::operator+= (const matrix<T>& m);
{ ... }

template<class T> matrix<T> operator + (const matrix& m, const matrix& n) \\ B
{
        matrix<T> p = m;
        p += n;
        return p;
}

**********
dmatrix.c:

#include "matrix.c"

template class matrix<double>;


This compiles, but at link time, I get:

test.o(.text+0x14d): undefined reference to `operator+(matrix<double> const &, matrix<double> const &)'

Okay, I figure, I'll just prepend "matrix<T>::" to "operator +" on line B
above and that'll fix it, right? Uh-uh:

matrix.h:(end of template): method `operator +' not found in class `matrix<double>'
matrix.c:B: `matrix<...>::operator +(matrix<...> &, matrix<...> &)' must take either zero or one argument

Now this is a pretty strange error, so I figure the obvious thing is to
move line A up into the template and modify it to look like the += def:

    matrix<T>& operator+= (const matrix<T>& m);
    matrix<T> operator+ (const matrix<T>& m, const matrix<T>& n);

I try this. It still insists on "either zero or one argument". So I oblige.

    matrix<T> operator+ (const matrix<T>& n);

and change matrix.c:

template<class T> matrix<T> operator + (const matrix& n) \\ B
{
        matrix<T> p = *this;
        p += n;
        return p;
}

This compiles, and it seems to resolve the symbol problems. (I actually
have quite a few more to fix, but I gotta go home!) (Yes, I worked this
last part out writing this!) But the question remains:

In the original version, the operator+ is declared outside of the class
definition, but in the template version, it seems to have to be declared
within the class definition. Is this really how things should work, or
did I miss something? If you don't know by now, I am just learning C++.
I've got Stroustrup's "C++ PL, 2nd ed., reprint April 95", so any references
to that would be useful. No, I haven't read the whole thing yet...

--
Andy Dustman / Computational Center for Molecular Structure and Design / UGA
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