Re: Basic problem with Inheritance

From:

=?ISO-8859-1?Q?Erik_Wikstr=F6m?= <Erik-wikstrom@telia.com>

Newsgroups:

comp.lang.c++

Date:

Thu, 16 Aug 2007 11:00:18 GMT

Message-ID:

<67Wwi.6263$ZA.2928@newsb.telia.net>

On 2007-08-16 11:55, Matthias Pfeifer wrote:

Hi there,

i am implementing a Point and Vector class and found that they have some
things in common. Therefore i introduced a common parent class called
ndimobj, that hosts an array of n values, offers some constructors and
the following operator

template<size_t n, typename T>
class ndimobj
{
<SNIP>
public:
    ndimobj<n, T>& operator= (const T s);
};

ndimobj is templated by the type of the elements that live in a given "n
dimensional space" (doubles, polynomials, etc...) and the dimension n.

Point is now derived from ndimobj like this

template<size_t n=3, typename T=float>
class Point : public ndimobj<n, T>
{
    public:
        double distance(Point<n,T> p)
        { <snip> }
};

If i do (in a test) program

Point <4, double> q;
q=3.0;

my compiler (g++) complains:

test.cc: 49: error no match for 'operator=' in 'q=3.0e+0'
Point.hpp: 22: note: candidates are: Point<4u, double>& Point<4u,
double>::operator=(const Point<4u, double>&)

Why is operator= not known. and why does g++ find this unsuitable
candidate?!
Please let me know if didn't give sufficiently information and i'll post
the complete interface of ndimobj and Point if it helps.

Because you have not defined the = operator, what exactly do you expect
q=3.0; to do? If this was a point in the mathematical sense what would
that statement mean? Nothing as far as I know, you can't assign a scalar
to a vector (an a point is a vector as far as math is concerned).

The candidate found by gcc is used like this:

Point <4, double> p;
Point <4, double> q;
p = q;

this = operator is automatically generated by the compiler for your
convenience.

--
Erik Wikstr?m