Re: is there a way to do this in C++?
This is a MIME GnuPG-signed message. If you see this text, it means that
your E-mail or Usenet software does not support MIME signed messages.
The Internet standard for MIME PGP messages, RFC 2015, was published in 1996.
To open this message correctly you will need to install E-mail or Usenet
software that supports modern Internet standards.
--=_mimegpg-commodore.email-scan.com-2909-1249856798-0001
Content-Type: text/plain; format=flowed; charset="US-ASCII"
Content-Disposition: inline
Content-Transfer-Encoding: 7bit
aaragon writes:
Hi,
I was wondering if it is possible to do the following trick in C++:
let's say you have a class template the represents a multi-dimensional
array:
template <int d, typename T = double>
class Array {
// rest of the class
};
Now, you would like to have different interfaces depending on the
dimension of the array. For example, if I have Array <2,T> we have a
matrix and it may be convenient to have functions "rows()" and "columns
()", which of course don't make sense for Array<1,T>.
Yes. It's called template specialization.
I would like to know if there is a way to change the interface of the
Array class WITHOUT having to inherit from another class. I was
thinking of some kind of template function defined in the body of the
Array class but is defined ONLY if you're instantiating Array<2,T>.
I tried using the enable_if template as follows:
template <bool B, class T = void>
struct enable_if { typedef T type; };
template <class T>
struct enable_if<false, T> {};
Well, there you go, that's an example of template specialization.
and inside the class:
typename enable_if<d == 2, size_t>::type rows() const
{ return n_[0]; }
template <class U>
typename enable_if<d == 2, size_t>::type columns() const
{ return n_[1]; }
but this still fails when instantiating Array<1,d>.
Thanks for your comments.
No, that's not how you go about specialization a template. You have to
provide a complete definition of your specialized template. A quick example:
template<int d, typename T=double>
class Array {
public:
class index {
public:
size_t i[d];
};
private:
class Comparator {
public:
bool operator()(const index &a, const index &b)
{
size_t i;
for (i=0; i<d; i++)
{
if (a.i[i] < b.i[i]) return true;
if (a.i[i] > b.i[i]) return false;
}
return false;
}
};
std::map<index, T, Comparator> theArray;
public:
Array();
~Array();
T get(const index &i) { return theArray[i]; }
};
That would be, more or less, the generic definition of your multidimensional
array template. I can't think, off the top of my head, of a particularly
clean way to define an n-dimensional vector, except as this kind of a sparse
map.
To specialize an instance of Array in the two-dimensional case:
template<typename T>
class Array<2, T> {
std::vector< std::vector<T> > theArray;
public:
Array();
~Array();
T get(size_t r, size_t c) { return theArray[r][c]; }
};
You would, of course, need to provide the actual definitions of all the
template class methods, for both the generic template and the specialized
one. The above, for all practical matters, are two distinct classes. The
only difference is that instantiating Array<1> or Array<3> gets you the
first class, while Array<2> gets you the specialized class.
Note that instantiating Array<n>, where n is not a constant expression, is
not allowed. The compiler needs to know which actual class needs to be
instantiated.
--=_mimegpg-commodore.email-scan.com-2909-1249856798-0001
Content-Type: application/pgp-signature
Content-Transfer-Encoding: 7bit
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.9 (GNU/Linux)
iEYEABECAAYFAkp/TR4ACgkQx9p3GYHlUOLfiACeMwQ05gDJS0q1is99J1CsilgX
/VgAn2bIErkbhWu8IUf9EIGtw5cgZYcC
=tvbc
-----END PGP SIGNATURE-----
--=_mimegpg-commodore.email-scan.com-2909-1249856798-0001--