Re: template error
On Jul 30, 8:11 am, red floyd <no.spam.h...@its.invalid> wrote:
Jerry Coffin wrote:
There are times that a using directive (or at least a using
declaration) can be useful, and there's no real alternative
to it.
The canonical example is something like a sort routine. If
swap has been specialized for the type being sorted, we want
to use that swap. If swap has not been specialized for the
type, we want to fall back on using std::swap instead.
We can get that with code something like this:
using std::swap;
template <class T>
void sort(/* ... */) {
// ...
// x1 and x2 are T's.
if (x2 < x1)
swap(x1, s2);
// ...
}
Isn't this the one case where the user is allowed to add to
std::? That is, specializing a template in std::? Why not
just specialize std::swap<> for your type T, instead? e.g.:
class my_expensive_to_swap_class {
// ...
public:
void swap(T&);
};
template<>
std::swap<my_expensive_to_swap_class>(
my_expensive_to_swap_class& x,
my_expensive_to_swap_class& y)
{
x.swap(y);
}
// etc....
That's what I'd generally recommend, but it doesn't work if the
class in question is actually a template; you can specialize a
function in std:: over a single type, but you can't provide an
overload which would work with a template type (officially, at
least---in practice, you almost certainly can).
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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