Re: template class specialization trouble

From:
=?ISO-8859-1?Q?Daniel_Kr=FCgler?= <daniel.kruegler@googlemail.com>
Newsgroups:
comp.lang.c++.moderated
Date:
Mon, 7 Apr 2008 16:39:00 CST
Message-ID:
<6afb6ebd-87b2-4f8c-88a6-63214c45fed8@s13g2000prd.googlegroups.com>
On 7 Apr., 18:13, Road Dog <no...@nowhere.com> wrote:

This seems to work fine:

template <typename A, typename B, typename C>
class D
{

};

template <typename A, typename C>
class D<A, void, C>
{

};

but this:

template <typename A, typename B, void (A::*MF)(B)>
class D
{

};

template <typename A, void (A::*MF)()>
class D<A, void, MF>
class D
{

};

produces:

test4.cc:17: error: invalid parameter type 'void'
test4.cc:12: error: in declaration 'template<class T, class W, void
(T::* MF)(W)> struct Whatever'
test4.cc:17: error: '<type error>' is not a valid type for a template
constant parameter

This:

class C<A, void, void (T::*MF)()>

is no better.

Is there some way to specialize on 'void' here ?


Not exactly with the primary class template, that
you provided above. The point is that any function
type R()(A1) with typenames R and A1 will never
match with something that actually has the pattern
R()() which is essentially the same as R()(void) in
C++, see [dcl.fct]/2:

"[..] The parameter list (void) is equivalent to the
empty parameter list. Except for this special case,
void shall not be a parameter type [..]"

Therefore, without variadic template parameters, it
is impossible to express an empty parameter set given
any type argument, e.g. C++0x would allow you to write
a template like this

template<class R, class ... Types> R f(Types ... args);

*including* the case where args corresponds to the empty
parameter set.

Now being in C++03, what about the following approach,
which has some further advantages, because it can be
specialized for even more cases:

template <typename T, T MF>
class D; // Declared, not defined

template<typename A, void (A::*MF)()>
class D<void(A::*)(), MF>
{
};

template<typename A, typename B, void (A::*MF)(B)>
class D<void(A::*)(B), MF>
{
};

struct W {
   void foo();
   void bar(double);
};

D<void(W::*)(), &W::foo> wf;
D<void(W::*)(double), &W::bar> wb;

HTH & Greetings from Bremen,

Daniel Kr?gler

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