Re: Template argument (pointer to member function) initialization with NULL

["vagran.ast" <vagran dot ast at gmail dot com>]

On 11/14/2011 11:28 PM, Jonathan Wakely wrote:
> On 14 November 2011 20:22, vagran.ast wrote:
> > Hi,
> >
> > In the following code:
> >
> > class A {
> > public:
> >     void SomeMethod() { }
> > };
> >
> > template<class T, void (T::*SomeMethod)() = 0>
> > class B {
> >
> > };
> >
> > B<A>  b1; // error: could not convert template argument '0' to 'void
> > (A::*)()'
> >
> > B<A, 0>  b2; // error: could not convert template argument '0' to 'void
> > (A::*)()'
> >
> > void (A::*someMethod)() = 0; // OK
> >
> > there are two compilation errors. AFAIK per C++ standard 0 is a valid value
> > for a pointer to member
> > function. Variable of such type can be successfully initialized by 0 but
> > template arguments can not.
> > Is it desired behavior or a bug?
> I think G++ is correct, you need to cast the literal zero to the right
> type to prevent it being treated as an int in that context, or use
> C++11's nullptr.
I have tried both variants and still no luck:

B<A, static_cast<void (A::*)()>(0)> b2;
// error: '((void (A::*)())0)' is not a valid template argument for type 
'void (A::*)()'
// error: it must be a pointer-to-member of the form '&X::Y'
// error: could not convert template argument '((void (A::*)())0)' to 
'void (A::*)()'
// error: invalid type in declaration before ';' token

B<A, nullptr> b3; // error: could not convert template argument 
'nullptr' to 'void (A::*)()'

void (A::*someMethod)() = static_cast<void (A::*)()>(0);// OK
void (A::*someMethod2)() = nullptr;// OK

Seems the compiler wants only real method pointer for a template 
parameter however
the conversion from NULL to member function pointer is possible.