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

["vagran.ast" <vagran.ast@gmail.com>]

On 11/15/2011 11:30 AM, Jonathan Wakely wrote:
> On 15 November 2011 05:56, vagran.ast wrote:
>> 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.
> That's a bug, maybe related to http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35167
>
> Using nullptr works with GCC 4.7
>
> I think this should work too:
>
> typedef void (A::*pmf_type)();
> const pmf_type pmf_constant = 0;
> template<class T, void (T::*SomeMethod)() = pmf_constant>
> class B { };

I have checked the workaround:

typedef void (A::*pmf_type)();
const pmf_type pmf_constant = 0;
template<class T, void (T::*SomeMethod)() = pmf_constant>
class B { };

It doesn't work on 4.6 with the same error.

And with the latest snapshot of GCC 4.7:

class A {
public:
void SomeMethod()
{
}
};

template<class T, void(T::*SomeMethod)() = nullptr>
class B {

};

B<A> b1; //OK
B<A, static_cast<SomeMethod_t>(0)> b2; //OK
B<A, nullptr> b3; //OK
B<A, 0> b4; // error: could not convert template argument ‘0’ to ‘void 
(A::*)()Â’

So everything works as expected in 4.7. Thanks!