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::*)()’
So everything works as expected in 4.7. Thanks!
