* Template argument (pointer to member function) initialization with NULL
@ 2011-11-14 21:48 vagran.ast
2011-11-14 21:51 ` Jonathan Wakely
0 siblings, 1 reply; 5+ messages in thread
From: vagran.ast @ 2011-11-14 21:48 UTC (permalink / raw)
To: gcc-help
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?
gcc version 4.6.1
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Template argument (pointer to member function) initialization with NULL
2011-11-14 21:48 Template argument (pointer to member function) initialization with NULL vagran.ast
@ 2011-11-14 21:51 ` Jonathan Wakely
2011-11-15 13:15 ` vagran.ast
0 siblings, 1 reply; 5+ messages in thread
From: Jonathan Wakely @ 2011-11-14 21:51 UTC (permalink / raw)
To: vagran.ast; +Cc: gcc-help
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Template argument (pointer to member function) initialization with NULL
2011-11-14 21:51 ` Jonathan Wakely
@ 2011-11-15 13:15 ` vagran.ast
2011-11-15 19:55 ` Jonathan Wakely
0 siblings, 1 reply; 5+ messages in thread
From: vagran.ast @ 2011-11-15 13:15 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: gcc-help
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.
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Template argument (pointer to member function) initialization with NULL
2011-11-15 13:15 ` vagran.ast
@ 2011-11-15 19:55 ` Jonathan Wakely
2011-11-16 21:11 ` vagran.ast
0 siblings, 1 reply; 5+ messages in thread
From: Jonathan Wakely @ 2011-11-15 19:55 UTC (permalink / raw)
To: vagran.ast; +Cc: gcc-help
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 { };
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: Template argument (pointer to member function) initialization with NULL
2011-11-15 19:55 ` Jonathan Wakely
@ 2011-11-16 21:11 ` vagran.ast
0 siblings, 0 replies; 5+ messages in thread
From: vagran.ast @ 2011-11-16 21:11 UTC (permalink / raw)
To: Jonathan Wakely; +Cc: gcc-help
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!
^ permalink raw reply [flat|nested] 5+ messages in thread
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2011-11-14 21:48 Template argument (pointer to member function) initialization with NULL vagran.ast
2011-11-14 21:51 ` Jonathan Wakely
2011-11-15 13:15 ` vagran.ast
2011-11-15 19:55 ` Jonathan Wakely
2011-11-16 21:11 ` vagran.ast
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