SFINAE HasMethod Traits crashes on private methods

[Vinzenz Feenstra]

Hi,
I was asking myself if there is a SFINAE way to avoid the problem with
that code below.

It will check if a class got a Method with a given Signature and Name.
But if the Method is private it crashes while compilation, is there a
way to solve that?

BR

Vinzenz 'evilissimo' Feenstra

[CODE]

#include <iostream>

struct yes{ char _; };
struct no{ char _[2]; };

#define DEFINE_HAS_METHOD( MethodName , MethodTraitsDef )\
template< class ClassType>\
struct MethodName\
{\
static void dummy();\
private:\
MethodTraitsDef\
template <class U>\
static no MethTest(...);\
template <class U>\
static yes MethTest(MethodTraits<U>*);\
public:\
static const bool value = (sizeof(yes) ==
sizeof(MethTest<ClassType>(0)));\
}

// Creating a TraitsType for a Swap Method
#define HasSwap template <class U, U &(U::*Ptr)(U const & ) = &U::Swap> \
struct MethodTraits;

// Defining HasMethodTraits using TraitsType HasSwap
DEFINE_HAS_METHOD( HasMethodSwap , HasSwap );

struct FooBar
{
private:
FooBar & Swap( FooBar const &){ return *this;}
};
struct Foo
{
Foo & Swap( Foo const &){ return *this;}
};
struct Bar
{
Bar & SomeFunc( Bar const &){ return *this;}
};

int main()
{
// Will crash
std::cout << " Has FooBar the method Swap? " << (
HasMethodSwap<FooBar>::value ? " Yes " : " No " ) << std::endl;

// Works fine
std::cout << " Has Foo the method Swap? " << (
HasMethodSwap<Foo>::value ? " Yes " : " No " ) << std::endl;
std::cout << " Has Bar the method Swap? " << (
HasMethodSwap<Bar>::value ? " Yes " : " No " ) << std::endl;
}

[/CODE]

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[Vladimir Marko]

Vinzenz Feenstra wrote:
> Hi,
> I was asking myself if there is a SFINAE way to avoid the problem
with
> that code below.
>
> It will check if a class got a Method with a given Signature and
Name.
> But if the Method is private it crashes while compilation, is there a
> way to solve that?

[snip]

I just can't help. I must write that the active core issue
http://www.open-std.org/jtc1/­sc22/WG21/docs/cwg_active.html­#339
questions the validity of all has-member code I've ever seen.

Now to your problem. It comes down to the question whether
access check failure during template overload resolution
should cause deduction failure or compile-time error. IIRC the
standard does _not_ list this in 14.8.2/2 as a reason for
deduction failure thus making any references to inaccessible
or ambiguous members ill-formed.

I've made a lot of test with gcc 3.4.2 around this problem
but it all seems irrelevant when the standard simply rules out
any workaroud I can think of. The only useful output of my
tests is a bug report against gcc i'm about to submit.

Regards,
Vladimir Marko

[Branimir Maksimovic]

Vinzenz Feenstra wrote:
> Hi,
> I was asking myself if there is a SFINAE way to avoid the problem
with
> that code below.
>
> It will check if a class got a Method with a given Signature and
Name.
> But if the Method is private it crashes while compilation, is there a
> way to solve that?
>

> #define DEFINE_HAS_METHOD( MethodName , MethodTraitsDef )\
> template< class ClassType>\
> struct MethodName\
> {\
> static void dummy();\
> private:\
> MethodTraitsDef\
> template <class U>\
> static no MethTest(...);\
> template <class U>\
> static yes MethTest(MethodTraits<U>*);\
> public:\
> static const bool value = (sizeof(yes) ==
> sizeof(MethTest<ClassType>(0)));\
> }
>

Nothing more to add except that older versions of gcc
are confused if one referes to templated class itself in enum or static
const member initialization, for example g++ 3.2.2 segfaults no matter
whether
member function is private or not.
Following will cause compile time error, not segfault, thus showing
real error that all MethTest's are instantiated.

#define DEFINE_HAS_METHOD( MethodName , MethodTraitsDef )\

struct MethodName##Tmp{\

MethodTraitsDef\
template <class U>\
static no MethTest(...);\
template <class U>\
static yes MethTest(MethodTraits<U>*);\

};\

template< class ClassType>\
struct MethodName\
{\
static void dummy();\
private:\

public:\
static const bool value = \
(sizeof(yes) ==

sizeof(MethodName##Tmp::MethTest<ClassType>(0)));\
}

Interestingly sun CC 5.6 compiles without error, but always shows 'No'.

Greetings, Bane.