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From: pfultz2 on 21 May 2010 07:09
Now say I build an interface like this:
class IInterface
{
public:
virtual void foo() = 0;
virtual ~IInterface() {};
};

template<class T>
class Box : public virtual IInterface
{
T * x;

public:
Box()
{
}
Box(T * x)
{
this->x = x;
}

virtual void foo()
{
x->foo();
}
};

class Interface
{
private:
IInterface * x;

public:
template<class T>
Interface(T * x)
{
this->x = new Box<T>(x);
}

IInterface & operator*()
{
return *x;
}

IInterface * operator-> ()
{
return x;
}
};

Now say I have two classes like this:
class A
{
public:
void foo()
{
printf("foo is called\n");
}
};

class B
{
};

If I do this:
A a;
B b;
Interface interface1 = &a; //OK Compiles
Interface interface2 = &b;//Throws compile error because b does not
have a member named foo
So i would like to create a trait to query whether i can create an
interface from B or not, something like this:
bool aHas = has_interface<A>::value; //True
bool bHas = has_interface<B>::value; //False

Ultimately, I would like to use this for function overloading,
something like this:
template <class T>
typename enable_if<has_interface<T> >::type
process(cons T& x)
{
//Optimize using this interface
}

template <class T>
void process(cons T& x)
{
//The generic code when this interface doesnt exist
}

I though maybe i could use SFINAE to do this, but I cant figure out a
way to deduce this. Perhaps there is another way to do this. Basically
if it throws a compiler error on constructing the interface, i want it
to return a false value. Does any have any ideas?

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From: SG on 22 May 2010 04:07
On 22 Mai, 00:09, pfultz2 wrote:
> [...]
> Now say I have two classes like this:
>
> class A
> {
> public:
> void foo()
> {
> printf("foo is called\n");
> }
> };
>
> class B
> {
> };
>
> [...]
> So i would like to create a trait to query whether [T has a
> member function foo or not], something like this:
> bool aHas = has_interface<A>::value; //True
> bool bHas = has_interface<B>::value; //False
>
> [...]
> I though maybe i could use SFINAE to do this, but I cant figure out a
> way to deduce this. Perhaps there is another way to do this. Basically
> if it throws a compiler error on constructing the interface, i want it
> to return a false value. Does any have any ideas?

I'm not sure whether it's possible in C++03. But In C++0x this should
work:

typedef char one;
typedef char (&two)[2];

template<class T> T&& declval();

template<class T>
struct has_foo_member {
template<class U, class = decltype(declval<U>().foo()) >
static one test(U*);
static two test(...);
static const bool value = sizeof(test(declval<T*>()))==1;
};

unless some foo member function is private in which case you'll get a
compile-time error anyway.

Cheers,
SG


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From: pfultz2 on 23 May 2010 03:46
On May 22, 3:07 pm, SG <s.gesem...(a)gmail.com> wrote:
> On 22 Mai, 00:09, pfultz2 wrote:
>
>
>
> > [...]
> > Now say I have two classes like this:
>
> > class A
> > {
> > public:
> > void foo()
> > {
> > printf("foo is called\n");
> > }
> > };
>
> > class B
> > {
> > };
>
> > [...]
> > So i would like to create a trait to query whether [T has a
> > member function foo or not], something like this:
> > bool aHas = has_interface<A>::value; //True
> > bool bHas = has_interface<B>::value; //False
>
> > [...]
> > I though maybe i could use SFINAE to do this, but I cant figure out a
> > way to deduce this. Perhaps there is another way to do this. Basically
> > if it throws a compiler error on constructing the interface, i want it
> > to return a false value. Does any have any ideas?
>
> I'm not sure whether it's possible in C++03. But In C++0x this should
> work:
>
> typedef char one;
> typedef char (&two)[2];
>
> template<class T> T&& declval();
>
> template<class T>
> struct has_foo_member {
> template<class U, class = decltype(declval<U>().foo()) >
> static one test(U*);
> static two test(...);
> static const bool value = sizeof(test(declval<T*>()))==1;
> };
>
> unless some foo member function is private in which case you'll get a
> compile-time error anyway.
>
> Cheers,
> SG


Thanks, that works great. Here is how I rewrote it to work in gcc
without C++0x extensions:

template<class T> T declval();

template<class T>
struct has_member_foo
{
template<class U>
class selector
{

};

template<class U>
static char test(U * u, selector<typeof(declval<U>().foo())> *
dummy = 0);
static long test(...);

static const bool value = sizeof(test(declval<T*>()))==1;

};


It needs typeof to work, it doesnt work with sizeof. I wonder what the
difference is. If I use sizeof it will always return false.
Thanks,
Paul


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From: Michael Aaron Safyan on 23 May 2010 03:45
It is possible to use SFINAE to detect if a class has a member
function. See:
http://stackoverflow.com/questions/1966362/sfinae-to-check-for-inherited-member-functions

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