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Hotlips · Guest

Hi
I would like to taste a class and see whether or not it has a certain
member function (h).
If it has I would like to call it, else I would like to do something
else.
I have tried using some boost stuff like enable_if and
is_member_function_pointer but I have absolutely no clue what so ever
if this is the right direction I'm going in???????

A simple example of what I intent to do is:
#include <boost/utility/enable_if.hpp>
#include <boost/type_traits.hpp>
struct X
{
void f(){};
};

struct Y
{
void h(){};
};

// default behaviour
template <typename T, class Enable = void>
class Test
{
T t;
public:
Test(){t.f();}
};

// if the class T has a member function called h with the signature
"void ()" use this template
template <typename T>
class Test<T, typename
boost::enable_if<boost::is_member_function_pointer<T::h>::type > >
{
T t;
public:
Test(){t.h();}
};

int main()
{
Test<X> t1; // should call X::f
Test<Y> t2; // should call X::g
}

My VC++ 8.0 compiler issues some warnings and errors, looking like:
warning C4346: 'T::h' : dependent name is not a type prefix with
'typename' to indicate a type
error C2923: 'boost::is_member_function_pointer' : 'T::h' is not a
valid template type argument for parameter 'T'
error C2955: 'boost::is_member_function_pointer' : use of class
template requires template argument list

Of course I have misunderstood something, but what??? I have no clue at
all!

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Maxim Yegorushkin · Guest

Hotlips wrote:

Alberto Ganesh Barbati · Guest

Hotlips ha scritto:

Hotlips · Guest

Woooawww!
As proposed by Maxim Yegorushkin I looked into
http://groups.google.com/group/comp.lang.c++.moderated/msg/410c7680a9b31411
and found some very cryptic code that solved my problem (thanks,
Maxim!). As seen in the following I have purified it so i appears to
look like code from "C++ Templates, The Complete Guide" by Vandevoorde
and Josuttis and added some usage of boost::enable_if to select the
right template-class. A real meta programming SFINEA eye-opener for
me!!!! The next version of my database library at http://www.dtilib.dk
will contain similar code to distinct between a predefined Record class
containing a mandatory create-binding method or a anonymous record in
the form of a boost::tuple to host a row.

#include <boost/utility/enable_if.hpp>
#include <iostream>

template <typename T>
class has_foo
{
// foo member function is expected to have type void(T::*)()
template<void(T::*)()> struct wrap;

typedef char One;
typedef struct { char a[2]; } Two;

template<typename U> static One test(U*, wrap<&U::foo>* = 0);
static Two test(...);

public:
enum { value = 1 == sizeof(has_foo<T>::test((T*)0)) };
};

struct A { void zap(){std::cout << "A::zap\n";}};
struct B { void foo(){std::cout << "B::foo\n";}};

template <class T, class Enable = void>
struct Test
{
T t;
Test(){t.zap();}
};

template <class T>
struct Test<T, typename boost::enable_if<has_foo<T> >::type>
{
T t;
Test(){t.foo();}
};

void main()
{
bool b1 = has_foo<A>::value;
bool b2 = has_foo<B>::value;
Test<A> a;
Test<B> b;
}

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Maxim Yegorushkin · Guest

Hotlips wrote:

David Abrahams · Guest

"Maxim Yegorushkin" <maxim.yegorushkin (AT) gmail (DOT) com> writes:

Hotlips · Guest

Maxim Yegorushkin wrote: