Re: Functors

From:

Alan Johnson <awjcs@yahoo.com>

Newsgroups:

comp.lang.c++

Date:

Wed, 06 Jun 2007 02:15:12 -0700

Message-ID:

<YMadnX1Pav285vvbnZ2dnUVZ_r2onZ2d@comcast.com>

Jon Slaughter wrote:

I'm trying to mess with functors and the way I want it to work is that when
I create a functor it will automatically add itself to an array.

The attached code demonstrates what I mean. The problem is that its a bit
unsatisfactory and I'd like to improve it.

In the constructor of TClassA

Functors<TClassA, std::string> *Functor = new Functors<TClassA,
std::string>(this, &TClassA::Display);

Create a function template to do the work for you. The template
parameter can be inferred from the type passed to the function. Example:

template <class T>
void addFunctor(T * pt)
{
     vTab.push_back(new Functors<T, std::string>(pt, &T::Display));
}

Your constructor then becomes:
TClassA(int t)
     : prob(t)
{
     addFunctor(this);
}

Your original code:

template<class type>
class Functor
{
public:
virtual type operator()(type &)=0; // call using operator
};

template <class TClass, class type>
class Functors : public Functor<type>
{
private:
type (TClass::*fpt)(type &); // pointer to member function
TClass* pt2Object; // pointer to object
public:
Functors(TClass* _pt2Object, type (TClass::*_fpt)(type &))
{
pt2Object = _pt2Object;
fpt=_fpt;
};
// override operator "()"
virtual type operator()(type &v)
{
return (*pt2Object.*fpt)(v);
};
};

std::vector< Functor<std::string>* > vTab;
class TClassA{
private:
int prob;
public:
TClassA(int t)
{
prob = t;
Functors<TClassA, std::string> *Functor = new Functors<TClassA,
std::string>(this, &TClassA::Display);
vTab.push_back(Functor);
};
std::string Display(std::string &v)
{
std::string str;
std::stringstream out;
out << prob;
str = out.str();
v.insert(0, str);
return v;
};
};

--
Alan Johnson