wrapper of methods

Hello,
I need to use pointer to members and retrieve the value (a double) that these members should return. But I have a compile error:

Code:

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#include <vector>
class B {
	double _mean, _mean2;
	enum { cnt = 2 };
public:
	double (B::*fptr[cnt])(void) const;
	B() {
		fptr[0] = &B::getMean;
		fptr[1] = &B::getMean2;
	}
	double getMean() const { return _mean; }
	double getMean2() const { return _mean2; }
};
 
class A {
public:
	std::vector<B> _vec;
 
};
//main.cpp
	B objB;
	A objA;
	double db = objA._vec.at(0).fptr[0]; //here error; doens't B::mean() return a double?

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Code:

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//error:
 error C2440: 'initializing' : cannot convert from 'double (__thiscall B::* )(void) const' to 'double'

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Maybe i', using it in a wrong way.....how can I do this thing, please?

You can't use pointer to member without an object, because it doesn't point to a method of particular object, but to a class member.

Why do you need pointer to member here? Can't you overload an operator or use normal method?

Quote:

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Originally Posted by jacek

You can't use pointer to member without an object, because it doesn't point to a method of particular object, but to a class member.
Why do you need pointer to member here? Can't you overload an operator or use normal method?

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To be sincer, there I attached those simple methods for simplicity; but I need to attach 5 functions a bit more complex. what of these will be called depends from console parameters; then they are fixed at program start; what can be a situation where pointer to functions are needed?
Isn't ................._vec.at(0). an instance of a class? I don't understand.....

Can you give me an example of what you mean with overload an operator for this case?

thanks,

try calling the function returned:

Code:

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double db = (objA.*(objA._vec.at(0).fptr[0]))();

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(or some similar syntax ;-)

HTH

Quote:

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Originally Posted by mickey

Isn't ................._vec.at(0). an instance of a class? I don't understand.....

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Yes, it it, but "objA._vec.at(0).fptr[0]" is an expression which returns a pointer to member. It doesn't mean "invoke the method that fptr[0] points to". As caduel wrote you have to use special syntax to invoke that method.

Quote:

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Originally Posted by mickey

Can you give me an example of what you mean with overload an operator for this case?

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The user of your class wants to perform some action depending on integer value. If you make him use pointers to members everywhere the code will be error-prone and unreadable. It is better to hide this inside your class and give your user only a convenient method and an enum:

Code:

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double db = obj.get( B::GeometricMean );
// or using an operator:
double db = obj[ B::GeometricMean ];

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hello,
I understand the alternative but not why I shouldn't use pointertomembers.
Anyway: What do you suggest for this case? The same?

Code:

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class A {
public:
     trasformData1() { ......}
     trasformData2() {..........}
     trasformData3() { .....}
     //maybe here is better use another enum and one "TrasformData(enum);" ? I mean in the same way below....
//trasFormData1,2,3 here are very different
};
class B {
   vector<A> _va;
   enum trasf { trasf1, trasf2, trasf3; }
public:
   	void trasform( trasf  n) { 
		switch (n) {
			case trasf1 :
                        //loop on _va and call "_va.trasformData1();"
                        break;
			case trasf2 : "call _va.trasformData2();" break;
			case trasf3 :"call _va.trasformData3();" break;
			default: exit(-1) //error
		};	
	}
};
//main.cpp
B b;
//fill the vector
//the type of trasformation is chose at command line
b.trasform(B::Trasf1);

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Quote:

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Originally Posted by mickey

why I shouldn't use pointertomembers.

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They're ugly, so if you want ot use them, hide them inside the class.

Quote:

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Originally Posted by mickey

Anyway: What do you suggest for this case? The same?

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How about this?

Code:

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TransformationFactory::TransformationFactory() 
{
   transformations[ SomeTransformation ] = new SomeTransformation();
   ...
}
 
const Transformation * TransformationFactory::get( TransformationType type ) 
{
   return transformations[ type ];
}
 
...
 
transformedVector = factory.get( SomeTransformation )->transform( originalVector );

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Quote:

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Originally Posted by jacek

How about this?

Code:

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TransformationFactory::TransformationFactory() 
{
   transformations[ SomeTransformation ] = new SomeTransformation();
   ...
}
const Transformation * TransformationFactory::get( TransformationType type ) 
{
   return transformations[ type ];
}
transformedVector = factory.get( SomeTransformation )->transform( originalVector );

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Sorry, but I don't understand your example. My trasformation1,2,3 inside A are methods and not object (the do a normalization of the data).....maybe you understood they're objects? Isn't this a "pointer to member" way? I thought that you didn't suggest to use pointer to member..

Quote:

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Originally Posted by mickey

My trasformation1,2,3 inside A are methods and not object (the do a normalization of the data).....

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Yes and my proposition is to change them into objects.

Quote:

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Originally Posted by mickey

Isn't this a "pointer to member" way? I thought that you didn't suggest to use pointer to member..

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It's very similar, but you can avoid that ugly syntax. Pointers to members will be OK too if you hide them behind some nice interface.