Double dispatch (makes no sense!)

S

saneman

I am trying to use double dispatch in the below code:

#include <iostream>

class Box;
class Sphere;


class geometry_type
{
public:
virtual void haps(geometry_type* other)=0;
virtual void collideWith(Box* other)=0;
virtual void collideWith(Sphere* other) = 0;
};

class Box : public geometry_type
{
public :
Box() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Box collision with Box" << std::endl;
}


};

class Sphere : public geometry_type
{
public :
Sphere() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Sphere collision with Box" << std::endl;
}


};


void narrow_phase(geometry_type* G1, geometry_type* G2)
{
G1->haps(G2);
}

int main()
{
Box* BoxA = new Box();
Sphere* SphereA = new Sphere();

BoxA->collideWith(BoxA);
SphereA->collideWith(BoxA);

// Should print box collide with box.
narrow_phase(BoxA, BoxA);

// Should print sphere collide with box.
narrow_phase(SphereA, BoxA);

// Should print box collide with sphere.
narrow_phase(BoxA, SphereA);

return 0;

}


But when I run main I get:

Box collision with Box
Sphere collision with Box
Box collision with Box
Sphere collision with Sphere
Sphere collision with Box


What goes on in the last two calls?
 
A

Andrey Tarasevich

saneman said:
...
class Box : public geometry_type
{
public :
Box() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
Click to expand...

Huh? This should be "Box collision with Sphere", shouldn't it?
}

virtual void collideWith(Box* other) {
std::cout << "Box collision with Box" << std::endl;
}


};

class Sphere : public geometry_type
{
public :
Sphere() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Sphere collision with Box" << std::endl;
}


};


void narrow_phase(geometry_type* G1, geometry_type* G2)
{
G1->haps(G2);
}

int main()
{
Box* BoxA = new Box();
Sphere* SphereA = new Sphere();

BoxA->collideWith(BoxA);
SphereA->collideWith(BoxA);

// Should print box collide with box.
narrow_phase(BoxA, BoxA);

// Should print sphere collide with box.
narrow_phase(SphereA, BoxA);
Click to expand...

No. Your 'haps' method essentially "swaps" the objects involved in the
collision (take a closer look at it): 'G1' from 'narrow_phase' becomes
'this' in 'haps'. So, it should print "Box collision with Sphere". And
it will, if you fix the error mentioned above.
// Should print box collide with sphere.
narrow_phase(BoxA, SphereA);
Click to expand...

Again, no. For the very same reason it should print "Sphere collision
with Box". And that exactly what it prints.
return 0;

}


But when I run main I get:

Box collision with Box
Sphere collision with Box
Box collision with Box
Sphere collision with Sphere
Sphere collision with Box


What goes on in the last two calls?
Click to expand...

See above.
 
H

hurcan solter

I am trying to use double dispatch in the below code:

#include <iostream>

class Box;
class Sphere;


class geometry_type
{
public:
virtual void haps(geometry_type* other)=0;
virtual void collideWith(Box* other)=0;
virtual void collideWith(Sphere* other) = 0;
};

class Box : public geometry_type
{
public :
Box() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}
Click to expand...
Here is a typo;it should read' Box collision with sphere'

virtual void collideWith(Box* other) {
std::cout << "Box collision with Box" << std::endl;
}


};

class Sphere : public geometry_type
{
public :
Sphere() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Sphere collision with Box" << std::endl;
}


};


void narrow_phase(geometry_type* G1, geometry_type* G2)
{
G1->haps(G2);
}

int main()
{
Box* BoxA = new Box();
Sphere* SphereA = new Sphere();

BoxA->collideWith(BoxA);
SphereA->collideWith(BoxA);

// Should print box collide with box.
narrow_phase(BoxA, BoxA);

// Should print sphere collide with box.
narrow_phase(SphereA, BoxA);

// Should print box collide with sphere.
narrow_phase(BoxA, SphereA);

return 0;

}


But when I run main I get:

Box collision with Box
Sphere collision with Box
Box collision with Box
Sphere collision with Sphere
Sphere collision with Box


What goes on in the last two calls?
Click to expand...

Last one is correct, you switch the parameters
inside the haps. Late there :)
 
D

Daniel Pitts

saneman said:
I am trying to use double dispatch in the below code:

#include <iostream>

class Box;
class Sphere;


class geometry_type
{
public:
virtual void haps(geometry_type* other)=0;
virtual void collideWith(Box* other)=0;
virtual void collideWith(Sphere* other) = 0;
};

class Box : public geometry_type
{
public :
Box() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Box collision with Box" << std::endl;
}


};

class Sphere : public geometry_type
{
public :
Sphere() {}

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}

virtual void collideWith(Sphere* other) {
std::cout << "Sphere collision with Sphere" << std::endl;
}

virtual void collideWith(Box* other) {
std::cout << "Sphere collision with Box" << std::endl;
}


};


void narrow_phase(geometry_type* G1, geometry_type* G2)
{
G1->haps(G2);
}

int main()
{
Box* BoxA = new Box();
Sphere* SphereA = new Sphere();

BoxA->collideWith(BoxA);
SphereA->collideWith(BoxA);

// Should print box collide with box.
narrow_phase(BoxA, BoxA);

// Should print sphere collide with box.
narrow_phase(SphereA, BoxA);

// Should print box collide with sphere.
narrow_phase(BoxA, SphereA);

return 0;

}


But when I run main I get:

Box collision with Box
Sphere collision with Box
Box collision with Box
Sphere collision with Sphere
Sphere collision with Box


What goes on in the last two calls?
Click to expand...
Because your Box::collideWith(Sphere*) has a bug in it. Has nothing to
do with your double dispatch, which appears to be working perfectly.
 
S

saneman

hurcan said:
Here is a typo;it should read' Box collision with sphere'



Last one is correct, you switch the parameters
inside the haps. Late there :)
Click to expand...

thanks! Yes it was a bit late :) I am wondering why the baseclass
should be declared as abstract. I have tried to change it to:


class geometry_type
{
public:

virtual void haps(geometry_type* other)
{
other->collideWith(this);
}


virtual void collideWith(Box* other)
{
std::cout << "Box collision" << std::endl;
}

virtual void collideWith(Sphere* other)
{
std::cout << "Sphere collision" << std::endl;
}





// virtual void haps(geometry_type* other)=0;
// virtual void collideWith(Box* other)=0;
// virtual void collideWith(Sphere* other) = 0;

};

But then I get:


test2.cpp: In member function ‘virtual void
geometry_type::haps(geometry_type*)’:
test2.cpp:16: error: no matching function for call to
‘geometry_type::collideWith(geometry_type* const)’
test2.cpp:19: note: candidates are: virtual void
geometry_type::collideWith(Box*)
test2.cpp:24: note: virtual void
geometry_type::collideWith(Sphere*)


Is there some rule that the baseclass must be abstract?
 
J

James Kanze

hurcan solter wrote:
thanks! Yes it was a bit late :) I am wondering why the baseclass
should be declared as abstract.
Click to expand...

It doesn't have to be.
I have tried to change it to:

class geometry_type
{
public:
Click to expand...
virtual void haps(geometry_type* other)
{
other->collideWith(this);
Click to expand...

And what should this call? Function overload resolution only
concerns static types.
}
Click to expand...
virtual void collideWith(Box* other)
{
std::cout << "Box collision" << std::endl;
}
Click to expand...
virtual void collideWith(Sphere* other)
{
std::cout << "Sphere collision" << std::endl;
}
Click to expand...
// virtual void haps(geometry_type* other)=0;
// virtual void collideWith(Box* other)=0;
// virtual void collideWith(Sphere* other) = 0;
};
Click to expand...
But then I get:

test2.cpp: In member function ?virtual void
geometry_type::haps(geometry_type*)?:
test2.cpp:16: error: no matching function for call to
?geometry_type::collideWith(geometry_type* const)?
test2.cpp:19: note: candidates are: virtual void
geometry_type::collideWith(Box*)
test2.cpp:24: note: virtual void
geometry_type::collideWith(Sphere*)
Click to expand...
Is there some rule that the baseclass must be abstract?
Click to expand...

No. But there is a rule that when you call a function, one of
the function found with name lookup must be callable with the
static types of your arguments.

Think about it for a moment. What you're asking for is
basically double dispatch. Or at least, dispatch on the dynamic
type of an argument, not on the object the function is called
on.
 

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