Nico Schlömer - 6 months ago 27

C++ Question

(*Note:* The corresponding gist is available here.)

I have a number of methods which are derived from

`std::unary_function<K::Point_3, K::FT>`

`typedef K::Point_3 Point;`

`Function`

I now need to add a number of instances of derived classes (example:

`MySphere`

`Function_vector`

`#include <CGAL/Exact_predicates_inexact_constructions_kernel.h>`

#include <CGAL/Implicit_to_labeling_function_wrapper.h>

typedef CGAL::Exact_predicates_inexact_constructions_kernel K;

typedef K::FT FT;

class Function: public std::unary_function<K::Point_3, K::FT>

{

public:

typedef K::Point_3 Point; // necessary

// I'd rather not define operator() here

virtual K::FT operator()(K::Point_3 p) const {return 0.0;}

};

class MySphere: public Function

{

public:

virtual K::FT operator()(K::Point_3 p) const {return 3.14;}

};

typedef CGAL::Implicit_multi_domain_to_labeling_function_wrapper<Function>

Function_wrapper;

typedef Function_wrapper::Function_vector Function_vector;

int main()

{

MySphere f1;

Function_vector v;

v.push_back(f1);

std::cout << f1(CGAL::ORIGIN) << std::endl; // MySphere::operator()

std::cout << v[0](CGAL::ORIGIN) << std::endl; // Function::operator() :(

return 0;

}

Problem:

`Function_vector`

`Function`

`operator()`

`std::unary_function`

`MySphere`

`Function_vector`

`MySphere`

`Function`

`Function`

`operator()`

`MySphere`

How to have

`v[0]`

`MySphere::operator()`

Answer

Since you put a `Function`

object into the vector, and therefore have *object slicing*, you simply can't. Virtual function *needs* pointers or references to properly work with an inheritance tree.

The only advice I can give is that you rethink your design.