How can I simplify C++ functions with function pointers to class member functions?

Question

I give the following example to illustrate my question:

class Abc
{
public:
  int height();
  int width();
  int area();

};

typedef std::vector<class Abc> AbcArray;

void obtain_selected_list_based_on_area(AbcArray& objArray, 
    std::vector<int> &listIndexArray)
{
   std::vector<int> areaArray;
   for(int i=0; i<objArray.size(); i++)
            areaArray.push_back(objArray[i].area());

   function_select_list(areaArray,listIndexArray);
}

void obtain_selected_list_based_on_height(AbcArray& objArray, 
    std::vector<int> &listIndexArray)
{
   std::vector<int> areaArray;
   for(int i=0; i<objArray.size(); i++)
            areaArray.push_back(objArray[i].height());

   function_select_list(areaArray,listIndexArray);
}

void obtain_selected_list_based_on_width(AbcArray& objArray, 
    std::vector<int> &listIndexArray)
{
   std::vector<int> areaArray;
   for(int i=0; i<objArray.size(); i++)
            areaArray.push_back(objArray[i].width());

   function_select_list(areaArray,listIndexArray);
}

As you can see from the above code snippet, we decide to select the list based on three different criteria. In order to do that, we have to write three functions: obtain_selected_list_based_on_area , obtain_selected_list_based_on_height , and obtain_selected_list_based_on_width .
I just wondering how I can simplify it.
For me there one solutions. One is to use function pointer:

void obtain_select_list(AbcArray& objArray, std::vector<int> &listSel, FUN fun)
{
    std::vector<int> areaArray;
       for(int i=0; i<objArray.size(); i++)
                areaArray.push_back(objArray[i].width());

       function_select_list(areaArray,listIndexArray);

}

However, I cannot make it work because the function pointer is pointing to the member of a class.
Do you have some ideas on how to make it work? Moreover, any other solutions for this problem?

Answer 1

Not exactly function pointers, but rather class method pointers.

Additionally, using C++11 range iteration makes the resulting syntax much simpler, and easier to read:

void obtain_selected_list_based_on(AbcArray& objArray, 
    std::vector<int> &listIndexArray,
    int (Abc::*get_something)())
)
{
   std::vector<int> areaArray;

   for (const auto &v:objArray)
            areaArray.push_back((v.*get_something)());

   function_select_list(areaArray,listIndexArray);
}

This can be invoked as:

obtain_selected_list_based_on(objArray,
                              listIndexArray,
                              &Abc::height);

// or

obtain_selected_list_based_on(objArray,
                              listIndexArray,
                              &Abc::width);

// or

obtain_selected_list_based_on(objArray,
                              listIndexArray,
                              &Abc::area);

Your class methods should probably be const themselves, which would change the parameter type to int (Abc::*get_something)() const .

Answer 2

I think something like this should work.

void <template T> obtain_select_list(AbcArray& objArray, std::vector<int> &listSel, T (*fun)(const Abc&))
{
    std::vector<T> areaArray;
       for(int i=0; i<objArray.size(); i++)
                areaArray.push_back(fun(objArray[i]));

       function_select_list(areaArray,listIndexArray);

}

Just to be more general i used a template one, so that if you need the function to return things different from an int you can still use it.

I wasn't able to test it, it compiles but i don't know it if works, but it should.