pointer to member function

Question

I am trying to generalize the functions filterX() and filterY() in the following class Table to function filter().

The functions filterX() and filterY() only differ in the function they call inside the procedure. While filterX() calls getX(), filterY() calls getY().

#include <iostream>
#include <string>
#include <vector>

using namespace std;

class Row 
{
    public:
        void add(string x, string y, int val);
        string getX()   const { return d_x; } 
        string getY()   const { return d_y; } 
        int    getVal() const { return d_val; } 

    private:
        string d_x;
        string d_y;
        int    d_val;
};

class Table
{
    public:
        void add(string x, string y, int val);
        vector<int> filterX(string s);
        vector<int> filterY(string s);
    private:
        vector<Row> d_table;
};


//--------------------class Row----------------------------
void Row::add(string x, string y, int val)
{
    d_x   = x;
    d_y   = y;
    d_val = val;
}


//-------------------class Table---------------------------

void Table::add(string x, string y, int val)
{
    Row r;
    r.add(x, y, val);
    d_table.push_back(r);
}

vector<int> Table::filterX(string s)
{
    vector<int> result;
    vector<Row>::iterator it;
    for(it = d_table.begin(); it != d_table.end(); ++it) {
        if(it->getX() == s) {
            int val = it->getVal();
            result.push_back(val);
        }
    }
    return result;
}


vector<int> Table::filterY(string s)
{
    vector<int> result;
    vector<Row>::iterator it;
    for(it = d_table.begin(); it != d_table.end(); ++it) {
        if(it->getY() == s) {
            int val = it->getVal();
            result.push_back(val);
        }
    }
    return result;
}

int main()
{
    Table t;
    t.add("x1", "y1", 1);
    t.add("x1", "y2", 2);
    t.add("x2", "y1", 3);
    t.add("x2", "y2", 4);

    vector<int> vx = t.filterX("x1");
    vector<int> vy = t.filterY("y2");

    vector<int>::const_iterator it;

    cout << "Matching X" << endl;
    for(it = vx.begin(); it != vx.end(); ++it)
        cout << *it << "\t";
    cout << endl;

    cout << "Matching Y" << endl;
    for(it = vy.begin(); it != vy.end(); ++it)
        cout << *it << "\t";
    cout << endl;

    return 0;
}

I tried pointer to member function but got bogged down by compiler errors. For the following example, I would like to have the following main() if that is possible:

int main()
{
    Table t;
    t.add("x1", "y1", 1);
    t.add("x1", "y2", 2);
    t.add("x2", "y1", 3);
    t.add("x2", "y2", 4);

    // instead of filterX, need to pass getX
    // to a function named filter       
    vector<int> vx = t.filter("x1", getX);
    vector<int> vy = t.filter("y2", getY);

    return 0;
}

Answer 1

A member function requires a pointer to the object instance. That is, think of getX as string Row::getX(const Table *this). You need to bind the member function with an instance placeholder.

E.g. using tr1,

vector<int> vx = t.filter("x1", std::bind(&Row::getX, std::placeholders::_1));

The binding creates a functor that takes in a Row object, assuming that your filter function is correctly defined. You should show code for filter. I presume it should be:

template <class function>
vector<int> Table::filter(const string &s, function f)
{
    vector<int> result;
    for (vector<Row>::const_iterator it = d_table.begin(), tEnd = d_table.end();
        it != tEnd; ++it)
    {
        if (s == f(*it)) result.push_back(it->getVal());
    }
    return result;
}

Answer 2

Here is the syntax for the way you want:

vector<int> Table::filter(string s, string (Row::*get)() const)
{                                  //^^^^^^^^^^^^^^^^^^^^^^^^^^^ member pointer
  ...
  if(((*it).*get)() == s) {  // call using the (*it). and not it->
  ... 
}

Call it as:

vector<int> vx = t.filter("x1", &Row::getX);
vector<int> vy = t.filter("y2", &Row::getY);