指向成员函数的函数指针数组

Question

I have the following code

#include<iostream>
using namespace std;
class operate
{
    int x;
    int y;
public:
    operate(int _x, int _y):x(_x), y(_y)
    {}
    void add(const char* ch)
    {
        cout<<ch<<" "<<x+y;
    }
    void subtract(const char* ch)
    {
        cout<<ch<<" "<<x-y;
    }
    void multiply(const char* ch)
    {
        cout<<ch<<" "<<x*y;
    }
};
int main()
{
    void (operate::*fptr[3])(const char*);
    operate obj(2,3);
    fptr[0] = &(operate.add);              //problem
    fptr[1] = &(operate.multiply);         //problem
    fptr[2] = &(operate.subtract);         //problem
    (obj.*fptr[0])("adding");
    (obj.*fptr[1])("multiplying");
    (obj.*fptr[2])("subtracting");
}

It seems I am not assigning the member functions to function pointer array properly. How can I solve this. I'm using VS2010

Answer 1

The dot (member-of) operator is used for accessing members of an object. For classes and namespaces, you have to use the :: operator. Also, don't parenthesize, since & has lower precedence than :: and it's more readable like

fptr[0] = &operate::add;

Answer 2

This should do the job

void testFP()
{
    typedef void (operate::*memFP)(const char*); 
    memFP fptr[3];

    fptr[0] = &operate::add;
    fptr[1] = &operate::multiply;
    fptr[2] = &operate::subtract;
    operate op(42, 42);

    (op.*(fptr[0]))("adding");
}