Function pointers and Optimization flag

[angelo]

Hello, I am new to this group so hello to everybody.
I start with a problem which occur when the optimization
flag is enabled (-O1, ..., -O3) when compiling the following
code which contain function pointers. The code compile
with no problems and works correctly when no
optimization is selected. When I turn-on the optimizer,
the program produces a non-correct result. I used gcc
4.0.3 under Linux Ubuntu.
Thanks in advance for any help or suggestion.

#include <iostream>
//---------------------------------------------------------------------------
// Calc2 use a function pointer initialized at the constructor to point
to
// the selected operation.
// + No need for inheritance
// - Indirection of pointer, hard syntax
//---------------------------------------------------------------------------
class Calc2 {
public:
    Calc2(const char op) {
        switch(op) {
            case '+' : _f = &Calc2::_add; break;
            case '*' : _f = &Calc2::_mul; break;
            case '/' : _f = &Calc2::_div; break;
        }
    }
    int f(const int& a, const int& b) { (this->*_f)(a,b); }
private:
    int (Calc2::*_f)(const int& a, const int& b);
    int _add(const int& a, const int& b) { return (a + b); }
    int _mul(const int& a, const int& b) { return (a * b); }
    int _div(const int& a, const int& b) { return (a / b); }
};

// Main program
int main()
{
    Calc2 calc2('+');
    std::cout << calc2.f(6, 2) << std::endl;
}