Example:
header:
-------------------------
#include <string>
class NewClass {
public:
    bool _distinct;
    int foo();
    operator std::string() const;
};
-------------------------
source:
-------------------------
#include "NewClass.h"
using namespace std;
NewClass::foo(){
    if (_distinct){
        return 1;
    }
    return 0;
}

NewClass::operator string() const{
    std::string res = "SELECT ";
    if (_distinct){
        res = "";
    }
    return res;
}
-------------------------
member _distinct is not resolved in NewClass::operator string()