How to use a member function inherited from base class? - c++

Question

I have a derived class named TimeWithDate inherited from Date class and Time class.
I try to use the member function by using ::.
like this:
int subtract(TimeWithDate& other_date){
return Date::subtract(other_date) + Time::subtract(other_date);
}
but I got this warning:
Error: a nonstatic member reference must be relative to a specific object.
Then I tried this way:
int subtract(TimeWithDate& other_date){
return *(Date*)this.subtract(other_date) + *(Time*)this.subtract(other_date);
}
and got this warning:
Error: 'this' may only be used inside a nonstatic member function.
What should I do?
whole code
#include<iostream>
using namespace std;
class Time
{
int hour, second, minute;
public:
Time();
Time(int h, int m, int s);
void set(int h, int m, int s);
void increment();
void display();
bool equal(Time &other_time);
bool less_than(Time &other_time);
int subtract(Time &another);
};
class Date
{
int year, month, day;
public:
Date();
Date(int y, int m, int d);
void increment();
bool equal(Date &another);
int subtract(Time &another);
};
class TimeWithDate : public Time, public Date
{
public:
bool compare(TimeWithDate&);
void increment();
int subtract(TimeWithDate&);
};
bool TimeWithDate::compare(TimeWithDate &other_date){
if (Date::equal(other_date) && Time::equal(other_date))
return true;
else return false;
}
void TimeWithDate::increment(){
Time::increment();
Time zero(0, 0, 0);
if (Time::equal(zero))
Date::increment();
}
int subtract(TimeWithDate& other_date){
return Date::subtract(other_date) + Time::subtract(other_date);
}

Answer 1

subtract() should be a member function of class TimeWithDate. It appears that you have it as a non-member/ static function. So, this pointer is no more available in that function.

Answer 2

You need parse your whole code, below works fine in my computer(VS2012).
#include <iostream>
using namespace std;
class Base1
{
public:
void print(const char *str){ cout << "base1 " << str << endl; }
};
class Base2
{
public:
void print(const char *str){ cout << "base2 " << str << endl; }
};
class Derived : public Base1, public Base2
{
public:
void print(const char *str);
};
void Derived::print(const char *str)
{
cout << "Derived " << str << endl;
Base1::print(str);
Base2::print(str);
}
int main()
{
Derived d;
d.print("hello");
return 0;
}