The Implicit Parameter

Consider the implementation of the hours() member function of the Time class shown here:

int Time::hours() const
{
    return m_hours;
}

int main()
{
    Time t;             				// a Time object
    cout << t.hours() << endl; 	        // value of t::m_hours
}

The hours() member function does not contain a local variable named m_hours. But, the function still compiles and runs correctly. Why?

In a member function, you may directly access and manipulate any or all of the class's data members by referring to them by name. You don't need to indicate that m_hours is a data member, nor do you specify which Time object it belongs to.

C++ assumes that all data members are the data members of the receiver object, and so the line return m_hours means “return the value of the m_hours data member of the object on which this function was invoked.” In such a case, the receiver object is known as the implicit parameter, passed to every member function.

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