How can a value from the derived class be assigned to a variable in base class?

Answer 1

Remember that derived classes can access the public and protected members of their base class, but none of the private members. If the member variable in the base class is protected (rather than private), then the derived class can assign it directly. However, this is bad style in OOP as it undermines data-hiding. A protected mutator (set accessor) would be a better option.

Answer 2

We use the class constructor initialiser list to transfer values to the base class constructor.

Consider the following class definitions:

class A {

private:

int m_x;

int m_y;

public:

A (int, int);

// ...

};

class B : public A {

B (int, int);

// ...

};

Class A has two data members. Being built-in types, we must initialise those members during construction otherwise they'd be left in an uninitialised state. We could initialise them via the constructor body:

A::A (int x, int y) {

m_x = x;

m_y = y;

}

However, this is inefficient because both m_x and m_y must exist before we enter the constructor body, so we are actually instantiating both variables first and then initialising them via assignment. Although this costs very little when initialising built-in types, for more complex types initialisation through assignment can add an unacceptable overhead.

It is always more efficient to initialise variables (including member data) at the point of instantiation as soon as we have a value to assign to them. We achieve that in class constructors by using an initialiser list:

A::A (int x, int y): m_x {x}, m_y {y} {}

The constructor initialiser list begins with a colon (:) and is followed by a comma-separated list of initialisers, followed by the (now empty) constructor body. Initialiser lists can also be used to initialise base classes, thus the B class constructor can be implemented in the same way, as follows:

B::B (int x, int y): A {x, y} {}

Note how the initialiser, A {x, y}, invokes the public base class constructor A::A (int, int).