Pure Virtual Methods, And Interfaces

See also

From A One-Day Overview Of C++

• Key To Polymorphism: virtual

Polymorphic Usage Of Objects

• Derived classes behave as-a sensor ⟶ virtual

• Possibly many derived classes ⟶ there are many sensor types out there, one class for each

• In the current form, the base class Sensor has an implementation of get_value()

Sensor Base Class: One Among Many?

• Questions

  • What sensor kind does Sensor (the base class) implement?

  • Why does it implement anything at all?

  • (Alas, implementations are the business of derived classes)

• Answer

  • Sensor is an interface, not a class

  • C++ does not differentiate, but others (e.g. Java and C#) do

  • Derived classes realize that interface

  • Interfaces don’t implement anything

Towards Interfaces: Pure Virtual Methods

• Pure virtual method: = 0; (like “no implementation”, or NULL pointer)

• Class Sensor becomes abstract

• ⟶ cannot be instantiated

code/pure-virtual-methods/sensors.h

#include <iostream>

class Sensor
{
public:
    virtual double get_value() const = 0;              // <-- purposely *no* implementation
};

class MySensor : public Sensor
{
public:
    MySensor(double base, double correction)
    : _base{base},
      _correction{correction} {}

    double get_value() const override                  // <-- overriding method
    {
        double value = _base + _correction;
        std::cout << "MySensor::get_value(): " << value << std::endl;
        return value;
    }
private:
    double _base;
    double _correction;
};

code/pure-virtual-methods/main.cpp

#include "sensors.h"
#include <iostream>

int main()
{
    MySensor ms{37.3, 0.25};

    Sensor* ps = &ms;

    double value = ps->get_value();
    std::cout << value << std::endl;

    return 0;
}

$ ./code/pure-virtual-methods/cxx-inher-oo-pure-virtual-methods
MySensor::get_value(): 37.55
37.55