Non-Virtual, But Related

Overview

• Goal: eliminate virtual

• Need dynamic dispatch though (want to store base types polymorphically)

• Base class

• static_cast<>, based on a type field that is stored in base class

• Manual virtual dispatch (see code/unrelated/virtual.h): switch based on type field.

Sensor Definitions

code/related-no-virtual/sensors.h

#pragma once

enum class DerivedType
{
    SENSOR1,
    SENSOR2,
};

class Sensor
{
public:
    Sensor(DerivedType type) : type{type} {}
    const DerivedType type;
};

class Sensor1 : public Sensor
{
public:
    Sensor1(double temperature)
    : Sensor{DerivedType::SENSOR1},
      _temperature{temperature} {}

    double get_temperature() { return _temperature; }

private:
    double _temperature;
};

class Sensor2 : public Sensor
{
public:
    Sensor2(double temperature)
    : Sensor{DerivedType::SENSOR2},
      _temperature{temperature} {}

    double get_temperature() { return _temperature; }

private:
    double _temperature;
};

Virtual Access

code/related-no-virtual/virtual.h

#pragma once

#include "sensors.h"

static inline double get_temperature(Sensor* s)
{
    switch (s->type) {
        case DerivedType::SENSOR1: {
            Sensor1* s1 = static_cast<Sensor1*>(s);
            return s1->get_temperature();
        }
        case DerivedType::SENSOR2: {
            Sensor2* s2 = static_cast<Sensor2*>(s);
            return s2->get_temperature();
        }
    }
    return -273.15; // <--- never reached
}

Average Across Many

code/related-no-virtual/avg.h

#pragma once

#include "virtual.h"

double average(auto& sensors)  // <--- abbreviated function template
{
    double sum = 0;
    unsigned long num = 0;
    for (auto& s: sensors) {
        ++num;
        sum += get_temperature(s);
    }
    return sum/num;
}

Main Program

code/related-no-virtual/main.cpp

#include "sensors.h"
#include "avg.h"

#include <iostream>

int main()
{
    Sensor1 s1{37.1};
    Sensor2 s2{42.666};

    Sensor* sensors[] = {&s1, &s2};
    
    std::cout << average(sensors) << std::endl;
    return 0;
}