Exercise: Sensor Interface

See also

• C++ “Interfaces”

• CMake: Local Build

Project Setup

TODO: this needs more thinking

Problem: Polymorphic Thermometer Usage

ConstantSensor and RandomSensor are completely unrelated - they don’t share a common base class. They just look similar. This programming technique, Duck Typing, is heavily used in dynamic languages such as Python (see Abstract Base Classes (abc), And Duck Typing). It is not appropriate in a strongly tyoed language like C++ though, as we will see.

The following program uses an instance of each in an array of a hypothetical interface/base class to measure two temperature points. It calculates the average of both points, and outputs that.

/trainings/material/soup/cxx-code/sensors-core/bin/sensors-avg.cpp

#include <sensor-const.h>
#include <sensor-random.h>

#include <iostream>

int main()
{
    ConstantSensor cs(42.7);
    RandomSensor rs(7, 42);

    Sensor* sensors[2];
    sensors[0] = &cs;
    sensors[1] = &rs;

    double sum = 0;
    unsigned int count = 0;
    for (int i=0; i<2; i++) {
        count++;
        sum += sensors[i]->get_temperature();
    }

    double avg = sum/count;
    std::cout << avg << std::endl;

    return 0;
}

The Exercise

• Download the program into the bin/ directory, and register it with that directory’s CMakeLists.txt file:

  • sensors-avg.cpp

• In addition to the program, download two tests into the tests/ directory, and register them with that directory’s CMakeLists.txt file:

  sensor-const-is-a-sensor.cpp

  #include <sensor-const.h>
  
  #include <gtest/gtest.h>
  
  
  TEST(sensor_const_suite, is_a_sensor)
  {
      ConstantSensor cs(36.4);
  
      Sensor* s = &cs;                                   // <--- ConstantSensor is-a Sensor
  
      // avoid "unused' warning
      (void)s;
  }
  

  sensor-random-is-a-sensor.cpp

  #include <sensor-random.h>
  
  #include <gtest/gtest.h>
  
  
  TEST(sensor_random_suite, is_a_sensor)
  {
      RandomSensor rs(36.4, 42.3);
  
      Sensor* s = &rs;                     // <--- ConstantSensor is-a Sensor
  
      // avoid "unused" warning
      (void)s;
  }
  

• It won’t build

• Add the missing Sensor interface (create a file sensors/sensor.h to contain it), and make the program compile and run. In UML, the new sensor hierarchy would look like follows:

  

• Run the program

  $ ./bin/cxx-exercises-sensors-avg
  27.5731
  

• Run the tests

  $ ./tests/cxx-exercises-sensors-core--suite
  Running main() from /home/jfasch/work/jfasch-home/googletest/googletest/src/gtest_main.cc
  [==========] Running 4 tests from 2 test suites.
  [----------] Global test environment set-up.
  [----------] 2 tests from sensor_const_suite
  [ RUN      ] sensor_const_suite.basic
  [       OK ] sensor_const_suite.basic (0 ms)
  [ RUN      ] sensor_const_suite.is_a_sensor
  [       OK ] sensor_const_suite.is_a_sensor (0 ms)
  [----------] 2 tests from sensor_const_suite (0 ms total)
  
  [----------] 2 tests from sensor_random_suite
  [ RUN      ] sensor_random_suite.basic
  [       OK ] sensor_random_suite.basic (0 ms)
  [ RUN      ] sensor_random_suite.is_a_sensor
  [       OK ] sensor_random_suite.is_a_sensor (0 ms)
  [----------] 2 tests from sensor_random_suite (0 ms total)
  
  [----------] Global test environment tear-down
  [==========] 4 tests from 2 test suites ran. (0 ms total)
  [  PASSED  ] 4 tests.
  

Topics Covered