Dependency injection

dependency_injection.cpp

// https://www.youtube.com/watch?v=zYPb7oBU5_E

#include "time_interface.h"
#include "output_interface.h"

class alarm_clock {
public:
  alarm_clock(const time_interface&, output_interface&)
  // ...
  private:
  // ...
  const time_interface& time_;
  output_interface& output_;
};

void alarm_clock::check_alarm() {
  if (!is_active_) return;
  if (now_is_approx_alarm_time()) {
    ring_alarm();
  }
}

bool alarm_clock::now_is_approx_alarm_time() const {
  auto diff = abs(time_.now() - alarm_time_);
  return (diff < 1s);
}

void alarm_clock::ring_alarm() {
  output_ << "Alarm\n";
}

main.cpp

int main() {
  time_impl t; // implements the time_interface
  output_console o; // implements the output_interface
  alarm_clock alarm(t, o);
  // ...
  return 0;
}

output_interface.h

class output_interface {
public:
  virtual ~output_interface() = default;
  virtual output_interface& operator<<(const string&) = 0;
};

test.cpp

TEST(alarm, checkAlarmAfter10Minutes) {
  time_fake t; // implements the time_interface
  output_fake o; // implements the output interface
  alarm_clock alarm(t, o);
  
  auto ring_time = t.now() + 10min;
  clock.set_alarm(ring_time);
  
  t = t.now() + 10min; // We can increase our time
  EXPECT_EQ(o.value(), "Alarm\n"); // We can check the output
}

time_interface.h

#include <chrono>

class time_interface {
public:
  using time_point = std::chrono::time_point<std::chrono::system_clock>;
  
  virtual ~time_interface() = default;
  virtual time_point now() const = 0;
};