Mocking clock in C++ unit test

I'm working on a project that does things depend on how much time has elapsed. Specifically for testing purposes, I need to call a method twice a second. The basic idea of the test is something like this: Class c; // Call the do_something() method 2x/second for 5 seconds for( int num_second = 0; num_second < 5; num_second++ ){ c.do_something(); std::this_thread::sleep_for(std::chrono::milliseconds(500)); c.do_something(); std::this_thread::sleep_for(std::chrono::milliseconds(500)); } In the do_something() method, we then do some operations based on time, similar to the following pseudo-code: void class::do_something(){ now = create_time_from(std::chrono::system_clock::now()); if( now.second() - last_time.second() >= 1 ){ // The second has rolled over - do some special handling code } // Do something important here last_time = now; } Unfortunately this means that the unit tests can be unreliable, as depending on how loaded the system is the do_something() method may not be called at the correct time, resulting in the special handling code not running. Specifically, since it should be called 2x/second, sometimes it gets called only 1x/second due to delays caused by the sleep. I would like to mock the clock to make this more reliable, but I'm not quite sure how to do it. There are two options that I see: Add in a method to the public API of the class that lets you set the clock. I don't want to do this, since this is required only for tests and adding methods just for tests is generally bad. I could extend the class just for testing, providing a mocked clock for testing somehow. This would potentially involve modifying the public API slightly(for example adding a virtual protected method to get the clock to use). Is there a better way to mock the clock in this case?

May 27, 2025 - 10:30

I'm working on a project that does things depend on how much time has elapsed. Specifically for testing purposes, I need to call a method twice a second. The basic idea of the test is something like this:

Class c;

// Call the do_something() method 2x/second for 5 seconds
for( int num_second = 0; num_second < 5; num_second++ ){
  c.do_something();
  std::this_thread::sleep_for(std::chrono::milliseconds(500));
  c.do_something();
  std::this_thread::sleep_for(std::chrono::milliseconds(500));
}

In the do_something() method, we then do some operations based on time, similar to the following pseudo-code:

void class::do_something(){
  now = create_time_from(std::chrono::system_clock::now());
  if( now.second() - last_time.second() >= 1 ){
    // The second has rolled over - do some special handling code
  }
  // Do something important here
  last_time = now;
}

Unfortunately this means that the unit tests can be unreliable, as depending on how loaded the system is the do_something() method may not be called at the correct time, resulting in the special handling code not running. Specifically, since it should be called 2x/second, sometimes it gets called only 1x/second due to delays caused by the sleep.

I would like to mock the clock to make this more reliable, but I'm not quite sure how to do it. There are two options that I see:

Add in a method to the public API of the class that lets you set the clock. I don't want to do this, since this is required only for tests and adding methods just for tests is generally bad.
I could extend the class just for testing, providing a mocked clock for testing somehow. This would potentially involve modifying the public API slightly(for example adding a virtual protected method to get the clock to use).

Is there a better way to mock the clock in this case?