Google Testing Blog: TotT: Avoiding friend Twister in C++

TotT: Avoiding friend Twister in C++

Tuesday, October 30, 2007

(resuming our testing on the toilet posts...)
In a previous episode, we extracted methods to simplify testing in Python. But if these extracted methods make the most sense as private class members, how can you write your production code so it doesn't depend on your test code? In Python this is easy; but in C++, testing private members requires more friend contortions than a game of Twister®.


// my_package/dashboard.h
class Dashboard {
 private:
  scoped_ptr<Database> database_;  // instantiated in constructor

  // Declaration of functions GetResults(), GetResultsFromCache(),
  // GetResultsFromDatabase(), CountPassFail()

  friend class DashboardTest; // one friend declaration per test
                              // fixture
};

You can apply the Extract Class and Extract Interface refactorings to create a new helper class containing the implementation. Forward declare the new interface in the .h of the original class, and have the original class hold a pointer to the interface. (This is similar to the Pimpl idiom.) You can distinguish between the public API and the implementation details by separating the headers into different subdirectories (/my_package/public/ and /my_package/ in this example):


// my_package/public/dashboard.h
class ResultsLog;  // extracted helper interface
class Dashboard {
 public:
  explicit Dashboard(ResultsLog* results) : results_(results) { }
 private:
  scoped_ptr<ResultsLog> results_;
};

// my_package/results_log.h
class ResultsLog {
 public:
  // Declaration of functions GetResults(),
  // GetResultsFromCache(),
  // GetResultsFromDatabase(), CountPassFail()
};

// my_package/live_results_log.h
class LiveResultsLog : public ResultsLog {
 public:
  explicit LiveResultsLog(Database* database)
      : database_(database) { }
};

Now you can test LiveResultsLog without resorting to friend declarations. This also enables you to inject MockResultsLog instance when testing the Dashboard class. The functionality is still private to the original class, and the use of a helper class results in smaller classes with better-defined responsibilities.