This document discusses refactoring strategies for C++ code to enhance testability by addressing common hard-to-test patterns. It highlights the importance of using frameworks like Googletest and Googlemock, and describes characteristics of both hard-to-test and easy-to-test code while providing examples of refactorings for various scenarios. The author shares insights on dependency management, abstract design, and clean architecture principles, emphasizing the need for solid coding practices and effective unit testing.

1. Refactoring for
testability in C++
Hard-to-test patterns in C++ and
how to refactor them

2. About me Dimitrios Platis
● Grew up in Rodos, Greece
● Software Engineer @ Edument,
Gothenburg
● Course responsible @ Gothenburg
University
● Interests:
○ Embedded systems
○ Software Architecture
○ API Design
○ Open source software & hardware
○ Robots, Portable gadgets, IoT
○ 3D printing
○ Autonomous Driving
● Website: https://platis.solutions

5. DIT112-V19
DIT112-V20

6. Agenda
1. GoogleTest & GoogleMock
2. Ηard-to-test code
3. Easy-to-test code
4. Patterns to refactor

7. GoogleTest &
GoogleMock

8. The de facto (?) unit test framework for C++
● xUnit test framework
● Runs on multiple platforms
● Open source
○ https://github.com/google/googletest
● Rich documentation & community
● Easy to integrate with CMake
● No fancy functionality
✔ Mocks, assertions, parameterized tests
✖ DI container, "spy"

9. Hard-to-test code

10. When is code hard to test?
● High complexity
● Tight coupling with other components
● Dependence on global/static states
● Sequential coupling
● ...more!

11. When
it's not fun
● Create fakes containing a lot of logic
● Change visibility of member functions and attributes
● Test things that have been tested before
● Sporadic fails
● Execution takes ages

12. Easy-to-test code

13. When is code easy to test?
● Low complexity
● SOLID
● Prefers composition over inheritance
● Abstracted dependencies
● Functional code without side-effects
● ...more!

14. Patterns to refactor

15. Grain of salt
● Treat these examples as generic guidelines
○ Won't necessarily apply to your project
● Maybe worse performance
○ 3 rules of optimization
● Possibly incompatible with your domain constraints
○ ISO 26262, AUTOSAR, MISRA
● Perhaps easier/cheaper with more advanced test frameworks
○ Fruit

16. Core philosophy
Getting own
dependencies
Managing own
lifecycle
Getting own
configuration
Ignorantly controlling self
Dependencies
injected
Lifecycle
managed from
outside
Configuration
injected
Inversion of Control
Adopted from picocontainer.com article

17. Files

18. Why is it difficult?
bool write(const std::string& filePath,
const std::string& content)
{
std::ofstream outfile(filePath.c_str(),
std::ios::trunc);
if (outfile.good())
{
outfile << content << std::endl;
}
return outfile.good();
}
std::optional<std::string>
read(const std::string& filePath)
{
std::ifstream fileToRead(filePath);
std::stringstream buffer;
buffer << fileToRead.rdbuf();
if (buffer.good())
{
return std::make_optional(buffer.str());
}
return std::nullopt;
}

19. How would you test this?
bool FileEncoder::encode(const std::string& filePath) const
{
const auto validFileContents = read(filePath);
if (!validFileContents)
{
return false;
}
auto encodedFileContents = validFileContents.value();
// Do something with file contents
const auto wroteFileSuccessfully
= write(filePath + ".encoded", encodedFileContents);
return wroteFileSuccessfully;
}

20. Refactor
Abstract
struct FileReader {
virtual ~FileReader() = default;
virtual std::optional<std::string>
read(const std::string& filePath) const = 0;
};
struct FileWriter {
virtual ~FileWriter() = default;
virtual bool
write(const std::string& filePath,
const std::string& content) const = 0;
};
Inject
struct FileEncoder {
FileEncoder(FileReader& fileReader,
FileWriter& fileWriter);
bool encode(const std::string& filePath) const;
private:
FileReader& mFileReader;
FileWriter& mFileWriter;
};

21. Hard-coded
dependencies

22. Why is it difficult?
struct DirectionlessOdometer
{
DirectionlessOdometer(int pulsePin,
int pulsesPerMeter);
double getDistance() const;
protected:
const int mPulsesPerMeter;
int mPulses{0};
MyInterruptServiceManager mInterruptManager;
};
struct DirectionalOdometer
: public DirectionlessOdometer
{
DirectionalOdometer(int directionPin,
int pulsePin,
int pulsesPerMeter);
private:
MyPinReader mPinReader;
const int mDirectionPin;
};

23. How would you test this?
DirectionlessOdometer::DirectionlessOdometer(
int pulsePin, int pulsesPerMeter)
: mPulsesPerMeter{pulsesPerMeter}
{
mInterruptManager.triggerOnNewPulse(
pulsePin, [this]() { mPulses++; });
}
double DirectionlessOdometer::getDistance() const
{
return mPulses == 0 ?
0.0 :
static_cast<double>(mPulsesPerMeter) / mPulses;
}
DirectionalOdometer::DirectionalOdometer(
int directionPin,
int pulsePin,
int pulsesPerMeter)
: DirectionlessOdometer(pulsePin, pulsesPerMeter)
, mDirectionPin{directionPin}
{
mInterruptManager.triggerOnNewPulse(
pulsePin,
[this]() {
mPinReader.read(mDirectionPin) ?
mPulses++ :
mPulses--;
});
}

24. Refactor
Abstract common functionality & inject
struct Encoder
{
virtual ~Encoder() = default;
virtual void incrementPulses() = 0;
virtual void decrementPulses() = 0;
virtual double getDistance() const = 0;
};
struct DirectionlessOdometer
{
DirectionlessOdometer(Encoder& encoder,
InterruptServiceManager& ism,
int pulsePin);
double getDistance() const;
private:
Encoder& mEncoder;
};
Abstract dependencies & inject
struct DirectionalOdometer
{
DirectionalOdometer(Encoder& encoder,
InterruptServiceManager& ism,
PinReader& pinReader,
int directionPin,
int pulsePin);
double getDistance() const;
private:
Encoder& mEncoder;
PinReader& mPinReader;
};

25. Time

26. Why is it difficult?
set(gtest_run_flags --gtest_repeat=1000)

27. How would you test this?
struct PowerController
{
PowerController(PinManager& pinManager,
InterruptManager& interruptManager);
bool turnOn();
private:
PinManager& mPinManager;
std::condition_variable mConditionVariable;
std::atomic<bool> mPulseReceived{false};
std::mutex mRunnerMutex;
};
bool PowerController::turnOn()
{
mPinManager.setPin(kPin);
std::this_thread::sleep_for(1s);
mPinManager.clearPin(kPin);
std::unique_lock<std::mutex> lk(mRunnerMutex);
mConditionVariable.wait_for(
lk,
10s,
[this]() { return mPulseReceived.load(); });
return mPulseReceived.load();
}

28. Refactor
struct TimeKeeper
{
virtual ~TimeKeeper() = default;
virtual void
sleepFor(std::chrono::milliseconds ms) const = 0;
};
struct AsynchronousTimer
{
virtual ~AsynchronousTimer() = default;
virtual void
schedule(std::function<void()> task,
std::chrono::seconds delay) = 0;
};
bool PowerController::turnOn() {
mPinManager.setPin(kPin);
mTimeKeeper.sleepFor(1s);
mPinManager.clearPin(kPin);
mAsynchronousTimer.schedule(
[this]() {
mPulseTimedOut = true;
mConditionVariable.notify_one(); }, 10s);
std::unique_lock<std::mutex> lk(mRunnerMutex);
mConditionVariable.wait(lk, [this]() {
return mPulseReceived.load() ||
mPulseTimedOut.load(); });
mPulseTimedOut = false;
return mPulseReceived.load();
}

29. Domain logic dependent
on application logic

30. Why is it difficult?
● "The dependency rule"
○ Clean architecture
● Circular dependencies
● Domain layer eventually becomes
unsustainable to develop or test
● (C++ specific) Macro madness
Variant A Variant B Variant C
Platform

31. How would you test this?
struct CommunicationManager
{
CommunicationManager(SerialPortClient& serial);
void sendViaSerial(std::string message);
private:
SerialPortClient& mSerialPortClient;
int mSequenceNumber{0};
};
void
CommunicationManager::sendViaSerial(std::string message)
{
#if defined(FOO_PRODUCT)
mSerialPortClient.send(
std::to_string(mSequenceNumber++) + ":" + message);
#elif defined(BAR_PRODUCT)
mSerialPortClient.send("M:" + message + ",");
#else
#error Did you forget to define a product?
#endif
}

32. Refactor
struct SerialFormatter {
virtual ~SerialFormatter() = default;
virtual std::string
format(std::string in) = 0;
};
std::string
BarSerialFormatter::format(std::string in) {
return "M:" + in + ",";
}
std::string
FooSerialFormatter::format(std::string in) {
return std::to_string(mSequenceNumber++) +
":" + input;
}
struct CommunicationManager {
CommunicationManager(
SerialPortClient& serialPortClient,
SerialFormatter& serialFormatter);
void sendViaSerial(std::string message);
private:
SerialPortClient& mSerialPortClient;
SerialFormatter& mSerialFormatter;
};
void
CommunicationManager::sendViaSerial(std::string message) {
mSerialPortClient.send(mSerialFormatter.format(message));
}

33. Refactoring workshop
● 19th of May
● Deeper look
● More patterns
○ Including the dreaded singleton
● Unit tests
● More Q&A
● Hands-on exercises
● 2 hours
● 399 SEK

34. Takeaways
More patterns & working code examples:
platisd/refactoring-for-testability-cpp
Contact me: dimitris@platis.solutions
● Unit tests should be atomic and run fast
● Verify your code (only)
● It should be fun
○ Not much up-front effort
● Abstract & inject
○ Care about what and not how
● Write SOLID code
● Follow OOP best practices
● Follow CPP core guidelines