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Implementing Unit Tests

For Mail2Voice Next, we introduced unit tests. Units tests are useful to ensure that every classes/methods/functions behave the way we think they are supposed to. It helps the developers to design their code when writing the tests before coding the actual implementation. It helps the developers verifying that the code still works after modifications. Ultimately, it helps developers to detect regressions in the code. Unit tests are one the first tools to use in Quality Assurance.

The unit tests written for Mail2Voice are based on QTest suite.

HOWTO writing a unit test for a class

Each class of Mail2Voice must have its dedicated unit tests to check every methods in relevant scenarios (to test edge cases, side effects, etc.).


Let's say we have a class named MyClass with the following header :

class MyClass
    MyClass(); // Constructor initializing members
    void increment(); // Add 1 to m_count
    void setText(const QString& text); // Set m_text to text
    int count(); // Returns current m_count value
    QString text(); // Returns current m_text value
    int m_count;
    QString m_text;

MyClass is a very simple class that does two things : incrementing an internal variable and setting an internal text. Nothing special here, isn't it? Why should we care about testing such a trivial code?

Wait, mistakes are common and even if you are sure of your code, are you sure that no one else won't introduce errors in your code? That, is where unit tests are useful: ensuring your code is correct and will stay correct over time. Saving you a lot of time of debugging in the future.

What to test?

So, what do we have to test exactly? Basically every methods:

  • first, the constructor because it is in charge of initializing an object of type MyClass,
  • the count() and text() methods to make sure they return correct values,
  • the increment() and setText(const QString& text) to verify they alter members correctly.

To write a unit test, you have to ask yourself what behavior is expected for each method. Let's do this exercise:

  • MyClass(): the constructor must initialize the object, so it must initialize the members m_count and m_text. But, to which values? This have to be specified somewhere otherwise the objects could be inconsistent.
    • For our example, we will pretend that m_count must be initialized to 0 and m_text to “Default text”;
  • increment(): this must add just 1 to m_count. But m_count is an int, meaning it can be negative. What if we increment m_count until 2147483647 (assuming int is on 32 bits) and then increment again? Does the increment method will just put m_count to the minimum negative value (-2147483648)? Or will it refuse to increment again, leaving m_count to 2147483647?
    • For our example, we choose the second option: never go to negative value.
  • setText(const QString& text): now you get it, we will assume this method accept any strings except empty ones (it will do nothing in this case).
  • count() and text() methods must always return the current value.

Coding the unit tests

In the Mail2Voice Qt project, we have a subproject named unittests. For each class, there is a dedicated test class. The header of the test class will be like this:

#include "myClass.h"
#include "propsTester.h"
#include <QObject>
class TestMyClass : public QObject, public PropsTester<MyClass>
    Q_OBJECT // Because we derive from QObject and want Qt features
        explicit TestMyClass(QObject *parent = 0);
    // Below are the unit tests. Each slot will be called via the QTest suite.
    private Q_SLOTS:
        void default_constructor();
        void increment();
        void incrementMax();
        void setText();
        void setTextEmpty();

The PropsTester class is a convenience to check properties against their default values. While it is not mandatory, it is highly recommended to ease the test of class members integrity.

The implementation of TestMyClass will look like this:

#include <QTest>
#include "test_myClass.h"
TestMyClass::TestMyClass(QObject *parent) : QObject(parent)
    // m_propsCheckList stores lambdas functions that check the default value of class members.
    // Each lambda function is associated with a name ("count" and "text" here) representing the tested class member.
    m_propsChecklist.append({"count", [](const MyClass& myClass) { QVERIFY2(myClass.count() == 0, "Count has not been initialized to 0"); }});
    m_propsChecklist.append({"text", [](const MyClass& myClass) { QVERIFY2(myClass.text() == QString("Default text"), "Text is not set to default."); }});
void TestMyClass::default_constructor()
    MyClass myClass; // We just create a myClass object...
    checkDefaultProperties(myClass, {}); //... and use this method to check that the members values are set correctly.
                                         // Note that the second parameter is a list of members to NOT check.
void TestMyClass::increment()
    MyClass myClass;
    for(int i = 1; i <= 10; ++i)
        QVERIFY2(myClass.count() == i, "Count is incorrect.");
        checkDefaultProperties(myClass, {"count"}); // We check that all members are set to default values except m_count
void TestMyClass::incrementMax()
    MyClass myClass;
    for(int i = 0; i < 2147483647; ++i)
    QVERIFY2(myClass.count() == 2147483647, "Count is incorrect."); // Check we are at maximum
    myClass.increment(); // Increment again to see if m_count stays at 2147483647
    checkDefaultProperties(myClass, {"count"}); // Check that nothing else has changed due to side effects
void TestMyClass::setText()
    MyClass myClass;
    myClass.setText("Hello world!");
    QVERIFY2(myClass.text() == "Hello world!", "Text incorrect."); // Check the text has been set properly
    checkDefaultProperties(myClass, {"text"}); // Check that nothing else has changed due to side effects
void TestMyClass::setTextEmpty()
    MyClass myClass;
    QVERIFY2(myClass.text() == "Default text", "Text has been changed."); // Check that the text has not been changed at all.

Then, in the main.cpp file of the unittests subproject, you have to instantiate a TestMyClass object and add it to the list of tests to run:

int main( int argc, char *argv[])
    int ret = 0;
    TestContact tstContact;
    TestEmail tstEmail;
    TestAccount tstAccount;
    TestAttachment tstAttachment;
    TestServerSettings tstServerSettings;
    TestMyClass tstMyClass; // <-- instantiation
    ret = executeTests(argc, argv,
                       &tstMyClass}); // Add test to the suite
    return ret;

Running unit tests

Well, that is the simplest part, just select the unittests subproject in QtCreator, compile it and run it!

unit_tests.txt · Last modified: 2020/08/01 11:10 (external edit)