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QTest: Data-Driven Unit Tests Are Hard To Understand

Today, I looked at the data-driven unit tests I had written nearly four weeks ago. It took me a couple of minutes to understand the tests again. Understanding my own tests should have been much easier.

Data-driven unit tests in QTest have a fundamental problem. The data goes in one function, say testWriteFrame_data, and the test case working on the data goes in another function, say testWriteFrame. I must go back and forth to understand the test case. While going back and forth, I typically forget one piece of information. So, I must do another round trip.

So, I sat down and converted each row of the table created by testWriteFrame_data into a test function of its own. The resulting test cases are much easier to understand. They have about the same code size as the original solution. But see for yourself.

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QCOMPARE with Custom Qt Types

When you write unit tests, you will have to compare the actual value and the expected value. A simplified example with QStrings would look like this.

void MyTest::testQCompare() {
    auto actualStr = QString{"abba"};
    auto expectedStr = QString{"juhu"};
    QCOMPARE(actualStr, expectedStr);

When you run this unit test, the QTest framework will print the actual value "abba" and the expected value "juhu". This is often enough to know what went wrong.

FAIL!  : MyTest::testQCompare() Compared values are not the same
   Actual   (actualStr)  : "abba"
   Expected (expectedStr): "juhu"

If two values of a custom type differ, the QTest framework will only print the first line: Compared values are not the same. This is not very helpful. How can you make QCOMPARE print the actual and expected for custom types as well?

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