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QTest

Applying TDD to Classes Accessing Files

We start with unit tests reading a file line by line with QFile. Unit tests accessing the file system are not considered unit tests, as they may be slow and may depend on each other in surprising ways (see A Set of Unit Testing Rules by Michael Feathers). In a first step, we encapsulate QFile in a class TextFile and provide a fake implementation representing the file as a list of strings kept in main memory. In a second step, we introduce an interface, from which the product and fake implementations derive. We can now apply TDD to classes accessing files.

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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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