1// Copyright (C) 2019 The Qt Company Ltd.
2// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
5 \page qttest-best-practices.qdoc
7 \title Qt Test Best Practices
9 \brief Guidelines for creating Qt tests.
11 We recommend that you add Qt tests for bug fixes and new features. Before
12 you try to fix a bug, add a \e {regression test} (ideally automatic) that
13 fails before the fix, exhibiting the bug, and passes after the fix. While
14 you're developing new features, add tests to verify that they work as
17 Conforming to a set of coding standards will make it more likely for
18 Qt autotests to work reliably in all environments. For example, some
19 tests need to read data from disk. If no standards are set for how this
20 is done, some tests won't be portable. For example, a test that assumes
21 its test-data files are in the current working directory only works for
22 an in-source build. In a shadow build (outside the source directory), the
23 test will fail to find its data.
25 The following sections contain guidelines for writing Qt tests:
28 \li \l {General Principles}
29 \li \l {Writing Reliable Tests}
30 \li \l {Improving Test Output}
31 \li \l {Writing Testable Code}
32 \li \l {Setting up Test Machines}
35 \section1 General Principles
37 The following sections provide general guidelines for writing unit tests:
41 \li \l {Give Test Functions Descriptive Names}
42 \li \l {Write Self-contained Test Functions}
43 \li \l {Test the Full Stack}
44 \li \l {Make Tests Complete Quickly}
45 \li \l {Use Data-driven Testing}
46 \li \l {Use Coverage Tools}
47 \li \l {Select Appropriate Mechanisms to Exclude Tests}
48 \li \l {Avoid Q_ASSERT}
51 \section2 Verify Tests
53 Write and commit your tests along with your fix or new feature on a new
54 branch. Once you're done, you can check out the branch on which your work
55 is based, and then check out into this branch the test-files for your new
56 tests. This enables you to verify that the tests do fail on the prior
57 branch, and therefore actually do catch a bug or test a new feature.
59 For example, the workflow to fix a bug in the \c QDateTime class could be
60 like this if you use the Git version control system:
63 \li Create a branch for your fix and test:
64 \c {git checkout -b fix-branch 5.14}
65 \li Write a test and fix the bug.
66 \li Build and test with both the fix and the new test, to verify that
67 the new test passes with the fix.
68 \li Add the fix and test to your branch:
69 \c {git add tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp src/corelib/time/qdatetime.cpp}
70 \li Commit the fix and test to your branch:
71 \c {git commit -m 'Fix bug in QDateTime'}
72 \li To verify that the test actually catches something for which you
73 needed the fix, checkout the branch you based your own branch on:
74 \c {git checkout 5.14}
75 \li Checkout only the test file to the 5.14 branch:
76 \c {git checkout fix-branch -- tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp}
78 Only the test is now on the fix-branch. The rest of the source tree
80 \li Build and run the test to verify that it fails on 5.14, and
81 therefore does indeed catch a bug.
82 \li You can now return to the fix branch:
83 \c {git checkout fix-branch}
84 \li Alternatively, you can restore your work tree to a clean state on
86 \c{git checkout HEAD -- tests/auto/corelib/time/qdatetime/tst_qdatetime.cpp}
89 When you're reviewing a change, you can adapt this workflow to check that
90 the change does indeed come with a test for a problem it does fix.
92 \section2 Give Test Functions Descriptive Names
94 Naming test cases is important. The test name appears in the failure report
95 for a test run. For data-driven tests, the name of the data row also appears
96 in the failure report. The names give those reading the report a first
97 indication of what has gone wrong.
99 Test function names should make it obvious what the function is trying to
100 test. Do not simply use the bug-tracking identifier, because the identifiers
101 become obsolete if the bug-tracker is replaced. Also, some bug-trackers may
102 not be accessible to all users. When the bug report may be of interest to
103 later readers of the test code, you can mention it in a comment alongside a
104 relevant part of the test.
106 Likewise, when writing data-driven tests, give descriptive names to the
107 test-cases, that indicate what aspect of the functionality each focuses on.
108 Do not simply number the test-case, or use bug-tracking identifiers. Someone
109 reading the test output will have no idea what the numbers or identifiers
110 mean. You can add a comment on the test-row that mentions the bug-tracking
111 identifier, when relevant. It's best to avoid spacing characters and
112 characters that may be significant to command-line shells on which you may
113 want to run tests. This makes it easier to specify the test and tag on \l{Qt
114 Test Command Line Arguments}{the command-line} to your test program - for
115 example, to limit a test run to just one test-case.
117 \section2 Write Self-contained Test Functions
119 Within a test program, test functions should be independent of each other
120 and they should not rely upon previous test functions having been run. You
121 can check this by running the test function on its own with
122 \c {tst_foo testname}.
124 Do not re-use instances of the class under test in several tests. Test
125 instances (for example widgets) should not be member variables of the
126 tests, but preferably be instantiated on the stack to ensure proper
127 cleanup even if a test fails, so that tests do not interfere with
130 \section2 Test the Full Stack
132 If an API is implemented in terms of pluggable or platform-specific backends
133 that do the heavy-lifting, make sure to write tests that cover the
134 code-paths all the way down into the backends. Testing the upper layer API
135 parts using a mock backend is a nice way to isolate errors in the API layer
136 from the backends, but it is complementary to tests that run the actual
137 implementation with real-world data.
139 \section2 Make Tests Complete Quickly
141 Tests should not waste time by being unnecessarily repetitious, by using
142 inappropriately large volumes of test data, or by introducing needless
145 This is particularly true for unit testing, where every second of extra
146 unit test execution time makes CI testing of a branch across multiple
147 targets take longer. Remember that unit testing is separate from load and
148 reliability testing, where larger volumes of test data and longer test
151 Benchmark tests, which typically execute the same test multiple times,
152 should be located in a separate \c tests/benchmarks directory and they
153 should not be mixed with functional unit tests.
155 \section2 Use Data-driven Testing
157 \l{Chapter 2: Data Driven Testing}{Data-driven tests} make it easier to add
158 new tests for boundary conditions found in later bug reports.
160 Using a data-driven test rather than testing several items in sequence in
161 a test saves repetition of very similar code and ensures later cases are
162 tested even when earlier ones fail. It also encourages systematic and
163 uniform testing, because the same tests are applied to each data sample.
165 When a test is data-driven, you can specify its data-tag along with the
166 test-function name, as \c{function:tag}, on the command-line of the test to
167 run the test on just one specific test-case, rather than all test-cases of
168 the function. This can be used for either a global data tag or a local tag,
169 identifying a row from the function's own data; you can even combine them as
170 \c{function:global:local}.
172 \section2 Use Coverage Tools
174 Use a coverage tool such as \l {Coco} or \l {gcov}
175 to help write tests that cover as many statements, branches, and conditions
176 as possible in the function or class being tested. The earlier this is done
177 in the development cycle for a new feature, the easier it will be to catch
178 regressions later when the code is refactored.
180 \section2 Select Appropriate Mechanisms to Exclude Tests
182 It is important to select the appropriate mechanism to exclude inapplicable
183 tests: \l QSKIP(), using conditional statements to exclude parts of a test
184 function, or not building the test for a particular platform.
186 Use QSKIP() to handle cases where a whole test function is found at run-time
187 to be inapplicable in the current test environment. When just a part of a
188 test function is to be skipped, a conditional statement can be used,
189 optionally with a \c qDebug() call to report the reason for skipping the
192 Test functions or data rows of a data-driven test can be limited to
193 particular platforms, or to particular features being enabled using
194 \c{#if}. However, beware of \l moc limitations when using \c{#if} to
195 skip test functions. The \c moc preprocessor does not have access to
196 all the \c builtin macros of the compiler that are often used for
197 feature detection of the compiler. Therefore, \c moc might get a different
198 result for a preprocessor condition from that seen by the rest of your
199 code. This may result in \c moc generating meta-data for a test slot that
200 the actual compiler skips, or omitting the meta-data for a test slot that
201 is actually compiled into the class. In the first case, the test will
202 attempt to run a slot that is not implemented. In the second case, the
203 test will not attempt to run a test slot even though it should.
205 If an entire test program is inapplicable for a specific platform or unless
206 a particular feature is enabled, the best approach is to use the parent
207 directory's build configuration to avoid building the test. For example, if
208 the \c tests/auto/gui/someclass test is not valid for \macOS, wrap its
209 inclusion as a subdirectory in \c{tests/auto/gui/CMakeLists.txt} in a
214 add_subdirectory(someclass)
218 or, if using \c qmake, add the following line to \c tests/auto/gui.pro:
221 mac*: SUBDIRS -= someclass
224 See also \l {Chapter 6: Skipping Tests with QSKIP}
225 {Skipping Tests with QSKIP}.
227 \section2 Avoid Q_ASSERT
229 The \l Q_ASSERT macro causes a program to abort whenever the asserted
230 condition is \c false, but only if the software was built in debug mode.
231 In both release and debug-and-release builds, \c Q_ASSERT does nothing.
233 \c Q_ASSERT should be avoided because it makes tests behave differently
234 depending on whether a debug build is being tested, and because it causes
235 a test to abort immediately, skipping all remaining test functions and
236 returning incomplete or malformed test results.
238 It also skips any tear-down or tidy-up that was supposed to happen at the
239 end of the test, and might therefore leave the workspace in an untidy state,
240 which might cause complications for further tests.
242 Instead of \c Q_ASSERT, the \l QCOMPARE() or \l QVERIFY() macro variants
243 should be used. They cause the current test to report a failure and
244 terminate, but allow the remaining test functions to be executed and the
245 entire test program to terminate normally. \l QVERIFY2() even allows a
246 descriptive error message to be recorded in the test log.
248 \section1 Writing Reliable Tests
250 The following sections provide guidelines for writing reliable tests:
253 \li \l {Avoid Side-effects in Verification Steps}
254 \li \l {Avoid Fixed Timeouts}
255 \li \l {Beware of Timing-dependent Behavior}
256 \li \l {Avoid Bitmap Capture and Comparison}
259 \section2 Avoid Side-effects in Verification Steps
261 When performing verification steps in an autotest using \l QCOMPARE(),
262 \l QVERIFY(), and so on, side-effects should be avoided. Side-effects
263 in verification steps can make a test difficult to understand. Also,
264 they can easily break a test in ways that are difficult to diagnose
265 when the test is changed to use \l QTRY_VERIFY(), \l QTRY_COMPARE() or
266 \l QBENCHMARK(). These can execute the passed expression multiple times,
267 thus repeating any side-effects.
269 When side-effects are unavoidable, ensure that the prior state is restored
270 at the end of the test function, even if the test fails. This commonly
271 requires use of an RAII (resource acquisition is initialization) class
272 that restores state when the function returns, or a \c cleanup() method.
273 Do not simply put the restoration code at the end of the test. If part of
274 the test fails, such code will be skipped and the prior state will not be
277 \section2 Avoid Fixed Timeouts
279 Avoid using hard-coded timeouts, such as QTest::qWait() to wait for some
280 conditions to become true. Consider using the \l QSignalSpy class,
281 the \l QTRY_VERIFY() or \l QTRY_COMPARE() macros, or the \c QSignalSpy
282 class in conjunction with the \c QTRY_ macro variants.
284 The \c qWait() function can be used to set a delay for a fixed period
285 between performing some action and waiting for some asynchronous behavior
286 triggered by that action to be completed. For example, changing the state
287 of a widget and then waiting for the widget to be repainted. However,
288 such timeouts often cause failures when a test written on a workstation is
289 executed on a device, where the expected behavior might take longer to
290 complete. Increasing the fixed timeout to a value several times larger
291 than needed on the slowest test platform is not a good solution, because
292 it slows down the test run on all platforms, particularly for table-driven
295 If the code under test issues Qt signals on completion of the asynchronous
296 behavior, a better approach is to use the \l QSignalSpy class to notify
297 the test function that the verification step can now be performed.
299 If there are no Qt signals, use the \c QTRY_COMPARE() and \c QTRY_VERIFY()
300 macros, which periodically test a specified condition until it becomes true
301 or some maximum timeout is reached. These macros prevent the test from
302 taking longer than necessary, while avoiding breakages when tests are
303 written on workstations and later executed on embedded platforms.
305 If there are no Qt signals, and you are writing the test as part of
306 developing a new API, consider whether the API could benefit from the
307 addition of a signal that reports the completion of the asynchronous
310 \section2 Beware of Timing-dependent Behavior
312 Some test strategies are vulnerable to timing-dependent behavior of certain
313 classes, which can lead to tests that fail only on certain platforms or that
314 do not return consistent results.
316 One example of this is text-entry widgets, which often have a blinking
317 cursor that can make comparisons of captured bitmaps succeed or fail
318 depending on the state of the cursor when the bitmap is captured. This,
319 in turn, may depend on the speed of the machine executing the test.
321 When testing classes that change their state based on timer events, the
322 timer-based behavior needs to be taken into account when performing
323 verification steps. Due to the variety of timing-dependent behavior, there
324 is no single generic solution to this testing problem.
326 For text-entry widgets, potential solutions include disabling the cursor
327 blinking behavior (if the API provides that feature), waiting for the
328 cursor to be in a known state before capturing a bitmap (for example, by
329 subscribing to an appropriate signal if the API provides one), or
330 excluding the area containing the cursor from the bitmap comparison.
332 \section2 Avoid Bitmap Capture and Comparison
334 While verifying test results by capturing and comparing bitmaps is sometimes
335 necessary, it can be quite fragile and labor-intensive.
337 For example, a particular widget may have different appearance on different
338 platforms or with different widget styles, so reference bitmaps may need to
339 be created multiple times and then maintained in the future as Qt's set of
340 supported platforms evolves. Making changes that affect the bitmap thus
341 means having to recreate the expected bitmaps on each supported platform,
342 which would require access to each platform.
344 Bitmap comparisons can also be influenced by factors such as the test
345 machine's screen resolution, bit depth, active theme, color scheme,
346 widget style, active locale (currency symbols, text direction, and so
347 on), font size, transparency effects, and choice of window manager.
349 Where possible, use programmatic means, such as verifying properties of
350 objects and variables, instead of capturing and comparing bitmaps.
352 \section1 Improving Test Output
354 The following sections provide guidelines for producing readable and
358 \li \l {Test for Warnings}
359 \li \l {Avoid Printing Debug Messages from Autotests}
360 \li \l {Write Well-structured Diagnostic Code}
363 \section2 Test for Warnings
365 Just as when building your software, if test output is cluttered with
366 warnings you will find it harder to notice a warning that really is a clue
367 to the emergence of a bug. It is thus prudent to regularly check your test
368 logs for warnings, and other extraneous output, and investigate the
369 causes. When they are signs of a bug, you can make warnings trigger test
372 When the code under test \e should produce messages, such as warnings
373 about misguided use, it is also important to test that it \e does produce
374 them when so used. You can test for expected messages from the code under
375 test, produced by \l qWarning(), \l qDebug(), \l qInfo() and friends,
376 using \l QTest::ignoreMessage(). This will verify that the message is
377 produced and filter it out of the output of the test run. If the message
378 is not produced, the test will fail.
380 If an expected message is only output when Qt is built in debug mode, use
381 \l QLibraryInfo::isDebugBuild() to determine whether the Qt libraries were
382 built in debug mode. Using \c{#ifdef QT_DEBUG} is not enough, as it will
383 only tell you whether \e{the test} was built in debug mode, and that does
384 not guarantee that the \e{Qt libraries} were also built in debug mode.
386 Your tests can (since Qt 6.3) verify that they do not trigger calls to
387 \l qWarning() by calling \l QTest::failOnWarning(). This takes the warning
388 message to test for or a \l QRegularExpression to match against warnings; if
389 a matching warning is produced, it will be reported and cause the test to
390 fail. For example, a test that should produce no warnings at all can
391 \c{QTest::failOnWarning(QRegularExpression(u".*"_s))}, which will match any
394 You can also set the environment variable \c QT_FATAL_WARNINGS to cause
395 warnings to be treated as fatal errors. See \l qWarning() for details; this
396 is not specific to autotests. If warnings would otherwise be lost in vast
397 test logs, the occasional run with this environment variable set can help
398 you to find and eliminate any that do arise.
400 \section2 Avoid Printing Debug Messages from Autotests
402 Autotests should not produce any unhandled warning or debug messages.
403 This will allow the CI Gate to treat new warning or debug messages as
406 Adding debug messages during development is fine, but these should be
407 either disabled or removed before a test is checked in.
409 \section2 Write Well-structured Diagnostic Code
411 Any diagnostic output that would be useful if a test fails should be part
412 of the regular test output rather than being commented-out, disabled by
413 preprocessor directives, or enabled only in debug builds. If a test fails
414 during continuous integration, having all of the relevant diagnostic output
415 in the CI logs could save you a lot of time compared to enabling the
416 diagnostic code and testing again. Epecially, if the failure was on a
417 platform that you don't have on your desktop.
419 Diagnostic messages in tests should use Qt's output mechanisms, such as
420 \c qDebug() and \c qWarning(), rather than \c stdio.h or \c iostream.h output
421 mechanisms. The latter bypass Qt's message handling and prevent the
422 \c -silent command-line option from suppressing the diagnostic messages.
423 This could result in important failure messages being hidden in a large
424 volume of debugging output.
426 \section1 Writing Testable Code
428 The following sections provide guidelines for writing code that is easy to
432 \li \l {Break Dependencies}
433 \li \l {Compile All Classes into Libraries}
436 \section2 Break Dependencies
438 The idea of unit testing is to use every class in isolation. Since many
439 classes instantiate other classes, it is not possible to instantiate one
440 class separately. Therefore, you should use a technique called
441 \e {dependency injection} that separates object creation from object use.
442 A factory is responsible for building object trees. Other objects manipulate
443 these objects through abstract interfaces.
445 This technique works well for data-driven applications. For GUI
446 applications, this approach can be difficult as objects are frequently
447 created and destructed. To verify the correct behavior of classes that
448 depend on abstract interfaces, \e mocking can be used. For example, see
449 \l {Googletest Mocking (gMock) Framework}.
451 \section2 Compile All Classes into Libraries
453 In small to medium sized projects, a build script typically lists all
454 source files and then compiles the executable in one go. This means that
455 the build scripts for the tests must list the needed source files again.
457 It is easier to list the source files and the headers only once in a
458 script to build a static library. Then the \c main() function will be
459 linked against the static library to build the executable and the tests
460 will be linked against the static libraries.
462 For projects where the same source files are used in building several
463 programs, it may be more appropriate to build the shared classes into
464 a dynamically-linked (or shared object) library that each program,
465 including the test programs, can load at run-time. Again, having the
466 compiled code in a library helps to avoid duplication in the description
467 of which components to combine to make the various programs.
469 \section1 Setting up Test Machines
471 The following sections discuss common problems caused by test machine setup:
474 \li \l {Screen Savers}
475 \li \l {System Dialogs}
476 \li \l {Display Usage}
477 \li \l {Window Managers}
480 All of these problems can typically be solved by the judicious use of
483 \section2 Screen Savers
485 Screen savers can interfere with some of the tests for GUI classes, causing
486 unreliable test results. Screen savers should be disabled to ensure that
487 test results are consistent and reliable.
489 \section2 System Dialogs
491 Dialogs displayed unexpectedly by the operating system or other running
492 applications can steal input focus from widgets involved in an autotest,
493 causing unreproducible failures.
495 Examples of typical problems include online update notification dialogs
496 on macOS, false alarms from virus scanners, scheduled tasks such as virus
497 signature updates, software updates pushed out to workstations, and chat
498 programs popping up windows on top of the stack.
500 \section2 Display Usage
502 Some tests use the test machine's display, mouse, and keyboard, and can
503 thus fail if the machine is being used for something else at the same
504 time or if multiple tests are run in parallel.
506 The CI system uses dedicated test machines to avoid this problem, but if
507 you don't have a dedicated test machine, you may be able to solve this
508 problem by running the tests on a second display.
510 On Unix, one can also run the tests on a nested or virtual X-server, such as
511 Xephyr. For example, to run the entire set of tests on Xephyr, execute the
515 Xephyr :1 -ac -screen 1920x1200 >/dev/null 2>&1 &
517 DISPLAY=:1 icewm >/dev/null 2>&1 &
520 DISPLAY=:1 make -k -j1 check
523 Users of NVIDIA binary drivers should note that Xephyr might not be able to
524 provide GLX extensions. Forcing Mesa libGL might help:
527 export LD_PRELOAD=/usr/lib/mesa-diverted/x86_64-linux-gnu/libGL.so.1
530 However, when tests are run on Xephyr and the real X-server with different
531 libGL versions, the QML disk cache can make the tests crash. To avoid this,
532 use \c QML_DISABLE_DISK_CACHE=1.
534 Alternatively, use the offscreen plugin:
537 TESTARGS="-platform offscreen" make check -k -j1
540 \section2 Window Managers
542 On Unix, at least two autotests (\c tst_examples and \c tst_gestures)
543 require a window manager to be running. Therefore, if running these
544 tests under a nested X-server, you must also run a window manager
547 Your window manager must be configured to position all windows on the
548 display automatically. Some windows managers, such as Tab Window Manager
549 (twm), have a mode for manually positioning new windows, and this prevents
550 the test suite from running without user interaction.
552 \note Tab Window Manager is not suitable for running the full suite of
553 Qt autotests, as the \c tst_gestures autotest causes it to forget its
554 configuration and revert to manual window placement.
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