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C++ Testing: Techniques, Tips and Tricks
Clare Macrae (She/her)
clare@claremacrae.co.uk
25 February 2020
Cpp Europe

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About Me
• C++ and Qt developer since 1999
• My mission: Sustainable and efficient testing and refactoring of legacy code
– Co-author of “Approval Tests for C++”
• Consulting & training
– https://claremacrae.co.uk
• All links from this talk via:
– github.com/claremacrae/talks

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Why?
• Share things I wish I knew earlier
– Some well known
– Some less so…
• High-level overview, with links
– Avoiding detail
• Anything unclear?
– Please ask!

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Assumptions
• Value of testing – safety net!
• No worries about types of tests
– (unit, integration, regression)

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Foundations

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Tip: Pick a Test Framework
• Lots to choose from, including:
• Get to know it well!

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Catch2 basic example
// Let Catch provide main():
#define CATCH_CONFIG_MAIN
#include <catch2/catch.hpp>
int Factorial( int number ) {
return number <= 1 ? 1 : Factorial( number - 1 ) * number;
}
TEST_CASE( "Factorial of 0 is 1" ) {
REQUIRE( Factorial(0) == 1 );
}
TEST_CASE( "Factorials of 1 and higher are computed" ) {
REQUIRE( Factorial(1) == 1 );
REQUIRE( Factorial(2) == 2 );
REQUIRE( Factorial(3) == 6 );
REQUIRE( Factorial(10) == 3628800 );
}

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But what about
Real World Code?

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Rarely that simple!
• Long methods
• Interdependent code
• Tests hard to set up
• Not designed to be testable

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Tip: Take any reasonable measure!
• … to get code tested!
• Don’t get hung up on perfect code…
• What does that mean?

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Tip: Take any reasonable measure!
• When you are stuck, write the test you want
• Then safely do whatever you need to your implementation – temporarily!
– Make method static to call from outside?
– Add getters – and maybe even setters?
– Public data?
• Hold your nose!
• Then refactor

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Prioritise Testable code
Over Perfect code

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How do I test private code?
Don’t!

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Tip: Don’t test private code
• Automated tests give freedom to safely change implementation
• Testing private code breaks that
• Only test observable – public – behaviours

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What could possibly go wrong?
• It’s easy to think your test is passing…
• I’ve been bitten by all of these:
– Forget to add test source to build
– IDE calls a different test
– Test is doing the wrong thing
– Test is accidentally disabled

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Never trust a test until
you have seen it fail!

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Book recommendations

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Summary: Foundations
• Use a test framework – get to know it well
• Take any reasonable measure to add tests
– Then improve it
• Test public behaviour – not private implementation
• Testable code over perfect code
• Never trust a test until you have seen it fail

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Private code revisited

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How do I test private code?
Don’t!

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But what if…
• Tiny real world example
• “GoToLineTool” widget
• User can type in to the spinner

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Testing goal
• Test that widget communicates changes correctly when user types in to widget
• lineNumberChanged()
• How to test that?

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Options – none are satisfactory
• Ideas
– Search the widget hierarchy for widgets by name
– Make the spinner widget data member public
– Provide a public accessor to the data member
– Make the test a friend of the class GoToLineTool class
• All if these require the test to be reworked if widget changes

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Goal: Make tests super easy to write
• And super easy to maintain…
• Test code accessing widgets directly makes that hard

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Technique: Separate concerns with Fixture
class GoToLineTool : public
QWidget
{
...
public/protected/private:
QSpinBox* spinBox();
QToolButton* goButton();
Implementation Tests – lots of them!Test Fixture
class GoToLineToolFixture
{
private:
GoToLineTool mGoToLineWidget;
public:
void typeCharacterIntoSpinner(
QChar character);
...
TEST_CASE_METHOD(
GoToLineToolFixture,
"GoToLineTool …")
{
// Ask Fixture to act
// on GoToLineTool
typeCharacterIntoSpinner('1');
}

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Summary: Fixtures
• Test Fixtures can make tests of complex code expressive
• And Maintainable

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Inheriting Legacy/Existing Code

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Typical Scenario
• I've inherited some
legacy code
• It's valuable
• I need to add feature
• Or fix bug
• How can I ever break
out of this loop?
Need to
change
the code
No tests
Not
designed
for testing
Needs
refactoring
to add
tests
Can’t
refactor
without
tests

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Typical Scenario
•
•
•
•
•
Need to
change
the code
No tests
Not
designed
for testing
Needs
refactoring
to add
tests
Can’t
refactor
without
tests
Topics of
this part

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Golden Master Test Setup
Input
Data
Existing
Code
Save
“Golden
Master”

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Golden Master Tests In Use
Input
Data
Updated
Code
Pass
Fail
Output
same?
Yes
No

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Approval Tests
TEST_CASE( "Factorials up to 10" )
{
// Create a container with values 0 ... 10 inclusive
std::vector<int> inputs(11);
std::iota(inputs.begin(), inputs.end(), 0);
// Act on all values in inputs container:
Approvals::verifyAll("Factorial", inputs,
[](auto i, auto& os)
{
os << i << "! => " << Factorial(i);
});
}

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More Info: Quickly Testing Legacy C++ Code
with Approval Tests

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Scenario: Golden Master is a log file
• Dates and times?
• Object addresses?
2019-01-29 15:27

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Options for unstable output
• Introduce date-time abstraction?
• Customised comparison function?
• Or: strip dates from the log file
– Credit: Llewellyn Falco

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Tip: Rewrite output file, before testing it
2019-01-29 15:27 [date-time-stamp]

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Summary: Legacy Code
• Approval Tests
• Be creative to make your tests work for you!
• After adding tests, start refactoring for maintainability

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Testing Qt User Interfaces

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Introducing ApprovalTests.cpp.Qt
•Goals
• Quickly start testing Qt code
– Useful even if you don’t use Approval Tests!
• Approval Tests support for Qt types
– Easy saving of state in Golden Master files
• https://github.com/approvals/ApprovalTests.cpp.Qt
• https://github.com/approvals/ApprovalTests.cpp.Qt.StarterProject
• v.0.0.1

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Example: Checking Table Contents
• Inherited complex code to set up a table
• Want to add at least a first test – of the text in the cells

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Verifying a QTableWidget
TEST_CASE("It approves a QTableWidget")
{
// A note on naming: QTableWidget is a concrete class that implements
// the more general QTableView. Here we create a QTableWidget,
// for convenience.
QTableWidget tableWidget;
populateTable(tableWidget);
ApprovalTestsQt::verifyQTableView(tableWidget);
}

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Approval file: .tsv

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Summary: ApprovalTests.cpp.Qt
• More on this and on using Fixtures to write Expressive, Maintainable Tests
– slideshare.net/ClareMacrae/quickly-testing-qt-desktop-applications

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Maintaining Tests

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Scenario: Tests too slow for CI
• “Builds take 2 to 3 days!” => No hope of Continuous Integration
• Most of that was running tests
• No prospect of speeding up some very slow tests

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Solution: Don’t run slow tests during day
• Mark some tests as Slow
– Filtering built in to Google Test Framework
• Teach CI system to run sub-set of tests
– Only run the Slow tests at night
– Everything else run on every commit
• Thanks to Michael Platings for making this happen!

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Tip: Never tolerate Flickering tests
• Tests that fail randomly
• Rapidly devalue other tests
• Fix them, or delete them!
• Especially don’t mix performance tests with unit tests

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Summary: Maintaining Tests
• Fast feedback from Continuous Integration
• Stomp on flickering tests

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Summary
• It’s never too late to start testing!
– Pick a framework and practice it
– Explore different types of testing
• You can test legacy code too!
• Keep maintaining your tests
• Even GUIs can be tested

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C++ Testing: Techniques, Tips and Tricks
• All links from this talk, and more, via:
– bit.ly/CppTestingTips
– github.com/claremacrae/talks
• Sustainable and efficient testing and refactoring of legacy code
• Consulting & Training
– https://claremacrae.co.uk
– clare@claremacrae.co.uk
• Any Questions?
• Any More Tips?