What is a Unit Test

• Allows automated testing of independent code units
• No replacement for other test categories
• Write them as early as possible - if possible before the code it tests
• Tests can serve as useful requirements specification
• Create a test for each bug you encounter (refacturing!)

Which problems does unit testing solve?

Code without tests (aka. legacy code) is extremely difficult to work with

Which problems does unit testing cause?

Essentially none, but writing unit tests ...

• seems to cost time
• demands discipline
• is far from trivial
• does not substitute other kinds of testing

What is a Test(case)

• Answers a single question about the code it is testing
• Run completely by itself (automation)
• Results must not require human interpretation
• Should be independent of other tests and their order of execution
• Can serve as documentation of how (not) to use your code

Flaky Tests

• Have different outcomes without changes to the code
• Erode confidence in testing processes
• Fix them (mute / skip them until they are fixed) or delete them

User's view (visible to the user)

• Functionality
• Reliability
• Usability
• Performance/ efficiency

Producer's view

• Portability
• Maintainability
• Testability (vs. legacy code)
• Transparency (is the code readable/ understandable?)

#include <stdint.h>
// Return the sum of all values from 1 to n.
uint64_t add_to(uint32_t n) {
    uint64_t sum = 0;
    for (uint32_t i = 1; i <= n; ++i) {
        sum += i;
    }
    return sum;
}

#include <stdint.h>
// Return the sum of all values from 1 to n.
uint64_t add_to(uint32_t n) {
    return (uint64_t) (n / 2.0) * (n + 1);
}

How can we apply the change without risking to destroy our application?

1. Define good test cases
2. Implement test cases using your project's unit testing library
3. Run the tests before the change to ensure they pass
4. Apply the change, then run the tests again

1. Define Good Test Cases

Using pencil and paper / brainstorming: define relevant test cases

• All code paths should be used
• Add one or more common cases
• Include relevant edge cases

25 is a possible common case. Add 26 as an even input. The edge cases should include 0 and 4294967295.
Expected results for these are 325, 351, 0 and 9223372034707292160.

Let's also add 500000000 (expected result 125000000250000000) as 4294967295 does not terminate with the current implementation.

2. Implement defined test cases

#include "gtest/gtest.h"

uint64_t add_to(uint32_t n);  // usually in header file

TEST(sum_to, fast_test) {
  EXPECT_EQ(add_to(0), 0);
  EXPECT_EQ(add_to(25), 325);
  EXPECT_EQ(add_to(26), 351);
  EXPECT_EQ(add_to(500000000), 125000000250000000);
}

// does not terminate for the old code version due to an overflow of the
// loop counter (the loop would terminate only at UINT_MAX + 1)
TEST(sum_to, slow_test) {
  EXPECT_EQ(add_to(4294967295), 9223372034707292160);
}

3. Run the Unittests Before the Change

clang++ --std=c++17 before_refactoring.cpp \
  test_sum_to.cpp -lgtest_main -lgtest -o run_tests
./run_tests # does not terminate; interrupt via [Ctrl]+[C]

What to do?

./run_tests --help
./run_tests --gtest_list_tests  # list all available tests
./run_tests --gtest_filter=sum_to.fast_test

4. Apply the Change, then Run the Tests Again

For the sake of the presentation, instead of applying the change, we use another source file.

clang++ --std=c++17 after_refactoring.cpp \
  test_sum_to.cpp -lgtest_main -lgtest -o run_tests
./run_tests

Now the tests are fast, but they are failing. Fix the code, then compile and run the tests again.

Installation

sudo apt install googletest
sudo mkdir /usr/src/googletest/build
sudo chown $USER /usr/src/googletest/build
cd /usr/src/googletest/build
cmake ..
make -j4
sudo make install
sudo ldconfig

For other operating systems, follow the instructions in project's README.

Documentation

Comprehensive documentation can be found in GoogleTest User’s Guide.