As part of publicising my C++ Now 2015 talk next week, here is part 9 of 20 from its accompanying Handbook of Examples of Best Practice for C++ 11/14 (Boost) libraries:
9. MAINTENANCE: Consider making it possible to use an XML outputting unit testing framework, even if not enabled by default
A very noticeable trend in the libraries reviewed above is that around half use good old C assert() and static_assert() instead of a unit testing framework.
There are many very good reasons not to use a unit testing framework by default, but there are few good reasons to not be able to use a unit testing framework at all. A big problem for the Boost release managers when your library cannot output XML indicating exactly which tests pass and which fail (including the static ones) is that all they get instead is failure to compile or failure to execute. This forces them to dig into compiler error diagnostics and unit test diagnostics respectively. It also makes what may be a very minor problem easily delegated appear as serious as the most serious possible problem because there is no way to quickly disambiguate without diving into potentially a debugger, so all these are good reasons to support some XML outputting unit testing framework which reports an XML entry one per test for each test case in every test suite in your library.
Let me give you an example with Boost.AFIO which executes about a hundred thousand tests for about 70 test platforms and configurations per commit. I once committed a change and noticed in the test matrix that only statically linked libraries were failing. The cause was immediately obvious to me: I had leaked ABI in a way that the unit tests which deliberately build mismatching versions of AFIO to ensure namespace version changes don't conflict had tripped, and without even having to investigate the error itself I knew to revisit my commit for ABI leakage. For someone less familiar with the library, a quick look into the failing test would have revealed the name of the failing test case and instantly realise it was an ABI leakage problem. This sort of extra information is a big win for anyone trying to get a release out the door.
There are big advantages for unit test stability analysis tooling as well. Jenkins CI can record the unit tests for thousands of builds, and if you have a test that regularly but rarely fails then Jenkins can flag such unstable tests. Atlassian tooling free for open source can display unit test aggregate statistics on a dashboard, and free web service tooling able to do ever more sophisticated statistical analysis which you once had to pay for is becoming ever more common.
Finally, specifically for Boost libraries we have an automated regression testing system which works by various end users uploading XML results generated by Boost.Test to an FTP site where a cron script regularly runs to generate static HTML tables of passing and failing tests. Needless to say, if your library was as useful as possible to that system everybody wins, and your library is not as useful to that system if it uses assert() and even static_assert() because the XML uploaded is a compiler error console log or an assert failure diagnostic instead of a detailed list of which tests passed and which failed.
Hopefully by now I have persuaded you to use an XML outputting unit test framework. If you are a Boost library, the obvious choice is to use Boost.Test. Despite its many problems, being slow to develop against and lack of maintenance in its release branch, Boost.Test is still a very competitive choice, and if you ignore the overly dense documentation and simply lift the pattern from this quick sample you'll be up and running very quickly:
#include "boost/test/unit_test.hpp" // Note the lack of angle brackets
BOOST_AUTO_TEST_SUITE(all) // Can actually be any name you like
BOOST_AUTO_TEST_CASE(works/spinlock, "Tests that the spinlock works as intended") // Note the forward slashes in the test name
// More BOOST_AUTO_TEST_CASE(), as many as is wanted
Already those familiar with Boost.Test will notice some unusual choices, but I'll come back to why shortly. For reference there are additional common tests in addition to BOOST_REQUIRE:
Check if expr is true, continuing the test case anyway if false.
Check if expr throws an exception, continuing the test case anyway if false.
Check if expr throws an exception of a specific type, continuing the test case anyway if false.
Check if expr does not throw an exception, continuing the test case anyway if false.
Check if expr is true, immediately exiting the test case if false.
Check if expr throws an exception, immediately exiting the test case if false.
Check if expr throws an exception of a specific type, immediately exiting the test case if false.
Check if expr does not throw an exception, immediately exiting the test case if false.
Log a message with the XML output.
If pred is false, log a message with the XML output.
If pred is false, log a warning message with the XML output.
Immediately exit this test case with a message.
Boost.Test provides an enormous amount of extra stuff (especially in its unstable branch) for all sorts of advanced testing scenarios, but for most software being developed in a person's free time most of those advanced testing facilities don't provide sufficient benefit for the significant added cost of implementation. Hence, for personally developed open source the above primitive checks, or a combination thereof into more complex solutions, is likely sufficient for 99% of C++ code. There is also a very specific reason I chose this exact subset of Boost.Test's functionality to suggest using here, because Boost.APIBind's lightweight header only Boost.Test emulation defines just the above subset and usefully does so into a header inside APIBind called "boost/test/unit_test.hpp" which is identical to the Boost.Test header path, so if you include just that header you get compatibility with APIBind and Boost.Test. In other words, by using the pattern just suggested you can:
1. With a macro switch turn on full fat Boost.Test.
2. For the default use Boost.APIBind's thin wrap of the CATCH header only unit testing library which I have forked with added thread safety support. CATCH is very convenient to develop against, provides pretty coloured console unit test output and useful diagnostics, and on request on the command line can also output JUnit format XML ready for consumption by almost every unit test XML consuming tool out there. Boost.Test theoretically can be used header only, but you'll find it's very hard on your compile times, whereas CATCH is always header only and has a minimal effect on compile time. CATCH also comes as a single kitchen sink header file, and APIBind includes a copy for you.
3. For those so motivated that they really want assert() and nothing more, simply wrap the above macros with calls to assert(). Your single unit test code base can now target up to three separate ways of reporting unit test fails.
Note if CATCH doesn't have enough features and Boost.Test is too flaky, another popular choice with tons of bells and whistles is Google Test. Like Boost.Test its Windows support is sadly also a bit flaky - in many ways for advanced testing scenarios the Microsoft Visual Studio test tooling is hard to beat on Windows, and now they are porting Visual Studio to all other platforms it may become the one to watch in the future - another good reason to get your C++ 11/14 codebase working perfectly on VS2015.
What are the problems with replacing asserts with a unit test framework?
1. Asserts are fast and don't synchronise threads. Unit test frameworks almost always must grab a mutex for every single check, even if that check passes, which can profoundly damage the effectiveness of your testing. The obvious workaround is to prepend an if statement of the test before every check, so if(!expr) BOOST_CHECK(expr); but be aware now only failures will be output into XML, and many CI parsers will consider zero XML test results in a test case to be a fatal error (workaround: always do a BOOST_CHECK(true) at the very end of the test).
2. Getting static_asserts to decay cleanly into a BOOST_CHECK without #ifdef-ing is not always obvious. The obvious beginning is:
#define BOOST_CHECK_MESSAGE(pred, msg) static_assert(pred, msg)
... and now use BOOST_CHECK_MESSAGE instead of static_assert directly. If your static assert is inside library implementation code, consider a macro which the unit tests override when being built with a unit test framework, but otherwise defaults to static_assert.
3. Asserts have no effect when NDEBUG is defined. Your test code may assume this for optimised builds, and a simple regex find and replace may not be sufficient.
Libraries implementing XML outputting unit testing with the Boost.Test macro API:
#cpp #cplusplus #cppnow #cppnow2015 #c++ #boostcpp #c++11 #c++14