Links To Examples Of Best Practice For C++ 11/14 Libraries

originally written by ​Niall Douglas May 2015

As part of preparing my ​C++ Now 2015 presentation "​A review of C++ 11/14 only Boost libraries" I examined ten C++ 11/14 mandatory libraries heading towards Boost which were:

Name	Authors	Min C++	Boost headers required	Entered Boost peer review queue	Description	Repository
Boost.Fiber	Oliver Kowalke	14	Context, Config	Conditionally accepted	a framework for micro-/userland-threads (fibers) scheduled cooperatively	​https://github.com/olk/boost-fiber
Boost.AFIO	Niall Douglas, Paul Kirth	11	none	2013-10	strongly ordered portable asynchronous filesystem and file i/o extending ASIO	​https://github.com/BoostGSoC13/boost.afio
Boost.DI	Krzysztof Jusiak	14	none	2015-01	provides compile time, macro free constructor dependency injection	​https://github.com/krzysztof-jusiak/di
Boost.Hana	Louis Dionne	14	none	2015-04	functional programming in C++ based around transformations of heterogeneous collections of types.	​https://github.com/ldionne/hana
Boost.Http	Vinícius dos Santos Oliveira	11	asio, filesystem, system, datetime, utility	2015-04	create applications that need to expose services through HTTP	​https://github.com/BoostGSoC14/boost.http
Boost.APIBind	Niall Douglas	11	none	not submitted yet	toolkit for modularizing Boost libraries, managing versioned API and ABI dependency binds, makes it easy for Boost libraries to switch between use of the C++ 11 STL and the Boost STL	​https://github.com/ned14/Boost.BindLib
Boost.Expected	Pierre Talbot, Vicente J. Botet Escriba	11	none	not submitted yet	category and functional programming in C++ based around the monadic expected<T, E>	​https://github.com/ptal/expected
Boost.Tick	Paul Fultz II	11	none	not submitted yet	trait introspection and concept creator for C++11	​https://github.com/pfultz2/Tick
Boost.Fit	Paul Fultz II	11	none	not submitted yet	provides utilities for functions and function objects	​https://github.com/pfultz2/Fit
Boost.Sqlpp11	Roland Bock	11	none	not submitted yet	A type safe embedded domain specific language for SQL queries and results in C++	​https://github.com/rbock/sqlpp11

There was very significant divergence in best practices between these libraries at the time of examination, and moreover the divergences were uneven possibly reflecting an uneven understanding of what best practice might be in C++ 11/14. Thence from my examination I have prepared the list below of what I felt were best practices in C++ 11/14 mandatory libraries with hyperlinks to the relevant source code files in the relevant libraries above. As what a "best practice" is or is not may vary according to opinion, I have attempted to provide a rationale for each of the suggestions below. It is up to library authors to weigh each suggestion and to decide whether to apply it, or not, to their library.

One of the most strikingly consistent features of these new libraries is the lack of dependence on Boost, even to the extent of not using the boost namespace as well as no Boost at all. The big advantage of this is modularity, so almost all of these libraries can be used standalone which is an oft requested feature by Boost end users. However this is likely not sustainable as more C++ 11/14 libraries become useful to other C++ 11/14 libraries and coupling therefore increases. I therefore dedicate significant effort below into how best to couple your library to other libraries which has very significantly improved with new C++ 11 features.

One will note that the list below is much more C++ 11/14 focused than Boost focused. This is because it is derived from the first crop of C++ 11/14 mandatory Boost libraries. This is not a handbook for writing Boost libraries or even C++ 11/14 Boost libraries, if you want that first start reading here (note some of the guidelines on that page don't really apply to C++ 11/14 libraries) and ​then read here and ​here.

I have tried to keep these points generic to all C++ 11/14 libraries in the hope that they are useful far outside Boost. I have also ordered them with what I consider the most important ("strongly consider") first and not as important ("consider") later.

1. Strongly consider using git and ​GitHub to host a copy of your library and its documentation

All the Boost libraries are on github, and all the free tooling exampled below integrates easily with github. Choosing github makes your life much easier. Note that as git is a distributed source code control system, you can keep a canonical master copy anywhere and write a script which autorefreshes the github copy, thus triggering any of the free tooling you have integrated there.

Github also provides free website hosting for HTML. Have a script automatically generate documentation and commit it to the gh-pages branch in your normal repo. This should present a copy of your HTML at http:// username.github.io/repository.

This is the script which generates the documentation for ​proposed Boost.AFIO, and indeed you can see the exact output generated by this script at ​http://boostgsoc13.github.io/boost.afio/. You may find it useful.

cd boost-local
rm -rf libs/afio/doc/doxy/doxygen_output/html
mkdir -p libs/afio/doc/doxy/doxygen_output/html
cd doc
../b2 -a ../libs/afio/doc
cd ../..
if [ ! -e publish ]; then
git clone -b gh-pages git@github.com:BoostGSoC13/boost.afio.git publish
fi
cd publish
git reset --hard b1414e11be50ff81124e2e1583f1bbb734ad9ead
cd ..
rm -rf publish/*
mkdir -p publish/doc/html
cp -a boost-local/doc/html/afio* publish/doc/html/
cp -a doc/src publish/doc/
cp -a doc/src/images/boost.png publish/
cp -af boost-local/doc/src publish/doc/
mkdir -p publish/libs/afio/doc
cp -a doc/* publish/libs/afio/doc/
cd boost-local/libs/afio/doc/doxy
doxygen
cd ../../../../../publish
cp -a ../boost-local/libs/afio/doc/doxy/doxygen_output/html .
cp -a ../Readme.md .
cp -a ../Readme.md Readme.html
echo '<html><head><title>Boost.AFIO documentation</title><meta http-equiv="refresh" content="300"/><body>
<h1>Boost.AFIO documentation</h1>
<p><a href="doc/html/afio.html">BoostBook format documentation</a></p>
<p><a href="html/index.html">Doxygen format documentation</a></p>
<p><a href="afio-stable.tar.bz2">Ready to go stable AFIO distribution with all git submodules (from master branch)</a></p>
<p></p>' > index.html
cat Readme.md | tail -n +4 >> index.html
echo '
</body></html>' >> index.html
git add .
git commit -a -m 'SHA stamp by Jenkins as building correctly' || true
git push -f

Some may wonder what the hard git reset to a SHA is for. This prevents the gh-pages branch continuously growing in storage by breaking history for the branch, and therefore making git clone times grow excessively. As the branch is purely for HTML publishing, breaking history like this is safe.

Other examples of libraries which use github for their documentation:

• ​https://olk.github.io/libs/fiber/doc/html/
• ​https://boostgsoc13.github.io/boost.afio/
• ​https://krzysztof-jusiak.github.io/di/boost/libs/di/doc/html/
• ​https://boostgsoc14.github.io/boost.http/

2. Strongly consider versioning your library's namespace using inline namespaces and requesting users to alias a versioned namespace instead of using it directly

C++ 11 adds a new feature called inline namespaces which are far more powerful than they first appear:

namespace boost { namespace afio { inline namespace v1 { /* stuff */ } } }
// Stuff is generated at the ABI link layer in boost::afio::v1
// But to the compiler everything boost::afio::v1::* appears identically in boost::afio::*
// INCLUDING for ADL and overload resolution
// In other words you can declare your code in boost::afio::v1, and use it as if declared in boost::afio.

// The other important new C++ 11 feature is namespace aliasing, so
namespace local { namespace afio = boost::afio; /* use afio::* and it all works */ }

The reason this pattern is so useful is because it greatly eases the lives of your end users and you the library maintainer in years to come when you need to break API compatibility. Let's take a case example: imagine the situation typical in 03 C++ libraries where library Boost.Foo uses dependent library Boost.AFIO:

namespace boost { namespace afio {
  struct foo {
    static void api(int i, double f);
  };
} }
...
namespace boost { namespace foo {
  boost::afio::api(1, 2);
} }

Imagine that you now release an API breaking refactor of Boost.AFIO, which would look like this:

namespace boost { namespace afio {
  struct foo {
    static void api(double f, int i);  // Oh dear, f and i have been swapped!
  };
} }
...
namespace boost { namespace foo {
  boost::afio::api(1, 2);  // This is probably now a bug!
} }

The users of Boost.Foo which uses boost::afio::foo::api() now finds that their library no longer passes its unit testing because foo::api() has changed from before. They will quite rightly throw up a fuss, and under Boost's present rules you will be asked to roll back your refactor until Boost.Foo has also been refactored to match your refactor. This causes inconvenience for you the maintainer of Boost.AFIO, the maintainer of Boost.Foo, and is a general pain for users. It also breaks modularity, increases coupling between libraries in a way which saddles you the maintainer of Boost.AFIO with the lack of maintenance or failure of timely maintenance of libraries dependent on AFIO, and I can't strongly enough recommend you don't blindly copy the 03 idiom of suggesting client code use your library directly using fully qualified namespacing.

The good news is we can make all this go away with inline namespaces and namespace aliasing, so consider this pattern instead:

namespace boost { namespace afio { inline namespace v1 {
  struct foo {
    static void api(int i, double f);
  };
} } }
...
namespace boost { namespace foo {
  // Probably somewhere in this library's config.hpp
  namespace afio = boost::afio;  // This is the key use change which needs to be strongly recommended to your library's users
  ...
  // In implementation code after config.hpp
  afio::api(1, 2);  // Note the no longer fully qualified use of afio. The local namespace alias is used to "symlink" to "the latest" version of Boost.AFIO
} }

Now imagine your refactor occurs as before:

namespace boost { namespace afio {
  // Probably defined by boost/afio.hpp which in turn includes boost/afio_v2.hpp
  inline namespace v2 {
    struct foo {
      static void api(double f, int i);  // new implementation
    };
  }
  // Probably defined by boost/afio_v1.hpp
  namespace v1 {
    struct foo {
      static void api(int i, double f);  // old implementation
    };
  }
} }
...
namespace boost { namespace foo {
  // Probably somewhere in this library's config.hpp
  namespace afio = boost::afio::v1;  // By changing this one single line we "fix" the problem. Earlier we included <boost/afio_v1.hpp> instead of <boost/afio.hpp>.
  ...
  // In implementation code after config.hpp
  afio::api(1, 2);  // And this finds the v1 AFIO implementation, not the v2 implementation
} }

What have we just achieved?

1. Library Boost.Foo dependent on Boost.AFIO no longer requires lots of refactoring work if Boost.AFIO is refactored. Just two lines changed in its config.hpp, something easy for the release managers to do.
2. Library Boost.AFIO can now be evolved far quicker than before, and simply keep shipping entire copies of legacy versions without problems with colliding namespaces. As end users get round to upgrading, legacy versions can be removed from the distro after a period of warning.

What are the problems with this technique?

1. You now need to ship multiple copies of your library, maintain multiple copies of your library, and make sure simultaneous use of multiple library versions in the same executable doesn't conflict. I suspect this cost is worth it for the added flexibility to evolve breaking changes for most library maintainers.
2. The above technique alone is insufficient for header only end users where multiple versions of your library must coexist within the same translation unit. With some additional extra work, it is possible to allow multiple header only library versions to also coexist in the same translation unit, but this is covered in a separate recommendation below.
3. Many end users are not used to locally aliasing a library namespace in order to use it, and may continue to directly qualify it using the 03 idiom. You may consider defaulting to not using an inline namespace for the version to make sure users don't end up doing this in ways which hurt themselves, but that approach has both pros and cons.

Some fun extra things this technique enables:

1. Something not so obvious above is that you can also stub out fake copies of dependencies where that dependency is missing in the current config. For example, imagine optional compression support where your config.hpp either namespace aliases to boost::foo::compression either a real compression library, or an internal stub copy which actually does nothing. Your code is then written to assume a compression library aliased at boost::foo::compression and need not consider if it's actually there or not. The advantages here for reducing coupling are very obvious.
2. This technique is highly extensible to allow dependency injection of STL11 vs Boost on a per-feature basis e.g. your user wants Boost.Thread instead of STL11 thread but only for threading, so your library can be so modular as to allow both options to end users. This is covered in a separate recommendation below.

3. Strong consider trying your library on Microsoft Visual Studio 2015

More than half the libraries reviewed had no support for Microsoft Visual Studio, and only supported GCC and clang. When the authors were asked why, in many cases it was simply assumed that MSVC didn't implement much C++ 11/14 and the authors hadn't attempted MSVC support.

This is in fact untrue. Here is a complete list of C++ 11/14 features which VS2015 does NOT support (everything else you can assume it supports, including a full and complete C++ 14 STL):

• Expression SFINAE (there are workarounds. Note the STL "turns on" magic Expression SFINAE support for those parts of the STL requiring it, so any Expression SFINAE done by the STL for you works as expected).
• Any serious constexpr use (try "#define constexpr" i.e. disable it completely. Most of the time your code will compile and work. Consider using a BOOST_CONSTEXPR macro thereafter). It is claimed by Microsoft that full C++ 11 constexpr conformance is present, but to be honest in my own code I don't find anything less than C++ 14 constexpr useful in practice.
• No two phase lookup. Reordering decls to make the compiler look up in a way which doesn't produce non-conforming outcomes is a straightforward, if annoying, workaround.
• MSVC's C99 support is still less than complete, but it's significantly more complete than before.
• MSVC's preprocessor is still non-conforming, but it's less broken than it has ever been.
• Variable templates.
• Non-static data member initialisers for aggregates.

VS2015 is a very highly conforming C++ 11/14 compiler. It meets or exceeds clang 3.3 on every C++ feature, so if your library can be compiled by clang 3.3 then it is highly likely it should compile, without too much work, on VS2015. VS2015 even has some support for C++ 1z (C++ 17) matching about what clang 3.5 provides with the main showstoppers being lack of relaxed constexpr and no variable templates. See ​http://blogs.msdn.com/b/vcblog/archive/2015/04/29/c-11-14-17-features-in-vs-2015-rc.aspx.

I am not claiming that you won't get surprises when you try getting MSVC to compile your code which you thought was standards compliant. MSVC is not an AST based compiler and uses heuristics to trigger partial AST compilation, and therefore has a unique processing model which exposes your assumptions about what you think or thought is valid C++. This is exactly why it is worthwhile getting your C++ 11/14 library working on MSVC because you will get a better quality, more standards conforming C++ library out of it.

4. 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 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 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 an arse to develop against and lack of maintenance, 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
{
  boost::spinlock::spinlock<bool> lock;
  BOOST_REQUIRE(lock.try_lock());
  BOOST_REQUIRE(!lock.try_lock());
  lock.unlock();
  
  std::lock_guard<decltype(lock)> h(lock);
  BOOST_REQUIRE(!lock.try_lock());
}
// More BOOST_AUTO_TEST_CASE(), as many as is wanted

BOOST_AUTO_TEST_SUITE_END()

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:

BOOST_CHECK(expr)

Check if expr is true, continuing the test case anyway if false.

BOOST_CHECK_THROWS(expr)

Check if expr throws an exception, continuing the test case anyway if false.

BOOST_CHECK_THROW(expr, type)

Check if expr throws an exception of a specific type, continuing the test case anyway if false.

BOOST_CHECK_NO_THROW(expr)

Check if expr does not throw an exception, continuing the test case anyway if false.

BOOST_REQUIRE(expr)

Check if expr is true, immediately exiting the test case if false.

BOOST_REQUIRE_THROWS(expr)

Check if expr throws an exception, immediately exiting the test case if false.

BOOST_REQUIRE_THROW(expr, type)

Check if expr throws an exception of a specific type, immediately exiting the test case if false.

BOOST_REQUIRE_NO_THROW(expr)

Check if expr does not throw an exception, immediately exiting the test case if false.

BOOST_TEST_MESSAGE(msg)

Log a message with the XML output.

BOOST_CHECK_MESSAGE(pred, msg)

If pred is false, log a message with the XML output.

BOOST_WARN_MESSAGE(pred, msg)

If pred is false, log a warning message with the XML output.

BOOST_FAIL(msg)

Immediately exit this test case with a message.

Boost.Test provides an enormous amount of extra stuff (especially in its unstable branch), but for 99% of C++ code the above is all you will ever need. 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", 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 slow, 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 is ​Google Test.

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:

#ifndef AM_USING_BOOST_TEST_FOR_STATIC_ASSERTS
#define BOOST_CHECK_MESSAGE(pred, msg) static_assert(pred, msg)
#endif

... 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.

5. Consider not doing compiler feature detection yourself

TODO