1. Andrea Valassi & Alejandro Álvarez IT-SDC White Area lecture 16 th April 2014 C++11 in practice Implications around Boost, ROOT, CORAL, COOL…

2. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture2 Foreword  C++11 is a new language in two complementary ways  1. It introduces radically new concepts and syntaxes (e.g. move semantics)  2. It provides a large number of new standard library classes  The first part of this white area lecture was mainly about new concepts  And was already very dense!  The second part of this lecture will be mainly about the new standard library  And release management when adding C++11 to existing software  Jacek’s course focuses mainly on new concepts  From the standard library, it covers (almost exclusively) smart pointers  It does not cover the new C++11 standard library classes for concurrent programming (some of which will only be very briefly mentioned here)

3. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture3 Outline  Adding C++11 to large inter-dependent software projects  Step 1: building the software with c++11 enabled  Step 2: using c++11 extensions in the software  The LCG Application Area “software stack” example  C++11 and Boost  What is Boost and why it has been so useful  C++11 in CORAL and COOL  Replace Boost by C++11 in CORAL and COOL  Issues with Boost in the COOL API in the port to ROOT6  Eventually get rid of Boost also in the internal implementation  List of C++11 classes we will be using to replace those from Boost

4. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture4 Adding C++11 to existing software  There are two logically separate steps involved  It is easier to take them one after the other (but can also do both in one go)  Step 1 – enable C++11 in the builds of ALL software packages involved  Compile code with -std=c++11  This is a prerequisite for using c++11 extensions in the code  Compiling ALL packages with this option (or without it) is the safest choice  “The C++98 language is ABI-compatible with the C++11 language, but several places in the library break compatibility. This makes it dangerous to link C++98 objects with C++11 objects.” [http://gcc.gnu.org/wiki/Cxx11AbiCompatibility]http://gcc.gnu.org/wiki/Cxx11AbiCompatibility  This is true even if none of your packages use any c++11 extensions!  See the recent discussion about c++11-compatible Grid client builds  Step 2 – start using C++11 extensions at leisure!  Provided there are no other incompatibilities (see next slide on ROOT5)…  And provided the compiler offers good C++11 support (gcc or clang – avoid icc...)  NB: There is no coming back (unless you use #ifdef’s and/or branches)

5. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture5 Example: adding C++11 to the LCG AA stack  Step 1 – enable C++11 in the builds of ALL software packages involved  Convention: disable c++11 until gcc46, enable c++11 as of gcc47 (and clang/icc)  The same release, e.g. LCGCMT_66, can have builds without (gcc46) and with (gcc48) c++11  All external packages are consistently rebuilt using the same build option  We waited for compilers with good enough c++11 support – and we use non-native compilers!  Step 2 – start using C++11 extensions at leisure?  Not yet: only in ROOT6-based stacks (ROOT5 does not support c++11)  Many #ifdef’s in CORAL/COOL code (ROOT5/ROOT6, CORAL2/CORAL3, c++11 compiler option?)  -std=c++11 means “#if defined __GXX_EXPERIMENTAL_CXX0X__ || __cplusplus >= 201103L” CORAL ROOT COOL PyCool PyROOT “Externals” (Boost, SQLite…) LHC exp. sw (Gaudi, Athena…) LCGCMT release LCG Application Area sw: CORAL & COOL are used by ATLAS and LHCb through “LCGCMT” configurations (built centrally by PH-SFT)

6. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture6 Before C++11, we have been using Boost!  What is Boost? [http://www.boost.org]http://www.boost.org  “Boost provides free peer-reviewed portable C++ source libraries.”  The keyword here is portable – use the same code on Linux, Mac, Windows…  “We emphasize libraries that work well with the C++ Standard Library. Boost libraries are intended to be widely useful, and usable across a broad spectrum of applications. The Boost license encourages both commercial and non- commercial use.”  This is software we can use freely in our context without paying a license…  “We aim to establish ‘existing practice’ and provide reference implementations so that Boost libraries are suitable for eventual standardization. Ten Boost libraries are included in the C++ Standards Committee's Library Technical Report (TR1) and in the new C++11 Standard. C++11 also includes several more Boost libraries in addition to those from TR1. More Boost libraries are proposed for TR2.”  Essentially, Boost was born as a working prototype for C++11!...  Boost was there well before C++11: for instance, boost::shared_ptr is used in COOL since 2004

7. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture7 Boost  C++11 in CORAL & COOL – why?  Replacing Boost by C++11 in the COOL and CORAL API was proposed and is being implemented in the context of the port of PyCool to ROOT6  Technicality: the old version of clang JIT used in the current ROOT6 beta does not support inline ASM (assembler) – this will only be supported by the new “MCJIT” (end 2014?)  PyCool is implemented in ROOT6 by feeding COOL public API headers to clang JIT, extracting the relevant “C++ reflection” information and exposing it in Python  COOL public API headers include Boost headers with inline ASM…  Moving the COOL API to C++11 was necessary to test PyCool with ROOT6 beta  Moving the CORAL API too (used by the COOL API) to C++11 is also ongoing  Well, but… replacing Boost by c++11 more generally is not a bad idea!  More modern code – more maintainable and easier to write and read  Removing dependency on a given Boost version may ease release management  Will also try to replace Boost by C++11 throughout CORAL and COOL internals  That said, my guess is that Boost will continue to do well and be very useful to many projects in the future…  “Window of opportunity” – the move to ROOT6 will be a major transition  Seize the moment and include this bunch of disruptive changes in the same release

8. 16 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture8 #include Boost  C++11 in CORAL & COOL – how? Algorithms C++11 – lambdas, Polymorphic types Not yet in C++11! – Used in a single CORAL file Containers C++11 – initializer lists Functions C++11 – std::function, std::bind Date and time C++11 – std::chrono Smart pointers C++11 – std::shared_ptr, std::unique_ptr Also: deprecated std::auto_ptr  std::unique_ptr (move semantics)! Filesystem Not yet in C++11! – Used in several CORAL files Random number generators C++11 – Concurrent programming C++11 – std::thread, std::mutex Tuples C++11 – std::tuple

9. 916 th April 2014A. Valassi and A. Álvarez – C++11 White Area Lecture Questions?