The objective of this session is to introduce [the Julia language](https://julialang.org) and its interfaces to C++. Julia is a high-level programming language with a focus on scientific computing. Julia leverages LLVM to achieve levels of performance comparable to C++. It is a strongly typed, but dynamic language with a strong focus on generics. Similarly to C++, function overloading is supported, resulting in a "multiple dispatch" system that chooses the correct function to call based on the types of the arguments, compiling specialized versions of the function as needed. However, unlike C++, the dispatch decision is delayed until runtime, eliminating the difference between compile time and runtime types. Julia is an excellent high-level companion to a C++ library (compared to e.g. Python). Its combination of high performance and almost zero-overhead FFI to C++ allow interfacing at any level of abstraction, even in inner loops (e.g. the inner loop of matrix multiply) without a significant performance penalty.
There are currently two methods of using C++ libraries within Julia: [Cxx.jl](https://github.com/Keno/Cxx.jl) to write the interfacing code as part of a Julia program, or [CxxWrap.jl](https://github.com/JuliaInterop/CxxWrap.jl) to write the interface in C++. Cxx.jl leverages Clang and LLVM to JIT compile inline C++ code and execute it on the julia execution engine, while CxxWrap.jl uses the existing Julia-C interface from C++. Cxx.jl allows tight integration between Julia and C++ code, interspersing both in the same source file, with arbitrarily nested control flow between the two languages. It also provides a C++ REPL and interactive use of C++ code, similar to Cling. Furthermore, it allows instantiating C++ templates on julia values. In CxxWrap.jl on the other hand, all glue code is in C++ and compiled into a shared library, akin to Boost.Python for Python. This is less flexible with regard to template parameters, but reduces the stress on the JIT-compiler. CxxWrap.jl also offers a few higher-level C++ constructs to facilitate calling into Julia from C++.
In our tutorial, we will start with some examples to introduce the basic Julia concepts, followed by interactive tutorials introducing the usage of both Cxx.jl and CxxWrap.jl. We will finish with a number of examples and benchmarks. By the end of the session, we hope you will agree that Julia is a fast, high-level language that offers excellent mechanisms to use existing C++ libraries without much effort.
Additional material for this talk can be found here:
https://github.com/barche/cppnow2018-julia
