Skip to content

Latest commit

 

History

History
40 lines (32 loc) · 2.01 KB

README.md

File metadata and controls

40 lines (32 loc) · 2.01 KB

cppnow2017

Repository for my C++Now 2017 talks.

Implementing variant visitation using lambdas

The addition of std::variant to the upcoming C++17 standard will introduce a "type-safe sum type" to the Standard Library. Variants model a "choice between types" - they essentially are type-safe "tagged unions".

The interface they expose, however, is often more cumbersome to use than it needs to be: defining exhaustive visitors requires the user to create a class with several operator() overloads, or to create a chain of > if constexpr(...) calls. Both solutions are not very elegant.

After a brief overview of std::variant and its usefulness, this talk will focus on the implementation of a "lambda-based in-place visitation" approach, where the user can visit a variant by simply providing a set > of lambdas on the spot. This will require implementing a way of overloading arbitrary function objects.

Recursive variant types will then be covered and the "lambda-based" visitation techniques will be applied to them. This will require implementing the "Y combinator" higher-order function to achieve > zero-runtime-overhead lambda recursion.

This talk is intended for developers familiar with C++11 and C++14 core language features (lambdas, variadic templates, auto, etc...). Prior knowledge of std::variant or sum types is not required.

You must type it three times

Lightning talk about the fact that code has to be manually copy-pasted three times to achieve noexcept-friendliness and SFINAE-friendliness. E.g.

template <typename F, typename... Ts>
auto log_and_call(F&& f, Ts&&... xs)
    noexcept(noexcept(
        std::forward<F>(f)(std::forward<Ts>(xs)...)
    ))
-> decltype(
    std::forward<F>(f)(std::forward<Ts>(xs)...)
)
{
    log << "calling `f`\n";
    return std::forward<F>(f)(std::forward<Ts>(xs)...);
}
  • YouTube Video (not yet available)