Add a slide on visitor pattern with lambdas

[sponce]

Originally posted by @sponce in #79 (comment)

[bernhardmgruber]

So this is essentially the overload pattern:

template<typename... Ts>
struct Overload : Ts ... { 
    using Ts::operator()...;
};
template<typename... Ts>
Overload(Ts...) -> Overload<Ts...>;

int main(){
    std::variant<char, long, float, int, double, long long> v;
    auto overloadSet = Overload{
        [](char) { return "char"; },
        [](int) { return "int"; },
        [](unsigned int) { return "unsigned int"; },
        [](long int) { return "long int"; },
        [](long long int) { return "long long int"; },
        [](auto) { return "unknown type"; },
    };
    std::visit(overloadSet, v);
}

(excerpted from here)

There is a lot of stuff going on here like variadic templates, variadic using declarations of member functions, deduction guides and aggregate initialization into base classes. I would still show it to beginners, because the code is short and the usage is easy. But understanding why this works requires more knowledge on the language rules.

[stale]

This issue or pull request has been automatically marked as stale because it has not had recent activity. Please manually close it, if it is no longer relevant, or ask for help or support to help getting it unstuck. Let me bring this to the attention of @klieret @wdconinc @michmx for now.