Confusing templates in C++17 example of std::visit

Question

When looking at std::visit() page in cppreference, https://en.cppreference.com/w/cpp/utility/variant/visit, I encountered the code I can't make sense of...

Here's the abbreviated version:

#include <iomanip>
#include <iostream>
#include <string>
#include <type_traits>
#include <variant>
#include <vector>

template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
template<class... Ts> overloaded(Ts...)->overloaded<Ts...>;

int main() {
    std::vector<std::variant<int,long,double,std::string>> vec = { 10, 15l, 1.5, "hello" };
    for (auto& v : vec) {
        std::visit(overloaded{
            [](auto arg) { std::cout << arg << ' '; },
            [](double arg) { std::cout << std::fixed << arg << ' '; },
            [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; },
            }, v);
    }
}

What do the two lines declaring overloaded, just above int main(), mean?

Thank you for explaining!

2019 Addition
After the two gentlemen below provided detailed explanations (thank you so much!), I've stumbled upon the same code in the very fine book C++17 in Detail - Learn the Exciting Features of The New C++ Standard! by Bartłomiej Filipek. Such a well written book!

Answer 1

What are the two lines declaring overloaded, just above int main(), mean?

The first one

template<class... Ts>
struct overloaded : Ts... 
 { using Ts::operator()...; };

is a classic class/struct declaration/definition/implementation. Valid from C++11 (because use variadic templates).

In this case, overloaded inherits from all template parameters and enables (using row) all inherited operator(). This is an example of Variadic CRTP.

Unfortunately the variadic using is available only starting from C++17.

The second one

template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;

is a "deduction guide" (see this page for more details) and it's a new C++17 feature.

In your case, the deduction guide says that when you write something as

auto ov = overloaded{ arg1, arg2, arg3, arg4 };

or also

overloaded ov{ arg1, args, arg3, arg4 };

ov becomes an overloaded<decltype(arg1), decltype(arg2), decltype(arg3), decltype(arg4)>

This permits you to write something as

overloaded
{
    [](auto arg) { std::cout << arg << ' '; },
    [](double arg) { std::cout << std::fixed << arg << ' '; },
    [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; },
}

that in C++14 was

auto l1 = [](auto arg) { std::cout << arg << ' '; };
auto l2 = [](double arg) { std::cout << std::fixed << arg << ' '; };
auto l3 = [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; }

overloaded<decltype(l1), decltype(l2), decltype(l3)> ov{l1, l2, l3};

-- EDIT --

As pointed by Nemo (thanks!) in the example code in your question there is another interesting new C++17 feature: the aggregate initialization of base classes.

I mean... when you write

overloaded
{
    [](auto arg) { std::cout << arg << ' '; },
    [](double arg) { std::cout << std::fixed << arg << ' '; },
    [](const std::string& arg) { std::cout << std::quoted(arg) << ' '; }
 }

you're passing three lambda functions to initialize three base classes of overloaded.

Before C++17, you could do this only if you wrote an explicit constructor to do it. Starting from C++17, it works automatically.

At this point, it seems to me that it can be useful to show a simplified full example of your overloaded in C++17 and a corresponding C++14 example.

I propose the following C++17 program

#include <iostream>

template <typename ... Ts>
struct overloaded : public Ts ...
 { using Ts::operator()...; };

template <typename ... Ts> overloaded(Ts...) -> overloaded<Ts...>;

int main ()
{
    overloaded ov
    {
        [](auto arg) { std::cout << "generic: " << arg << std::endl; },
        [](double arg) { std::cout << "double: " << arg << std::endl; },
        [](long arg) { std::cout << "long: " << arg << std::endl; }
    };
    ov(2.1);
    ov(3l);
    ov("foo");      
 }

and the best C++14 alternative (following also the bolov's suggestion of a "make" function and his recursive overloaded example) that I can imagine.

#include <iostream>

template <typename ...>
struct overloaded;

template <typename T0>
struct overloaded<T0> : public T0
{
    template <typename U0>
    overloaded (U0 && u0) : T0 { std::forward<U0>(u0) }
    { }
};

template <typename T0, typename ... Ts>
struct overloaded<T0, Ts...> : public T0, public overloaded<Ts ...>
{
    using T0::operator();
    using overloaded<Ts...>::operator();

    template <typename U0, typename ... Us>
    overloaded (U0 && u0, Us && ... us)
      : T0{std::forward<U0>(u0)}, overloaded<Ts...> { std::forward<Us>(us)... }
    { }
 };

template <typename ... Ts>
auto makeOverloaded (Ts && ... ts)
{
    return overloaded<Ts...>{std::forward<Ts>(ts)...};
}

int main ()
{
    auto  ov
    {
        makeOverloaded
        (
            [](auto arg) { std::cout << "generic: " << arg << std::endl; },
            [](double arg) { std::cout << "double: " << arg << std::endl; },
            [](long arg) { std::cout << "long: " << arg << std::endl; }
        )
    };
    ov(2.1);
    ov(3l);
    ov("foo");      
 }

I suppose that it's matter of opinion, but it seems to me that the C++17 version is a lot simpler and more elegant.

Answer 2

Ahh, I love this.

It's a way to concisely declare a struct with a call operator overloaded on the set of the template arguments call operators.

template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };

overloaded inherits from Ts... and uses all of their operator()

template<class... Ts> overloaded(Ts...)->overloaded<Ts...>;

This is a deduction guide so you don't specify the template parameters

The usage is as you see in the example.

It's a nice utility to create an overloaded set of multiple lambdas (and other function types).

---

Previous to C++17 you would have to use recursion to create overload. Not pretty:

template <class... Fs> struct Overload : Fs...
{
};

template <class Head, class... Tail>
struct Overload<Head, Tail...> : Head, Overload<Tail...>
{
    Overload(Head head, Tail... tail)
        : Head{head}, Overload<Tail...>{tail...}
    {}

    using Head::operator();
    using Overload<Tail...>::operator();
};


template <class F> struct Overload<F> : F
{
    Overload(F f) : F{f} {}

    using F::operator();
};


template <class... Fs> auto make_overload_set(Fs... fs)
{
    return Overload<Fs...>{fs...};
}

auto test()
{
    auto o = make_overload_set(
         [] (int) { return 24; },
         [] (char) { return 11; });

    o(2); // returns 24
    o('a'); // return 11
}

The main nuisance is that Overload because inherits is not an aggregate, so you need to do the recursion trick to create a constructor with all the types. In C++17 overloaded is an aggregate (yey) so constructing one works out of the box :). You also need to specify using::operator() for each of them.