NAME¶

std::any::type - std::any::type

Synopsis¶

const std::type_info& type() const noexcept; (since C++17)

Queries the contained type.

Parameters¶

(none)

Return value¶

The typeid of the contained value if instance is non-empty, otherwise typeid(void).

Example¶

The example demonstrates std::any visitor idiom with ability to register new
visitors at compile- and run-time.

// Run this code

#include <type_traits>
#include <any>
#include <functional>
#include <iomanip>
#include <iostream>
#include <typeindex>
#include <typeinfo>
#include <unordered_map>
#include <vector>

template<class T, class F>
inline std::pair<const std::type_index, std::function<void(std::any const&)>>
to_any_visitor(F const &f)
{
return {
std::type_index(typeid(T)),
[g = f](std::any const &a)
{
if constexpr (std::is_void_v<T>)
g();
else
g(std::any_cast<T const&>(a));
}
};
}

static std::unordered_map<
std::type_index, std::function<void(std::any const&)>>
any_visitor {
to_any_visitor<void>([]{ std::cout << "{}"; }),
to_any_visitor<int>([](int x){ std::cout << x; }),
to_any_visitor<unsigned>([](unsigned x){ std::cout << x; }),
to_any_visitor<float>([](float x){ std::cout << x; }),
to_any_visitor<double>([](double x){ std::cout << x; }),
to_any_visitor<char const*>([](char const *s)
{ std::cout << std::quoted(s); }),
// ... add more handlers for your types ...
};

inline void process(const std::any& a)
{
if (const auto it = any_visitor.find(std::type_index(a.type()));
it != any_visitor.cend()) {
it->second(a);
} else {
std::cout << "Unregistered type "<< std::quoted(a.type().name());
}
}

template<class T, class F>
inline void register_any_visitor(F const& f)
{
std::cout << "Register visitor for type "
<< std::quoted(typeid(T).name()) << '\n';
any_visitor.insert(to_any_visitor<T>(f));
}

auto main() -> int
{
std::vector<std::any> va { {}, 42, 123u, 3.14159f, 2.71828, "C++17", };

std::cout << "{ ";
for (const std::any& a : va) {
process(a);
std::cout << ", ";
}
std::cout << "}\n";

process(std::any(0xFULL)); //< Unregistered type "y" (unsigned long long)
std::cout << '\n';

register_any_visitor<unsigned long long>([](auto x) {
std::cout << std::hex << std::showbase << x;
});

process(std::any(0xFULL)); //< OK: 0xf
std::cout << '\n';
}

Possible output:¶

{ {}, 42, 123, 3.14159, 2.71828, "C++17", }
Unregistered type "y"
Register visitor for type "y"
0xf

See also¶

type_index wrapper around a type_info object, that can be used as index in
(C++11) associative and unordered associative containers
(class)