std::any::type

 

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Symbols index

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std::any

Member functions

any::any

any::~any

any::operator=

Modifiers

any::emplace

any::reset

any::swap

Observers

any::has_value

any::type

Non-member functions

swap(std::any)

any_cast

make_any

 

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 <any>
#include <functional>
#include <iomanip>
#include <iostream>
#include <type_traits>
#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));
}
 
int main()
{
    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 (C++11)

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