std::any_cast

#include <any>
#include <iostream>
#include <string>
#include <type_traits>
#include <utility>
 
int main()
{
 // Simple example
 auto a1 = std::any (12);
 std::cout << "1) a1 is int: " << std::any_cast<int>(a1) << '\n';
 
 try
 {
 auto s = std::any_cast<std::string >(a1); // throws
 }
 catch (const std::bad_any_cast & e)
 {
 std::cout << "2) " << e.what() << '\n';
 }
 
 // Pointer example
 if (int* i = std::any_cast<int>(&a1))
 std::cout << "3) a1 is int: " << *i << '\n';
 else if (std::string * s = std::any_cast<std::string >(&a1))
 std::cout << "3) a1 is std::string: " << *s << '\n';
 else
 std::cout << "3) a1 is another type or unset\n";
 
 // Advanced example
 a1 = std::string ("hello");
 auto& ra = std::any_cast<std::string &>(a1); // reference
 ra[1] = 'o';
 
 std::cout << "4) a1 is string: "
 << std::any_cast<const std::string &>(a1) << '\n'; // const reference
 
 auto s1 = std::any_cast<std::string &&>(std::move(a1)); // rvalue reference
 // Note: "s1" is a move-constructed std::string:
 static_assert(std::is_same_v <decltype(s1), std::string >);
 
 // Note: the std::string in "a1" is left in valid but unspecified state
 std::cout << "5) a1.size(): "
 << std::any_cast<std::string >(&a1)->size() // pointer
 << '\n'
 << "6) s1: " << s1 << '\n';
}

1) a1 is int: 12
2) bad any_cast
3) a1 is int: 12
4) a1 is string: hollo
5) a1.size(): 0
6) s1: hollo