C++: What is std::any in C++17?

1#include <any>
2
3std::any a = 42;           // int
4a = std::string("hello"); // string
5a = 3.14;                 // double
6
7// Access with std::any_cast
8try {
9    int val = std::any_cast<int>(a);  // Throws — a is double
10} catch (const std::bad_any_cast& e) {
11    std::cout << e.what();
12}
13
14// Safe access
15if (auto* p = std::any_cast<double>(&a)) {
16    std::cout << *p;  // 3.14
17}
18
19// Check type
20std::cout << a.type().name();  // double
21
22// Reset
23a.reset();
24std::cout << a.has_value();  // false