Std::any_cast (3) Linux Manual Page

std::any_cast – std::any_cast

Synopsis

template <class T>
(1)(since C++ 17)

    T any_cast(const any &operand);

template <class T>
(2)(since C++ 17)

    T any_cast(any &operand);

template <class T>
(3)(since C++ 17)

    T any_cast(any &&operand);

template <class T>
(4)(since C++ 17)

    const T *any_cast(const any *operand) noexcept;

template <class T>
(5)(since C++ 17)

    T *any_cast(any *operand) noexcept;

Performs type-safe access to the contained object.
Let U be std::remove_cv_t<std::remove_reference_t<T>>.
1) The program is ill-formed if std::is_constructible_v<T, const U&> is false.
2) The program is ill-formed if std::is_constructible_v<T, U&> is false.
3) The program is ill-formed if std::is_constructible_v<T, U> is false.

Parameters

operand – target any object

Return value

1-2) Returns static_cast<T>(*std::any_cast<U>(&operand))
3) Returns static_cast<T>(std::move(*std::any_cast<U>(&operand))).
4-5) If operand is not a null pointer, and the typeid of the requested T matches that of the contents of operand, a pointer to the value contained by operand, otherwise a null pointer.

Exceptions

1-3) Throws std::bad_any_cast if the typeid of the requested T does not match that of the contents of operand.

Example

// Run this code

#include <string>

#include <iostream>

#include <any>

#include <utility>
int main()

{
    // simple example
    auto a = std::any(12);
    std::cout << std::any_cast<int>(a) << '\n';
    try {
        std::cout << std::any_cast<std::string>(a) << '\n';
    } catch (const std::bad_any_cast &e) {
        std::cout << e.what() << '\n';
    }
    // pointer example
    if (int *i = std::any_cast<int>(&a)) {
        std::cout << "a is int: " << *i << '\n';
    } else if (std::string *s = std::any_cast<std::string>(&a)) {
        std::cout << "a is std::string: " << *s << '\n';
    } else {
        std::cout << "a is another type or unset\n";
    }
    // advanced example
    a = std::string("hello");
    auto &ra = std::any_cast<std::string &>(a); //< reference
    ra[1] = 'o';
    std::cout << "a: "
              << std::any_cast<const std::string &>(a) << '\n'; //< const reference
    auto b = std::any_cast<std::string &&>(std::move(a));       //< rvalue reference
    // Note: 'b' is a move-constructed std::string,
    // 'a' is left in valid but unspecified state
    std::cout << "a: " << *std::any_cast<std::string>(&a) //< pointer
              << "b: " << b << '\n';
}

Possible output:

  12
  bad any_cast
  a is int: 12
  a: hollo
  a: b: hollo

std::any_cast (3) Linux Manual Page