# std::any_cast

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< cpp‎ | utility‎ | any

Defined in header `<any>` |  |
template< class T > T any_cast( const any& operand ); | (1) | (since C++17)
template< class T > T any_cast( any& operand ); | (2) | (since C++17)
template< class T > T any_cast( any&& operand ); | (3) | (since C++17)
template< class T > const T* any_cast( const any* operand ) noexcept; | (4) | (since C++17)
template< class T > T* any_cast( any* operand ) noexcept; | (5) | (since C++17)

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.
4,5) The program is ill-formed if std::is_void_v <T> is true.

### 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 <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';
    }

Possible output:

    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

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3305 | C++17 | the behavior of overloads (4,5) was unclear if `T` is void | the program ill-formed in this case

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