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std::any_cast

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< cpp‎ | utility‎ | any
 
 
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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.

Contents

[edit] Parameters

operand - target any object

[edit] 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.

[edit] Exceptions

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

[edit] Example

#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<std::string const&>(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

[edit] 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