std:: any_cast

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C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
std::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<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

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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