NAME¶

std::any_cast - std::any_cast

Synopsis¶

Defined in header <any>
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.
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 the program ill-formed in
unclear if T is void this case