NAME

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