std::any_cast

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

Utilities library

Type support (basic types, RTTI, type traits)
Dynamic memory management
Error handling
Program utilities
Variadic functions
Library feature-test macros
Date and time
Function objects
initializer_list (C++11)
bitset
hash (C++11)
integer_sequence (C++14)
Relational operators (deprecated)
rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
Comparisons (C++20)
strong_order weak_order partial_order strong_equal weak_equal	strong_ordering weak_ordering partial_ordering strong_equality weak_equality	is_eqis_neqis_ltis_lteqis_gtis_gteq
common_comparison_category
Common vocabulary types
pair tuple (C++11) apply (C++17) make_from_tuple (C++17)	optional (C++17) any (C++17) variant (C++17)
Swap, forward and move
swap
exchange (C++14)
forward (C++11)
move (C++11)
move_if_noexcept (C++11)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
Type operations
declval (C++11)
as_const (C++17)

std::any

Member functions

any::any

any::~any

any::operator=

Modifers

any::emplace

any::reset

any::swap

Observers

any::has_value

any::type

Non-member functions

swap(std::any)

any_cast

make_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.

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';
    }

    // 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: hollo
a: b: hollo