std::any_cast

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Utilities library

Language support Type support (basic types, RTTI) Library feature-test macros(C++20) Program utilities Variadic functions initializer_list (C++11) is_constant_evaluated (C++20) is_within_lifetime (C++26) source_location (C++20) Coroutine support(C++20) Contract support(C++26) Three-way comparison three_way_comparablethree_way_comparable_with (C++20)(C++20) strong_ordering (C++20) weak_ordering (C++20) partial_ordering (C++20) common_comparison_category (C++20) compare_three_way_result (C++20) compare_three_way (C++20) strong_order (C++20) weak_order (C++20) partial_order (C++20) compare_strong_order_fallback (C++20) compare_weak_order_fallback (C++20) compare_partial_order_fallback (C++20)   is_eqis_ltis_lteq (C++20)(C++20)(C++20)   is_neqis_gtis_gteq (C++20)(C++20)(C++20)

General utilities Function objects Bit manipulation(C++20) bitset hash (C++11) Relational operators(deprecated in C++20) rel_ops::operator!=rel_ops::operator>    rel_ops::operator<=rel_ops::operator>= Integer comparison functions cmp_equalcmp_lesscmp_less_than (C++20)(C++20)(C++20)   cmp_not_equalcmp_greatercmp_greater_than (C++20)(C++20)(C++20) in_range (C++20) Swap and type operations swap ranges::swap (C++20) exchange (C++14) declval (C++11) to_underlying (C++23) forward (C++11) forward_like (C++23) move (C++11) move_if_noexcept (C++11) as_const (C++17) Common vocabulary types pair tuple (C++11) optional (C++17) any (C++17) variant (C++17) tuple_size (C++11) tuple_element (C++11) apply (C++17) make_from_tuple (C++17) expected (C++23)

 

std::any

Member functions

any::any

any::~any

any::operator=

Modifiers

any::emplace

any::reset

any::swap

Observers

any::has_value

any::type

Non-member functions

swap(std::any)

any_cast

make_any

Helper classes

bad_any_cast

 

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 exampleauto 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(conststd::bad_any_cast& e){std::cout<<"2) "<< e.what()<<'\n';}   // Pointer exampleif(int* i = std::any_cast<int>(&a1))std::cout<<"3) a1 is int: "<<*i <<'\n';elseif(std::string* s = std::any_cast<std::string>(&a1))std::cout<<"3) a1 is std::string: "<<*s <<'\n';elsestd::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<conststd::string&>(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 statestd::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