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Filesystem library (C++17) Regular expressions library (C++11) Concurrency support library (C++11) Technical specifications Symbols index External libraries

[edit]

Utilities library

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 .

Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
is_constant_evaluated (C++20)
is_within_lifetime (C++26)
initializer_list (C++11)
source_location (C++20)
Debugging support
is_debugger_present (C++26)
breakpoint_if_debugging (C++26)
breakpoint (C++26)
Three-way comparison
three_way_comparable three_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_eq is_lt is_lteq (C++20) (C++20) (C++20)
is_neq is_gt is_gteq (C++20) (C++20) (C++20)
General utilities
Date and time
Function objects
Formatting library (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_equal cmp_less cmp_less_than (C++20) (C++20) (C++20)
cmp_not_equal cmp_greater cmp_greater_than (C++20) (C++20) (C++20)
in_range (C++20)
Swap and type operations
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
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)
Elementary string conversions
to_chars (C++17)
from_chars (C++17)
chars_format (C++17)
to_chars_result (C++17)
from_chars_result (C++17)

Contents 1 Parameters 2 Return value 3 Exceptions 4 Example 5 Defect reports [ edit ] Parameters [ edit ] 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. [ edit ] Exceptions 1-3) Throws std::bad_any_cast if the `typeid` of the requested `T` does not match that of the contents of operand . [ edit ] 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'; 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'; 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 [edit] Defect reports The following behavior-changing defect reports were applied retroactively to previously published C++ standards.	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 Toolbox What links here Related changes Upload file Special pages Printable version Permanent link Page information In other languages Español 日本語 Русский 中文 This page was last modified on 3 February 2024, at 19:44. Privacy policy About cppreference.com Disclaimers

[ edit ] Parameters

[ edit ] 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

matches that of the contents of operand , a pointer to the value contained by operand, otherwise a null pointer.

[ edit ] Exceptions

1-3) Throws std::bad_any_cast if the typeid of the requested

does not match that of the contents of operand .

[ edit ] 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';
        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';
        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 
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

Applied to

Behavior as published

Correct behavior