std::any::any

 

C++

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Utilities library

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) 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 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_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) 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)

 

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

 

constexpr any() noexcept;	(1)	(since C++17)
any( const any& other );	(2)	(since C++17)
any( any&& other ) noexcept;	(3)	(since C++17)
template< class ValueType > any( ValueType&& value );	(4)	(since C++17)
template< class ValueType, class... Args > explicit any( std::in_place_type_t<ValueType>, Args&&... args );	(5)	(since C++17)
template< class ValueType, class U, class... Args > explicit any( std::in_place_type_t<ValueType>, std::initializer_list<U> il,               Args&&... args );	(6)	(since C++17)

Constructs a new any object.

1) Constructs an empty object.

2,3) Copies (2) or moves (3) content of other into a new instance, so that any content is equivalent in both type and value to those of other prior to the constructor call, or empty if other is empty. Formally,

2) If other is empty, the constructed object is empty. Otherwise, equivalent to any(std::in_place_type<T>, std::any_cast<const T&>(other)), where T is the type of the object contained in other.

3) If other is empty, the constructed object is empty. Otherwise, the constructed object contains either the object contained in other, or an object of the same type constructed from the object contained in other, considering that object as an rvalue.

4) Constructs an object with initial content an object of type std::decay_t<ValueType>, direct-initialized from std::forward<ValueType>(value).

• This overload participates in overload resolution only if std::decay_t<ValueType> is not the same type as any nor a specialization of std::in_place_type_t, and std::is_copy_constructible_v<std::decay_t<ValueType>> is true.

5) Constructs an object with initial content an object of type std::decay_t<ValueType>, direct-non-list-initialized from std::forward<Args>(args)....

• This overload participates in overload resolution only if std::is_constructible_v<std::decay_t<ValueType>, Args...> and std::is_copy_constructible_v<std::decay_t<ValueType>> are both true.

6) Constructs an object with initial content an object of type std::decay_t<ValueType>, direct-non-list-initialized from il, std::forward<Args>(args)....

• This overload participates in overload resolution only if std::is_constructible_v<std::decay_t<ValueType>, std::initializer_list<U>&, Args...> and std::is_copy_constructible_v<std::decay_t<ValueType>> are both true.

Template parameters

ValueType	-	contained value type
Type requirements
- std::decay_t<ValueType> must meet the requirements of CopyConstructible.

Parameters

other	-	another any object to copy or move from
value	-	value to initialize the contained value with
il, args	-	arguments to be passed to the constructor of the contained object

Exceptions

2,4-6) Throws any exception thrown by the constructor of the contained type.

Notes

Because the default constructor is constexpr, static std::anys are initialized as part of static non-local initialization, before any dynamic non-local initialization begins. This makes it safe to use an object of type std::any in a constructor of any static object.

Example

Run this code

#include <boost/core/demangle.hpp>
 
#include <any>
#include <initializer_list>
#include <iostream>
#include <memory>
#include <set>
#include <string>
#include <utility>
 
struct A
{
    int age;
    std::string name;
    double salary;
 
#if __cpp_aggregate_paren_init < 201902L
    // Required before C++20 for in-place construction
    A(int age, std::string name, double salary)
        : age(age), name(std::move(name)), salary(salary) {}
#endif
};
 
// Using abi demangle to print nice type name of instance of any holding 
void printType(const std::any& a)
{
    std::cout << boost::core::demangle(a.type().name()) << '\n';
}
 
int main()
{
    // Constructor #4: std::any holding int
    std::any a1{7};
 
    // Constructor #5: std::any holding A, constructed in place
    std::any a2(std::in_place_type<A>, 30, "Ada", 1000.25);
 
    // Constructor #6: std::any holding a set of A with custom comparison
    auto lambda = [](auto&& l, auto&& r){ return l.age < r.age; };
    std::any a3(
        std::in_place_type<std::set<A, decltype(lambda)>>,
        {
            A{39, std::string{"Ada"}, 100.25}, 
            A{20, std::string{"Bob"}, 75.5}
        }, 
        lambda);
 
    printType(a1);
    printType(a2);
    printType(a3);
}

Possible output:

int
A
std::set<A, main::{lambda(auto:1&&, auto:2&&)#1}, std::allocator<A> >

See also

operator=

assigns an any object (public member function)