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std::any::any

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

Contents

[edit] Template parameters

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

[edit] 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

[edit] Exceptions

2,4,5,6) Throws any exception thrown by the constructor of the contained type.

[edit] 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.

[edit] Example

#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> >

[edit] See also

assigns an any object
(public member function) [edit]