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std::any::emplace

template< class ValueType, class... Args >
std::decay_t<ValueType>& emplace( Args&&... args );
 (1) (since C++17) 
template< class ValueType, class U, class... Args >
std::decay_t<ValueType>& emplace( std::initializer_list<U> il, Args&&... args );
 (2) (since C++17)

Changes the contained object to one of type std::decay_t<ValueType> constructed from the arguments.

First destroys the current contained object (if any) by reset(), then:

1) constructs an object of type std::decay_t<ValueType>, direct-non-list-initialized from std::forward<Args>(args)..., as the contained object. 2) constructs an object of type std::decay_t<ValueType>, direct-non-list-initialized from il, std::forward<Args>(args)..., as the contained object.

Template parameters

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

Return value

A reference to the new contained object.

Exceptions

Throws any exception thrown by T's constructor. If an exception is thrown, the previously contained object (if any) has been destroyed, and *this does not contain a value.

Example

#include <algorithm>
#include <any>
#include <iostream>
#include <string>
#include <vector>
 
class Star
{
    std::string name;
    int id;
 
public:
    Star(std::string name, int id) : name{name}, id{id}
    {
        std::cout << "Star::Star(string, int)\n";
    }
 
    void print() const
    {
        std::cout << "Star{\"" << name << "\" : " << id << "};\n";
    }
};
 
int main()
{
    std::any celestial;
    // (1) emplace(Args&&... args);
    celestial.emplace<Star>("Procyon", 2943);
    const auto* star = std::any_cast<Star>(&celestial);
    star->print();
 
    std::any av;
    // (2) emplace(std::initializer_list<U> il, Args&&... args);
    av.emplace<std::vector<char>>({'C', '+', '+', '1', '7'} /* no args */);
    std::cout << av.type().name() << '\n';
    const auto* va = std::any_cast<std::vector<char>>(&av);
    std::for_each(va->cbegin(), va->cend(), [](char const& c) { std::cout << c; });
    std::cout << '\n';
}

Possible output:

Star::Star(string, int)
Star{"Procyon" : 2943};
St6vectorIcSaIcEE
C++17

See also

constructs an any object
(public member function)
destroys contained object
(public member function)

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