< cpp‎ | utility‎ | any
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
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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.


[edit] Template parameters

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

[edit] Return value

A reference to the new contained object.

[edit] 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.

[edit] Example

#include <algorithm>
#include <any>
#include <iostream>
#include <string>
#include <vector>
class Star
    std::string name;
    int id;
    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);
    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};

[edit] See also

constructs an any object
(public member function) [edit]
destroys contained object
(public member function) [edit]