std::any::emplace
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Common vocabulary types
template< class ValueType, class... Args >
std::decay_t <ValueType>& emplace( Args&&... args );
(1)
(since C++17)
std::decay_t <ValueType>& emplace( Args&&... args );
template< class ValueType, class U, class... Args >
std::decay_t <ValueType>& emplace( std::initializer_list <U> il, Args&&... args );
(2)
(since C++17)
std::decay_t <ValueType>& emplace( std::initializer_list <U> il, Args&&... args );
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.
- 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.
2) constructs an object of type std::decay_t <ValueType>, direct-non-list-initialized from il, std::forward <Args>(args)..., as the contained object.
- 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.
[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
Run this code
#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