std::variant<Types...>::emplace

Declarations

C++20

// 1)
template <class T, class... Args>
constexpr T& emplace(Args&&... args);
// 2)
template <class T, class U, class... Args>
constexpr T& emplace( std::initializer_list<U> il, Args&&... args );
// 3)
template <std::size_t I, class... Args>
constexpr std::variant_alternative_t<I, variant>& emplace( Args&&... args );
// 4)
template <std::size_t I, class U, class... Args>
constexpr std::variant_alternative_t<I, variant>&
    emplace( std::initializer_list<U> il, Args&&... args );

C++17

// 1)
template <class T, class... Args>
T& emplace(Args&&... args);
// 2)
template <class T, class U, class... Args>
T& emplace( std::initializer_list<U> il, Args&&... args );
// 3)
template <std::size_t I, class... Args>
std::variant_alternative_t<I, variant>& emplace( Args&&... args );
// 4)
template <std::size_t I, class U, class... Args>
std::variant_alternative_t<I, variant>&
    emplace( std::initializer_list<U> il, Args&&... args );

Creates a new value in-place, in an existing variant object

1. 
   

Equivalent to emplace<I>(std::forward<Args>(args)...), where I is the zero-based index of T in Types....

• This overload participates in overload resolution only if std::is_constructible_v<T, Args...> is true, and T occurs exactly once in Types...

2. 
   

Equivalent to emplace<I>(il, std::forward<Args>(args)...), where I is the zero-based index of T in Types....

• This overload participates in overload resolution only if std::is_constructible_v<T, std::initializer_list<U>&, Args...> is true, and T occurs exactly once in Types...

3. 
   

First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type T_I with the arguments std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception.

• This overload participates in overload resolution only if std::is_constructible_v<T_I, Args...> is true.
• It is a compile-time error if I is not less than sizeof...(Types).

4. 
   

First, destroys the currently contained value (if any). Then direct-initializes the contained value as if constructing a value of type T_I with the arguments il, std::forward<Args>(args).... If an exception is thrown, *this may become valueless_by_exception.

• This overload participates in overload resolution only if std::is_constructible_v<T_I, std::initializer_list<U>&, Args...> is true.
• It is a compile-time error if I is not less than sizeof...(Types).

Parameters

args - constructor arguments to use when constructing the new value il - initializer_list argument to use when constructing the new value

Return value

A reference to the new contained value.

Exceptions

1-4) Any exception thrown during the initialization of the contained value.

Example

#include <iostream>
#include <string>
#include <variant>
 
int main()
{
    std::variant<std::string> v1;
    v1.emplace<0>("abc"); // OK
    std::cout << std::get<0>(v1) << '\n';
    v1.emplace<std::string>("def"); // OK
    std::cout << std::get<0>(v1) << '\n';
 
    std::variant<std::string, std::string> v2;
    v2.emplace<1>("ghi"); // OK
    std::cout << std::get<1>(v2) << '\n';
    // v2.emplace<std::string>("abc"); -> Error
}

Result

abc
def
ghi

Defect reports The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
P2231R1	C++20	emplace was not constexpr while the required operations can be constexpr in C++20	made constexpr