[variant.mod]

# 22 General utilities library [[utilities]](./#utilities)

## 22.6 Variants [[variant]](variant#mod)

### 22.6.3 Class template variant [[variant.variant]](variant.variant#variant.mod)

#### 22.6.3.5 Modifiers [variant.mod]

[🔗](#lib:emplace,variant)

`template<class T, class... Args> constexpr T& emplace(Args&&... args);
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6244)

*Constraints*: is_constructible_v<T, Args...> is true, andT occurs exactly once in Types[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6249)

*Effects*: Equivalent to:return emplace<I>(std::forward<Args>(args)...); where I is the zero-based index of T in Types[.](#2.sentence-1)

[🔗](#lib:emplace,variant_)

`template<class T, class U, class... Args>
  constexpr T& emplace(initializer_list<U> il, Args&&... args);
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6265)

*Constraints*: is_constructible_v<T, initializer_list<U>&, Args...> is true,
and T occurs exactly once in Types[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6270)

*Effects*: Equivalent to:return emplace<I>(il, std::forward<Args>(args)...); where I is the zero-based index of T in Types[.](#4.sentence-1)

[🔗](#lib:emplace,variant__)

`template<size_t I, class... Args>
  constexpr variant_alternative_t<I, variant<Types...>>& emplace(Args&&... args);
`

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6286)

*Mandates*: I < sizeof...(Types)[.](#5.sentence-1)

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6290)

*Constraints*: is_constructible_v<TI, Args...> is true[.](#6.sentence-1)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6294)

*Effects*: Destroys the currently contained value if valueless_by_exception() is false[.](#7.sentence-1)

Then direct-non-list-initializes the contained value of type TI with the arguments std​::​forward<Args>(args)...[.](#7.sentence-2)

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6301)

*Postconditions*: index() is I[.](#8.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6305)

*Returns*: A reference to the new contained value[.](#9.sentence-1)

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6309)

*Throws*: Any exception thrown during the initialization of the contained value[.](#10.sentence-1)

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6313)

*Remarks*: If an exception is thrown during the initialization of the contained value,
the variant is permitted to not hold a value[.](#11.sentence-1)

[🔗](#lib:emplace,variant___)

`template<size_t I, class U, class... Args>
  constexpr variant_alternative_t<I, variant<Types...>>&
    emplace(initializer_list<U> il, Args&&... args);
`

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6327)

*Mandates*: I < sizeof...(Types)[.](#12.sentence-1)

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6331)

*Constraints*: is_constructible_v<TI, initializer_list<U>&, Args...> is true[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6335)

*Effects*: Destroys the currently contained value if valueless_by_exception() is false[.](#14.sentence-1)

Then direct-non-list-initializes the contained value of type TI with il, std​::​forward<Args>(args)...[.](#14.sentence-2)

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6342)

*Postconditions*: index() is I[.](#15.sentence-1)

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6346)

*Returns*: A reference to the new contained value[.](#16.sentence-1)

[17](#17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6350)

*Throws*: Any exception thrown during the initialization of the contained value[.](#17.sentence-1)

[18](#18)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L6354)

*Remarks*: If an exception is thrown during the initialization of the contained value,
the variant is permitted to not hold a value[.](#18.sentence-1)