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Use std::in_place for std::optional<T>, std::in_place_type<T> for std::any, and either std::in_place_type<T> or std::in_place_index<I> for std::variant. These C++17 tags tell a wrapper which value to construct and let you pass constructor arguments directly, rather than creating a temporary value first.
What the three in-place tags mean
std::in_place, std::in_place_type<T>, and std::in_place_index<I> are disambiguation tags introduced with these standard-library wrappers in C++17. They direct a constructor to create the contained object from the arguments that follow. Include the relevant wrapper header—<optional>, <any>, or <variant>—and <utility> for the in-place tags.
The choice of tag follows how the wrapper identifies its value: an optional already has one fixed type, an any stores a runtime-selected type, and a variant holds one alternative from a fixed list.
Constructing each wrapper in place
std::optional<T>: use std::in_place
The template parameter fixes the contained type, so there is no need to name it in the tag. Pass std::in_place followed by arguments accepted by T‘s constructor:
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#include <optional>
#include <utility>
struct Point {
Point(int x, int y) : x(x), y(y) {}
int x, y;
};
std::optional<Point> p(std::in_place, 3, 4);This constructs the Point inside p. The optional is engaged; its value is a Point with coordinates 3 and 4.
std::any: use std::in_place_type<T>
An std::any can hold different types at runtime, so its typed in-place constructor must be told which type to create. The type must meet the constructor requirements, including being copy-constructible.
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#include <any>
#include <utility>
std::any a(std::in_place_type<Point>, 3, 4);Here, a contains a Point. Unlike optional, std::any does not have a fixed contained type in its class template.
std::variant<Types...>: select by type or index
A variant’s alternatives form a closed set. If an alternative type occurs exactly once, select it with std::in_place_type<T>. If duplicate types or conversions make type-based selection ambiguous, use std::in_place_index<I>, where I is the alternative’s zero-based position.
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#include <string>
#include <variant>
#include <utility>
std::variant<Point, std::string> v(std::in_place_index<0>, 3, 4);
std::variant<Point, std::string> by_type(std::in_place_type<Point>, 3, 4);Both examples construct the Point alternative directly. The index form selects the first alternative, Point; the type form identifies that alternative by its unique type.
In-place constructor or emplace?
Use an in-place constructor when creating the wrapper and its initial value together. Use emplace when you already have a wrapper and want to create or replace its contained value. Each wrapper’s emplace function takes constructor arguments directly:
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p.emplace(5, 6);
a.emplace<Point>(7, 8);
v.emplace<Point>(9, 10);For optional, emplace(args...) constructs its fixed T. For any, emplace<T>(args...) names the runtime type. For variant, emplace<T>(args...) selects by type or emplace<I>(args...) selects by zero-based index.
| Wrapper | In-place constructor | emplace |
How the value is selected |
|---|---|---|---|
std::optional<T> |
std::optional<T>(std::in_place, args...) |
opt.emplace(args...) |
T is fixed by the class template. |
std::any |
std::any(std::in_place_type<T>, args...) |
a.emplace<T>(args...) |
Name runtime type T; it must be copy-constructible. |
std::variant<Ts...> |
std::variant<Ts...>(std::in_place_type<T>, args...) or std::in_place_index<I> |
v.emplace<T>(args...) or v.emplace<I>(args...) |
Choose a type that occurs once, or a zero-based alternative index. |
What replacement and constructor failure mean
emplace replaces the wrapper’s current contained value rather than updating it in place. For std::optional, the previous value is destroyed before the new one is constructed. If that construction throws, the optional is disengaged and has no value.
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Do not assume the same post-exception state for std::any or std::variant from optional’s guarantee: their behavior is governed by their own overload requirements and exception specifications. Consult the relevant reference for the specific operation and type when exception recovery matters.
Initializer-list arguments
Initializer-list construction uses dedicated overloads in these APIs. If the contained type is to be constructed from a braced list, pass the initializer list in the position required by the particular any constructor, optional constructor, or variant emplace overload; do not assume a braced list is interchangeable with an ordinary forwarded argument pack.
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