[utility.undefined]

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

## 22.2 Utility components [[utility]](utility#undefined)

### 22.2.9 Undefined behavior [utility.undefined]

[ð](#lib:unreachable)

`[[noreturn]] void unreachable();
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L696)

*Preconditions*: false is true[.](#1.sentence-1)

[*Note [1](#note-1)*:

This precondition cannot be satisfied, thus the behavior
of calling unreachable is undefined[.](#1.sentence-2)

â *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L704)

[*Example [1](#example-1)*: int f(int x) {switch (x) {case 0:case 1:return x; default: std::unreachable(); }}int a = f(1); // OK, a has value 1int b = f(3); // undefined behavior â *end example*]

[ð](#lib:observable_checkpoint)

`void observable_checkpoint() noexcept;
`

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/utilities.tex#L728)

*Effects*: Establishes an observable checkpoint ([[intro.abstract]](intro.abstract "4.1.2 Abstract machine"))[.](#3.sentence-1)
