[ptr.launder]

# 17 Language support library [[support]](./#support)

## 17.6 Dynamic memory management [[support.dynamic]](support.dynamic#ptr.launder)

### 17.6.5 Pointer optimization barrier [ptr.launder]

[🔗](#lib:launder)

`template<class T> constexpr T* launder(T* p) noexcept;
`

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3160)

*Mandates*: !is_function_v<T> && !is_void_v<T> is true[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3164)

*Preconditions*: p represents the address *A* of a byte in memory[.](#2.sentence-1)

An object *X* that is within its [lifetime](basic.life#def:lifetime "6.8.4 Lifetime [basic.life]") and whose type is [similar](conv.qual#def:similar_types "7.3.6 Qualification conversions [conv.qual]") to T is located at the address *A*[.](#2.sentence-2)

All bytes of storage that would be
reachable through ([[basic.compound]](basic.compound "6.9.4 Compound types")) the result
are reachable through p[.](#2.sentence-3)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3174)

*Returns*: A value of type T* that points to *X*[.](#3.sentence-1)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3178)

*Remarks*: An invocation of this function
may be used in a core constant expression
if and only if the (converted) value of its argument
may be used in place of the function invocation[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3185)

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

If a new object is created
in storage occupied by an existing object of the same type,
a pointer to the original object
can be used to refer to the new object
unless its complete object is a const object or it is a base class subobject;
in the latter cases,
this function can be used to obtain a usable pointer to the new object[.](#5.sentence-1)

See [[basic.life]](basic.life "6.8.4 Lifetime")[.](#5.sentence-2)

— *end note*]

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/support.tex#L3197)

[*Example [1](#example-1)*: struct X { int n; };const X *p = new const X{3};const int a = p->n;new (const_cast<X*>(p)) const X{5}; // p does not point to new object ([[basic.life]](basic.life "6.8.4 Lifetime")) because its type is constconst int b = p->n; // undefined behaviorconst int c = std::launder(p)->n; // OK — *end example*]