[out.ptr.t]

# 20 Memory management library [[mem]](./#mem)

## 20.3 Smart pointers [[smartptr]](smartptr#out.ptr.t)

### 20.3.4 Smart pointer adaptors [[smartptr.adapt]](smartptr.adapt#out.ptr.t)

#### 20.3.4.1 Class template out_ptr_t [out.ptr.t]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5423)

out_ptr_t is a class template used to adapt types
such as smart pointers ([[smartptr]](smartptr "20.3 Smart pointers"))
for functions that use output pointer parameters[.](#1.sentence-1)

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5428)

[*Example [1](#example-1)*:

#include <memory>#include <cstdio>int fopen_s(std::FILE** f, const char* name, const char* mode);

struct fclose_deleter {void operator()(std::FILE* f) const noexcept { std::fclose(f); }};

int main(int, char*[]) {constexpr const char* file_name = "ow.o";
 std::unique_ptr<std::FILE, fclose_deleter> file_ptr; int err = fopen_s(std::out_ptr<std::FILE*>(file_ptr), file_name, "r+b"); if (err != 0)return 1; // *file_ptr is validreturn 0;}unique_ptr can be used with out_ptr to be passed into an output pointer-style function,
without needing to hold onto an intermediate pointer value and
manually delete it on error or failure[.](#2.sentence-1)

â *end example*]

[ð](#lib:out_ptr_t)

namespace std {template<class Smart, class Pointer, class... Args>class out_ptr_t {public:constexpr explicit out_ptr_t(Smart&, Args...);
 out_ptr_t(const out_ptr_t&) = delete; constexpr ~out_ptr_t(); constexpr operator Pointer*() const noexcept; operator void**() const noexcept; private: Smart& s; // *exposition only* tuple<Args...> a; // *exposition only* Pointer p; // *exposition only*};}

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5480)

Pointer shall meet the [*Cpp17NullablePointer*](nullablepointer.requirements#:Cpp17NullablePointer "16.4.4.4 Cpp17NullablePointer requirements [nullablepointer.requirements]") requirements[.](#3.sentence-1)

If Smart is a specialization of shared_ptr andsizeof...(Args) == 0,
the program is ill-formed[.](#3.sentence-2)

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

It is typically a user error to reset a shared_ptr without specifying a deleter,
as shared_ptr will replace a custom deleter upon usage of reset,
as specified in [[util.smartptr.shared.mod]](util.smartptr.shared.mod "20.3.2.2.5 Modifiers")[.](#3.sentence-3)

â *end note*]

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5492)

Program-defined specializations of out_ptr_t that depend on at least one program-defined type
need not meet the requirements for the primary template[.](#4.sentence-1)

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5497)

Evaluations of the conversion functions
on the same object may conflict ([[intro.races]](intro.races "6.10.2.2 Data races"))[.](#5.sentence-1)

[ð](#lib:out_ptr_t,constructor)

`constexpr explicit out_ptr_t(Smart& smart, Args... args);
`

[6](#6)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5507)

*Effects*: Initializes s with smart,a with std​::​forward<Args>(args)..., and
value-initializes p[.](#6.sentence-1)

Then, equivalent to:

- [(6.1)](#6.1)

s.reset(); if the expression s.reset() is well-formed;

- [(6.2)](#6.2)

otherwise,s = Smart(); if is_constructible_v<Smart> is true;

- [(6.3)](#6.3)

otherwise, the program is ill-formed[.](#6.sentence-2)

[7](#7)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5533)

[*Note [2](#note-2)*:

The constructor is not noexcept to allow for a variety of non-terminating and safe implementation strategies[.](#7.sentence-1)

For example, an implementation can allocate
a shared_ptr's internal node in the constructor and
let implementation-defined exceptions escape safely[.](#7.sentence-2)

The destructor can then move the allocated control block in directly and
avoid any other exceptions[.](#7.sentence-3)

â *end note*]

[ð](#lib:out_ptr_t,destructor)

`constexpr ~out_ptr_t();
`

[8](#8)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5551)

Let SP be*POINTER_OF_OR*(Smart, Pointer) ([[memory.general]](memory.general "20.2.1 General"))[.](#8.sentence-1)

[9](#9)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5555)

*Effects*: Equivalent to:

- [(9.1)](#9.1)

if (p) { apply([&](auto&&... args) { s.reset(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));} if the expressions.reset(static_cast<SP>(p), std​::​forward<Args>(args)...) is well-formed;

- [(9.2)](#9.2)

otherwise,if (p) { apply([&](auto&&... args) { s = Smart(static_cast<SP>(p), std::forward<Args>(args)...); }, std::move(a));} if is_constructible_v<Smart, SP, Args...> is true;

- [(9.3)](#9.3)

otherwise, the program is ill-formed[.](#9.sentence-1)

[ð](#itemdecl:3)

`constexpr operator Pointer*() const noexcept;
`

[10](#10)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5590)

*Preconditions*: operator void**() has not been called on *this[.](#10.sentence-1)

[11](#11)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5594)

*Returns*: addressof(const_cast<Pointer&>(p))[.](#11.sentence-1)

[ð](#itemdecl:4)

`operator void**() const noexcept;
`

[12](#12)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5604)

*Constraints*: is_same_v<Pointer, void*> is false[.](#12.sentence-1)

[13](#13)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5608)

*Mandates*: is_pointer_v<Pointer> is true[.](#13.sentence-1)

[14](#14)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5612)

*Preconditions*: operator Pointer*() has not been called on *this[.](#14.sentence-1)

[15](#15)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5616)

*Returns*: A pointer value v such that:

- [(15.1)](#15.1)

the initial value *v is equivalent to static_cast<void*>(p) and

- [(15.2)](#15.2)

any modification of *v that is not followed by a subsequent modification of *this affects the value of p during the destruction of *this,
such that static_cast<void*>(p) == *v[.](#15.sentence-1)

[16](#16)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5629)

*Remarks*: Accessing *v outside the lifetime of *this has undefined behavior[.](#16.sentence-1)

[17](#17)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/memory.tex#L5634)

[*Note [3](#note-3)*:

reinterpret_cast<void**>(static_cast<Pointer*>(*this)) can be a viable implementation strategy for some implementations[.](#17.sentence-1)

â *end note*]
