NAME¶

std::out_ptr_t - std::out_ptr_t

Synopsis¶

Defined in header <memory>
template< class Smart, class Pointer, class... Args > (since C++23)
class out_ptr_t;

out_ptr_t is used to adapt types such as smart pointers for foreign functions that
output their results via a Pointer* (usually T** for some object type T) or void**
parameter.

out_ptr_t captures additional arguments on construction, provides storage for the
result to which such an aforementioned foreign function writes, and finally resets
the adapted Smart object with the result and the captured arguments when it is
destroyed.

out_ptr_t behaves as if it holds following non-static data members:

* a Smart& reference, which is bound to the adapted object on construction,
* for every T in Args..., a member of type T, which is an argument captured on
construction and used for resetting while destruction, and
* a member subobject that suitable for storing a Pointer within it and providing a
void* object, where the Pointer or void* object is generally exposed to a
foreign function for re-initialization.

Users can control whether each argument for resetting is captured by copy or by
reference, by specifying an object type or a reference type in Args... respectively.

Template parameters¶

Smart - the type of the object (typically a smart pointer) to adapt
Pointer - type of the object (typically a raw pointer) to which a foreign function
writes its result
Args... - type of captured arguments used for resetting the adapted object

Type requirements¶

-
Pointer must meet the requirements of NullablePointer.
-
The program is ill-formed if Smart is a std::shared_ptr specialization and
sizeof...(Args) == 0.

Specializations¶

Unlike most class templates in the standard library, 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.

This license allows a program-defined specialization to expose the raw pointer
stored within a non-standard smart pointer to foreign functions.

Member functions¶

constructor constructs an out_ptr_t
(C++23) (public member function)
operator= out_ptr_t is not assignable
[deleted](C++23) (public member function)
destructor resets the adapted smart pointer
(C++23) (public member function)
operator Pointer* converts the out_ptr_t to the address of the storage for output
operator void** (public member function)
(C++23)

Non-member functions¶

out_ptr creates an out_ptr_t with an associated smart pointer and resetting
(C++23) arguments
(function template)

Notes¶

out_ptr_t expects that the foreign functions do not used the value of the pointed-to
Pointer, and only re-initialize it. The value of the smart pointer before adaption
is not used.

The typical usage of out_ptr_t is creating its temporary objects by std::out_ptr,
which resets the adapted smart pointer immediately. E.g. given a setter function and
a smart pointer of appropriate type declared with int foreign_setter(T**); and
std::unique_ptr<T, D> up; respectively,

int foreign_setter(T**);
std::unique_ptr<T, D> up;

if (int ec = foreign_setter(std::out_ptr(up)))
return ec;

is roughly equivalent to

int foreign_setter(T**);
std::unique_ptr<T, D> up;
T* raw_p{};

int ec = foreign_setter(&raw_p);
up.reset(raw_p);
if (ec != 0)
return ec;

It is not recommended to create an out_ptr_t object of a storage duration other than
automatic storage duration, because such code is likely to produce dangling
references and result in undefined behavior on destruction.

out_ptr_t forbids the usage that would reset a std::shared_ptr without specifying a
deleter, because it would call std::shared_ptr::reset and replace a custom deleter
later.

Captured arguments are typically packed into a std::tuple<Args...>. Implementations
may use different mechanism to provide the Pointer or void* object they need hold.

Feature-test macro Value Std Feature
__cpp_lib_out_ptr 202106L (C++23) std::out_ptr, std::inout_ptr
202311L (C++26) freestanding std::out_ptr and std::inout_ptr

Example¶

This section is incomplete
Reason: no example

See also¶

inout_ptr_t interoperates with foreign pointer setters, obtains the initial pointer
(C++23) value from a smart pointer, and resets it on destruction
(class template)
unique_ptr smart pointer with unique object ownership semantics
(C++11) (class template)
shared_ptr smart pointer with shared object ownership semantics
(C++11) (class template)

Category:¶

* Todo no example